LECTURE NOTES IN HUMAN ANATOMY AND PHYSIOLOGY
Skeletal Muscle Groups
In our discussion, the muscles of the body will be grouped according to their location and their action. After you understand the meaning of a muscle’s name, try to correlate its name with the muscle’s location and the action it performs. Knowing the origin and insertion will also help you remember what the muscle does. Why? Because the insertion is on the bone that moves. Only then will you be able to understand the actions of the muscles listed in Tables 7.2–7.5. Scientific terminology is necessary because it allows all persons to know the exact action being described for that muscle. Also review the meaning of the terms arm and leg.
Muscles of the Head
The muscles of the head and neck are the first group of muscles we will study. The muscles of the head are responsible for facial expression and mastication (chewing). One muscle of the head and several muscles of the neck allow us to swallow. The muscles of the neck also move the head.
Muscles of Facial Expression
The muscles of facial expression are located on the scalp and face. These muscles are unusual in that they insert into and move the skin. Therefore, we expect them to move the skin and not a bone. The use of these muscles communicates to others whether we are surprised, angry, fearful, happy, and so forth.
Frontalis lies over the frontal bone; it raises the eyebrows and wrinkles the brow. Frequent use results in furrowing of the forehead.
Orbicularis oculi is a ringlike band of muscle that encircles (forms an orbit about) the eye. It causes the eye to close or blink, and is responsible for “crow’s feet” at the eye corners.
Orbicularis oris encircles the mouth and is used to pucker the lips, as in forming a kiss. Frequent use results in lines about the mouth.
Buccinator muscles are located in the cheek areas. When a buccinator contracts, the cheek is compressed, as when a person whistles or blows out air. Therefore, this muscle is called the “trumpeter’s muscle.” Important to everyday life, the buccinator helps hold food in contact with the teeth during chewing. It is also used in swallowing, as discussed next.
Zygomaticus extends from each zygomatic arch (cheekbone) to the corners of the mouth. It raises the corners of the mouth when a person smiles.
Muscles of Mastication
The muscles of mastication are used when we chew food or bite something. As you might expect, both of these muscles insert on the mandible. Each masseter has its origin on the zygomatic arch and its insertion on the mandible.
The masseter is a muscle of mastication (chewing) because it is a prime mover for elevating the mandible.
Each temporalis is a fan-shaped muscle that overlies the temporal bone. It is also a prime mover for elevating the mandible. The masseter and temporalis are synergists.
Musclesofthe Neck
Deep muscles of the neck (not illustrated) are responsible for swallowing. Superficial muscles of the neck move the head.
Swallowing
Swallowing is an important activity that begins after we chew our food. First, the tongue (a muscle) and the buccinators squeeze the food back along the roof of the mouth toward the pharynx. An important bone that functions in swallowing is the hyoid. The hyoid is the only bone in the body that does not articulate with another bone. Muscles that lie superior to the hyoid, called the suprahyoid muscles, and muscles that lie inferior to the hyoid, called the infrahyoid muscles, move the hyoid. These muscles lie deep in the neck and are not illustrated. The suprahyoid muscles pull the hyoid forward and upward toward the mandible. Because the hyoid is attached to the larynx, this pulls the larynx upward and forward. The epiglottis now lies over the glottis and closes the respiratory passages. Small palatini muscles (not illustrated) pull the soft palate backward, closing off the nasal passages. Pharyngeal constrictor muscles (not illustrated) push the bolus of food into the pharynx, which widens when the suprahyoid muscles move the hyoid. The hyoid bone and larynx are returned to their original positions by the infrahyoid muscles. Notice that the suprahyoid and infrahyoid muscles are antagonists.
Muscles that Move the Head
Two muscles in the neck are of particular interest: The sternocleidomastoid and the trapezius. Recall that flexion is a movement that closes the angle at a joint and extension is a movement that increases the angle at a joint. Recall that abduction is a movement away from the midline of the body, while adduction is a movement toward the midline. Also, rotation is the movement of a part around its own axis.
Sternocleidomastoid muscles ascend obliquely from their origin on the sternum and clavicle to their insertion on the mastoid process of the temporal bone. Which part of the body do you expect them to move? When both sternocleidomastoid muscles contract, flexion of the head occurs. When only one contracts, the head turns to the opposite side. If you turn your head to the right, you can see how the left sternocleidomastoid shortens, pulling the head to the right.
Each trapezius muscle is triangular, but together, they take on a diamond or trapezoid shape. The origin of a trapezius is at the base of the skull. Its insertion is on a clavicle and scapula. You would expect the trapezius muscles to move the scapulae, and they do. They adduct the scapulae when the shoulders are shrugged or pulled back. The trapezius muscles also help extend the head, however. The prime movers for head extension are actually deep to the trapezius.
Functions and Types of Muscles
All muscles, regardless of the particular type, can contract that is, shorten. When muscles contract, some part of the body or the entire body moves. Humans have three types of muscles: smooth, cardiac, and skeletal. The contractile cells of these tissues are elongated and therefore are called muscle fibers.
Smooth Muscle
Smooth muscle is located in the walls of hollow internal organs, and its involuntary contraction moves materials through an organ. Smooth muscle fibers are spindle-shaped cells, each with a single nucleus (uninucleated). The cells are usually arranged in parallel lines, forming sheets. Smooth muscle does not have the striations (bands of light and dark) seen in cardiac and skeletal muscle. Although smooth muscle is slower to contract than skeletal muscle, it can sustain prolonged contractions and does not fatigue easily.
Cardiac Muscle
Cardiac muscle forms the heart wall. Its fibers are uninucleated, striated, tubular, and branched, which allows the fibers to interlock at intercalated disks. Intercalated disks permit contractions to spread quickly throughout the heart. Cardiac fibers relax completely between contractions, which prevents fatigue. Contraction of cardiac muscle fibers is rhythmical; it occurs without outside nervous stimulation or control. Thus, cardiac muscle contraction is involuntary.
Skeletal Muscle
Skeletal muscle fibers are tubular, multinucleated, and striated. They make up the skeletal muscles attached to the skeleton. Skeletal muscle fibers can run the length of a muscle and therefore can be quite long. Skeletal muscle is voluntary because its contraction is always stimulated and controlled by the nervous system. In this chapter, we will explore why skeletal muscle (and cardiac muscle) is striated.
Connective Tissue Coverings
Muscles are organs, and as such they contain other types of tissues, such as nervous tissue, blood vessels, and connective tissue. Connective tissue is essential to the organization of the fibers within a muscle. First, each fiber is surrounded by a thin layer of areolar connective tissue called the endomysium. Blood capillaries and nerve fibers reach each muscle fiber by way of the endomysium. Second, the muscle fibers are grouped into bundles called fascicles. The fascicles have a sheath of connective tissue called the perimysium. Finally, the muscle itself is covered by a connective tissue layer called the epimysium. The epimysium becomes a part of the fascia,a layer of fibrous tissue that separates muscles from each other (deep fascia) and from the skin (superficial fascia). Collagen fibers of the epimysium continue as a strong, fibrous tendonthat attaches the muscle to a bone. The epimysium merges with the periosteum of the bone.
Functions of Skeletal Muscles
This chapter concerns the skeletal muscles, and therefore it is fitting to consider their functions independent of the other types of muscles: Skeletal muscles support the body. Skeletal muscle contraction opposes the force of gravity and allows us to remain upright. Some skeletal muscles are serving this purpose even when you think you are relaxed. Skeletal muscles make bones and other body parts move. Muscle contraction accounts not only for the movement of limbs but also for eye movements, facial expressions, and breathing. Skeletal muscles help maintain a constant body temperature. Skeletal muscle contraction causes ATP to break down, releasing heat that is distributed about the body. Skeletal muscle contraction assists movement in cardiovascular and lymphatic vessels. The pressure of skeletal muscle contraction keeps blood moving in cardiovascular veins and lymph moving in lymphatic vessels. Skeletal muscles help protect internal organs and stabilize joints. Muscles pad the bones that protect organs, and they have tendons that help hold bones together at joints.
Microscopic Anatomy and Contraction of Skeletal Muscle
We have already examined the structure of skeletal muscle as seen with the light microscope. As you know, skeletal muscle tissue has alternating light and dark bands, giving it a striated appearance. The electron microscope shows that these bands are due to the arrangement of myofilaments in a muscle fiber.
Muscle Fiber
A muscle fiber contains the usual cellular components, but special names have been assigned to some of these components. The plasma membrane is called the sarcolemma; the cytoplasm is the sarcoplasm; and the endoplasmic reticulum is the sarcoplasmic reticulum. A muscle fiber also has some unique anatomical characteristics. One feature is its T (for transverse) system; the sarcolemma forms T (transverse) tubules that penetrate, or dip down, into the cell so that they come into contact—but do not fuse—with expanded portions of the sarcoplasmic reticulum. The expanded portions of the sarcoplasmic reticulum are calcium storage sites. Calcium ions (Ca ), as we shall see, are essential for muscle contraction. The sarcoplasmic reticulum encases hundreds and sometimes even thousands of myofibrils, each about 1 m in diameter, which are the contractile portions of the muscle fibers. Any other organelles, such as mitochondria, are located in the sarcoplasm between the myofibrils. The sarcoplasm also contains glycogen, which provides stored energy for muscle contraction, and the red pigment myoglobin,which binds oxygen until it is needed for muscle contraction.
Myofibrils and Sarcomeres
Myofibrils are cylindrical in shape and run the length of the muscle fiber. The striations of skeletal muscle fibers are formed by the placement of myofilaments within units of myofibrils called sarcomeres. A sarcomere extends between two darklines called the Z lines. A sarcomere contains two types of protein myofilaments. The thick filaments are made up of a protein called myosin, and the thin filaments are made up of a protein called actin. Other proteins are also present. The I band is light colored because it contains only actin filaments attached to a Z line. The dark regions of the A band contain overlapping actin and myosin filaments, and its H zone has only myosin filaments.
Myofilaments
The thick and thin filaments differ in the following ways: Thick Filaments A thick filament is composed of several hundred molecules of the protein myosin. Each myosin molecule is shaped like a golf club, with the straight portion of the molecule ending in a double globular head, or crossbridge. Cross-bridges are slanted away from the middle of a sarcomere. Thin Filaments Primarily, a thin filament consists of two intertwining strands of the protein actin. Two other proteins, called tropomyosin and troponin, are also present, as we will discuss later in this section. Sliding Filaments We will also see that when muscles are innervated, impulses travel down a T tubule, and calcium is released from the sarcoplasmic reticulum. Now the muscle fiber contracts as the sarcomeres within the myofibrils shorten. When a sarcomere shortens, the actin (thin) filaments slide past the myosin (thick) filaments and approach one another. This causes the I band to shorten and the H zone to almost or completely disappear. The movement of actin filaments in relation to myosin filaments is called the sliding filament theory of muscle contraction. During the sliding process, the sarcomere shortens even though the filaments themselves remain the same length. ATP supplies the energy for muscle contraction. Although the actin filaments slide past the myosin filaments, it is the myosin filaments that do the work. Myosin filaments break down ATP and have crossbridges that pull the actin filaments toward the center of the sarcomere. Skeletal Muscle Contraction Muscle fibers are innervated—that is, they are stimulated to contract by motor neurons whose axons are found in nerves. The axon of one motor neuron has several branches and can stimulate from a few to several muscle fibers of a particular muscle. Each branch of the axon ends in an axon terminal that lies in close proximity to the sarcolemma of a muscle fiber. A small gap, called a synaptic cleft, separates the axon bulb from the sarcolemma. This entire region is called a neuromuscular junction. Axon terminals contain synaptic vesicles that are filled with the neurotransmitter acetylcholine (ACh). When nerve impulses traveling down a motor neuron arrive at an axon terminal, the synaptic vesicles release a neurotransmitter into the synaptic cleft. It quickly diffuses across the cleft and binds to receptors in the sarcolemma. Now the sarcolemma generates impulses that spread over the sarcolemma and down T tubules to the sarcoplasmic reticulum. The release of calcium from the sarcoplasmic reticulum causes the filaments within the sarcomeres to slide past one another. Sarcomere contraction results in myofibril contraction, which in turn results in muscle fiber, and finally muscle, contraction. The Role of Actin and Myosin shows the placement of two other proteins associated with an actin filament, which you will recall is composed of a double row of twisted actin molecules. Threads of tropomyosin wind about an actin filament, and troponin occurs at intervals along the threads. calcium ions (Ca ) that have been released from the sarcoplasmic reticulum combine with troponin. After binding occurs, the tropomyosin threads shift their position, and myosin binding sites are exposed.
The double globular heads of a myosin filament have ATP binding sites. The heads function as ATPase enzymes, splitting ATP into ADP and P . This reaction activates the head so that it will bind to actin. The ADP and P remain on the myosin heads until the heads attach to actin, forming a cross-bridge. Now, ADP and P are released, and this causes the cross-bridges to change their positions. This is the power stroke that pulls the thin filaments toward the middle of the sarcomere. When another ATP molecule binds to a myosin head, the cross-bridge is broken as the head detaches from actin. The cycle begins again; the actin filaments move nearer the center of the sarcomere each time the cycle is repeated. Contraction continues until nerve impulses cease and calcium ions are returned to their storage sites. The membranes of the sarcoplasmic reticulum contain active transport proteins that pump calcium ions back into the sarcoplasmic reticulum.
Skeletal Muscles of the Body
The human body has some 600 skeletal muscles, but this text will discuss only some of the most significant of these. First, let us consider certain basic principles of muscle contraction.
Basic Principles
When a muscle contracts, one bone remains fairly stationary, and the other one moves. The origin of a muscle is on the stationary bone, and the insertion of a muscle is on the bone that moves. Frequently, a body part is moved by a group of muscles working together. Even so, one muscle does most of the work, and this muscle is called the prime mover. For example, in flexing the elbow, the prime mover is the biceps brachii. The assisting muscles are called the synergists. The brachialis is a synergist that helps the biceps brachii flex the elbow. A prime mover can have several synergists. When muscles contract, they shorten. Therefore, muscles can only pull; they cannot push. However, muscles have antagonists, and antagonistic pairs work opposite one another to bring about movement in opposite directions. For example, the biceps brachii and the triceps brachii are antagonists; one flexes the forearm, and the other extends the forearm. Later on in our discussion, we will encounter other antagonistic pairs.
Naming Muscles
When learning the names of muscles, considering what the name means will help you remember it. The names of the various skeletal muscles are often combinations of the following terms used to characterize muscles:
1. Size. For example, the gluteus maximus is the largest muscle that makes up the buttocks. The gluteus minimus is the smallest of the gluteal muscles. Other terms used to indicate size are vastus (huge), longus (long), and brevis (short).
2. Shape. For example, the deltoid is shaped like a delta, or triangle, while the trapezius is shaped like a trapezoid. Other terms used to indicate shape are latissimus (wide) and teres (round).
3. Direction of fibers. For example, the rectus abdominis is a longitudinal muscle of the abdomen (rectus means straight). The orbicularis is a circular muscle around the eye. Other terms used to indicate direction are transverse (across) and oblique (diagonal).
4. Location. For example, the frontalis overlies the frontal bone. The external obliques are located outside the internal obliques. Other terms used to indicate location are pectoralis (chest), gluteus (buttock), brachii (arm), and sub (beneath). You should also review these directional terms: anterior, posterior, lateral, medial, proximal, distal, superficial, and deep.
Vertebral Column (Spine)
The vertebral column extends from the skull to the pelvis. It consists of a series of separate bones, the vertebrae, separated by pads of fibrocartilage called the intervertebral disks. The vertebral column is located in the middorsal region and forms the vertical axis. The skull rests on the superior end of the vertebral column, which also supports the rib cage and serves as a point of attachment for the pelvic girdle. The vertebral column also protects the spinal cord, which passes through a vertebral canal formed by the vertebrae. The vertebrae are named according to their location: seven cervical (neck) vertebrae, twelve thoracic (chest) vertebrae, five lumbar (lower back) vertebrae, five sacral vertebrae fused to form the sacrum, and three to five coccygeal vertebrae fused into one coccyx. When viewed from the side, the vertebral column has four normal curvatures, named for their location. The cervical and lumbar curvatures are convex anteriorly, and the thoracic and sacral curvatures are concave anteriorly. In the fetus, the vertebral column has but one curve, and it is concave anteriorly. The cervical curve develops three to four months after birth, when the child begins to hold the head up. The lumbar curvature develops when a child begins to stand and walk, around one year of age. The curvatures of the vertebral column provide more support than a straight column would, and they also provide the balance needed to walk upright. The curvatures of the vertebral column are subject to abnormalities. An abnormally exaggerated lumbar curvature is called lordosis, or “swayback.” People who are balancing a heavy midsection, such as pregnant women or men with “potbellies,” may have swayback. An increased roundness of the thoracic curvature is kyphosis, or “hunchback.” This abnormality sometimes develops in older people. An abnormal lateral (side-to-side) curvature is called scoliosis. Occurring most often in the thoracic region, scoliosis is usually first seen during late childhood.
Intervertebral Disks
The fibrocartilaginous intervertebral disks located between the vertebrae act as a cushion. They prevent the vertebrae from grinding against one another and absorb shock caused by such movements as running, jumping, and even walking. The disks also allow motion between the vertebrae so that a person can bend forward, backward, and from side to side. Unfortunately, these disks become weakened with age, and can slip or even rupture (called a herniated disk). A damaged disk pressing against the spinal cord or the spinal nerves causes pain. Such a disk may need to be removed surgically. If a disk is removed, the vertebrae are fused together, limiting the body’s flexibility.
Vertebrae
The vertebral arch forms the wall of a vertebral foramen (pl., foramina). The foramina become a canal through which the spinal cord passes. The vertebral spinous process (spine) occurs where two thin plates of bone called laminae meet. A transverse process is located where a pedicle joins a lamina. These processes serve for the attachment of muscles and ligaments. Articular processes (superior and inferior) serve for the joining of vertebrae. The vertebrae have regional differences. For example, as the vertebral column descends, the bodies get bigger and are better able to carry more weight. In the cervical region, the spines are short and tend to have a split, or bifurcation. The thoracic spines are long and slender and project downward. The lumbar spines are massive and square and project posteriorly. The transverse processes of thoracic vertebrae have articular facets for connecting to ribs. Atlas and Axis the first two cervical vertebrae are not typical. The atlas supports and balances the head. It has two depressions that articulate with the occipital condyles, allowing movement of the head forward and back. The axis has an odontoid process (also called the dens) that projects into the ring of the atlas. When the head moves from side to side, the atlas pivots around the odontoid process. Sacrum and Coccyx The five sacral vertebrae are fused to form the sacrum. The sacrum articulates with the pelvic girdle and forms the posterior wall of the pelvic cavity. The coccyx, or tailbone, is the last part of the vertebral column. It is formed from a fusion of three to five vertebrae.
The Rib Cage
The rib cage, sometimes called the thoracic cage, is composed of the thoracic vertebrae, ribs and associated cartilages, and sternum. The rib cage demonstrates how the skeleton is protective but also flexible. The rib cage protects the heart and lungs; yet it swings outward and upward upon inspiration and then downward and inward upon expiration. The rib cage also provides support for the bones of the pectoral girdle.
The Ribs
There are twelve pairs of ribs. All twelve pairs connect directly to the thoracic vertebrae in the back. After connecting with thoracic vertebrae, each rib first curves outward and then forward and downward. A rib articulates with the body of one vertebra and the transverse processes of two adjoining thoracic vertebra (called facet for tubercle of rib. The upper seven pairs of ribs connect directly to the sternum by means of costal cartilages. These are called the “true ribs,” or the vertebrosternal ribs. The next three pairs of ribs are called the “false ribs,” or vertebrochondral ribs, because they attach to the sternum by means of a common cartilage. The last two pairs are called “floating ribs,” or vertebral ribs, because they do not attach to the sternum at all.
The Sternum
The sternum, or breastbone, is a flat bone that has the shape of a blade. The sternum, along with the ribs, helps protect the heart and lungs. During surgery the sternum may be split to allow access to the organs of the thoracic cavity. The sternum is composed of three bones that fuse during fetal development. These bones are the manubrium, the body, and the xiphoid process. The manubrium is the superior portion of the sternum. The body is the middle and largest part of the sternum and the xiphoid process is the inferior and smallest portion of the sternum. The manubrium joins with the body of the sternum at an angle. This joint is an important anatomical landmark because it occurs at the level of the second rib, and therefore allows the ribs to be counted. Counting the ribs is sometimes done to determine where the apex of the heart is located—usually between the fifth and sixth ribs. The manubrium articulates with the costal cartilages of the first and second ribs; the body articulates costal cartilages of the second through tenth ribs; and the xiphoid processdoesn’t articulate with any ribs. The xiphoid process is the third part of the sternum. Composed of hyaline cartilage in the child, it becomes ossified in the adult. The variably shaped xiphoid process serves as an attachment site for the diaphragm, which separates the thoracic cavity from the abdominal cavity.
Appendicular Skeleton
The appendicular skeleton contains the bones of the pectoral girdle, upper limbs, pelvic girdle, and lower limbs.
Pectoral Girdle
The pectoral girdle (shoulder girdle) contains four bones: two clavicles and two scapulae. It supports the arms and serves as a place of attachment for muscles that move the arms. The bones of this girdle are not held tightly together; rather, they are weakly attached and held in place by ligaments and muscles. This arrangement allows great flexibility but means that the pectoral girdle is prone to dislocation.
Clavicles
The clavicles (collarbones) are slender and S-shaped. Each clavicle articulates medially with the manubrium of the sternum. This is the only place where the pectoral girdle is attached to the axial skeleton. Each clavicle also articulates with a scapula. The clavicle serves as a brace for the scapula and helps stabilize the shoulder. It is structurally weak, however, and if undue force is applied to the shoulder, the clavicle will fracture.
Scapulae
The scapulae (sing., scapula), also called the shoulder blades, are broad bones that somewhat resemble triangles. One reason for the pectoral girdle’s flexibility is that the scapulae are not joined to each other. Each scapula has a spine, as well as the following features:
acromion process, which articulates with a clavicle and provides a place of attachment for arm and chest muscles;
coracoid process, which serves as a place of attachment for arm and chest muscles;
glenoid cavity, which articulates with the head of the arm bone (humerus). The pectoral girdle’s flexibility is also a result of the glenoid cavity being smaller than the head of the humerus.
Upper Limb
The upper limb includes the bones of the arm (humerus), the forearm (radius and ulna), and the hand (carpals, metacarpals, and phalanges).
Humerus
The humerus (Fig. 6.12) is the bone of the arm. It is a long bone with the following features at the proximal end:
head, which articulates with the glenoid cavity of the scapula;
greater and lesser tubercles, which provide attachments for muscles that move the arm and shoulder;
intertubercular groove, which holds a tendon from the biceps brachii, a muscle of the arm;
deltoid tuberosity, which provides an attachment for the deltoid, a muscle that covers the shoulder joint. The humerus has the following features at the distal end:
capitulum, a lateral condyle that articulates with the head of the radius;
trochlea, a spool-shaped condyle that articulates with the ulna;
coronoid fossa, a depression for a process of the ulna when the elbow is flexed;
olecranon fossa, a depression for a process of the ulna when the elbow is extended.
Radius
The radius and ulna are the bones of the forearm. The radius is on the lateral side of the forearm (the thumb side). When you turn your hand from the “palms up” position to the “palms down” position, the radius crosses over the ulna, so the two bones are crisscrossed. Proximally, the radius has the following features:
head, which articulates with the capitulum of the humerus and fits into the radial notch of the ulna;
radial tuberosity, which serves as a place of attachment for a tendon from the biceps brachii;
Distally, the radius has the following features:
ulnar notch, which articulates with the head of the ulna;
styloid process, which serves as a place of attachment for ligaments that run to the wrist.
Ulna
The ulna is the longer bone of the forearm. Proximally, the ulna has the following features:
coronoid process, which articulates with the coronoid fossa of the humerus when the elbow is flexed;
olecranon process, the point of the elbow, articulates with the olecranon fossa of the humerus when the elbow is extended;
trochlear notch, which articulates with the trochlea of the humerus at the elbow joint;
radial notch, which articulates with head of the radius.
Distally, the ulna has the following features:
head, which articulates with the ulnar notch of the radius;
styloid process, which serves as a place of attachment for ligaments that run to the wrist.
Hand
Each hand has a wrist, a palm, and five fingers, or digits. The wrist, or carpus, contains eight small carpal bones, tightly bound by ligaments in two rows of four each. Where we wear a “wrist watch” is the distal forearm—the true wrist is the proximal part of what we generally call the hand. Only two of the carpals (the scaphoid and lunate) articulate with the radius. Anteriorly, the concave region of the wrist is covered by a ligament, forming the so-called carpal tunnel. Inflammation of the tendons running though this area causes them to compress a nerve and the result is a numbness known as carpal tunnel syndrome. Five metacarpal bones, numbered 1 to 5 from the thumb side of the hand toward the little finger, fan out to form the palm. When the fist is clenched, the heads of the metacarpals, which articulate with the phalanges, become obvious. The first metacarpal is more anterior than the others, and this allows the thumb to touch each of the other fingers. The fingers, including the thumb, contain bones called the phalanges. The thumb has only two phalanges (proximal and distal), but the other fingers have three each (proximal, middle, and distal).
Pelvic Girdle
The pelvic girdle contains two coxal bones (hipbones), as well as the sacrum and coccyx. The strong bones of the pelvic girdle are firmly attached to one another and bear the weight of the body. The pelvis also serves as the place of attachment for the lower limbs and protects the urinary bladder, the internal reproductive organs, and a portion of the large intestine.
Coxal Bones
Each coxal bone has the following three parts:
1. Ilium. The ilium, the largest part of a coxal bone, flares outward to give the hip prominence. The margin of the ilium is called the iliac crest. Each ilium connects posteriorly with the sacrum at a sacroiliac joint.
2. Ischium. The ischium is the most inferior part of a coxal bone. Its posterior region, the ischial tuberosity, allows a person to sit. Near the junction of the ilium and ischium is the ischial spine, which projects into the pelvic cavity. The distance between the ischial spines tells the size of the pelvic cavity. The greater sciatic notch is the site where blood vessels and the large sciatic nerve pass posteriorly into the lower leg.
3. Pubis. The pubis is the anterior part of a coxal bone. The two pubic bones join together at the pubic symphysis. Posterior to where the pubis and the ischium join together is a large opening, the obturator foramen, through which blood vessels and nerves pass anteriorly into the leg. Where the three parts of each coxal bone meet is a depression called the acetabulum, which receives the rounded head of the femur.
False and True Pelvises
The false pelvis is the portion of the trunk bounded laterally bythe flared parts of the ilium. This space is much larger than that of the true pelvis. The true pelvis, which is inferior to the false pelvis, is the portion of the trunk bounded by the sacrum, lower ilium, ischium, and pubic bones. The true pelvis is said to have an upper inlet and a lower outlet. The dimensions of these outlets are important for females because the outlets must be large enough to allow a baby to pass through during the birth process.
Sex Differences
Female and male pelvises) usually differ in several ways, including the following:
1. Female iliac bones are more flared than those of the male; therefore, the female has broader hips.
2. The female pelvis is wider between the ischial spines and the ischial tuberosities.
3. The female inlet and outlet of the true pelvis are wider.
4. The female pelvic cavity is shallower, while the male pelvic cavity is more funnel shaped.
5. Female bones are lighter and thinner.
6. The female pubic arch (angle at the pubic symphysis) is wider.
In addition to these differences in pelvic structure, male pelvic bones are larger and heavier, the articular ends are thicker, and the points of muscle attachment may be larger.
Lower Limb
The lower limb includes the bones of the thigh (femur), the kneecap (patella), the leg (tibia and fibula), and the foot (tarsals, metatarsals, and phalanges).
Femur
The femur, or thighbone, is the longest and strongest bone in the body. Proximally, the femur has the following features:
Head, which fits into the acetabulum of the coxal bone;
Greater and lesser trochanters, which provide a place of attachment for the muscles of the thighs and buttocks;
linea aspera, a crest that serves as a place of attachment for several muscles.
Distally, the femurhas the following features:
Medial and lateral epicondyles that serve as sites of attachment for muscles and ligaments;
Lateral and medial condyles that articulate with the tibia;
Patellar surface, which is located between the condyles on the anterior surface, articulates with the patella, a small triangular bone that protects the knee joint.
Tibia
The tibia and fibula are the bones of the leg. The tibia, or shinbone, is medial to the fibula. It is thicker than the fibula and bears the weight from the femur, with which it articulates. It has the following features:
Medial and lateral condyles, which articulate with the femur;
tibial tuberosity, where the patellar (kneecap) ligaments attach;
Anterior crest, commonly called the shin;
Medial malleolus, the bulge of the inner ankle, articulates with the talus in the foot.
Fibula
The fibula is lateral to the tibia and is more slender. It has a head that articulates with the tibia just below the lateral condyle. Distally, the lateral malleolus articulates with the talus and forms the outer bulge of the ankle. Its role is to stabilize the ankle. Foot Each foot has an ankle, an instep, and five toes (also called digits).
The ankle has seven tarsal bones; together, they are called the tarsus. Only one of the seven bones, the talus, can move freely where it joins the tibia and fibula. The largest of the ankle bones is the calcaneus,or heel bone. Along with the talus, it supports the weight of the body. The instep has five elongated metatarsal bones. The distal ends of the metatarsals form the ball of the foot. Along with the tarsals, these bones form the arches of the foot (longitudinal and transverse), which give spring to a person’s step. If the ligaments and tendons holding these bones together weaken, fallen arches, or “flat feet,” can result. The toes contain the phalanges. The big toe has only two phalanges, but the other toes have three each.
SYNOPSIS OF CHAPTER 1
1.1 The Human Body
A. Anatomy is the study of the structure of body parts, and physiology is the study of the function of these parts. Structure is suited to the function of a part.
B. The body has levels of organization that progress from atoms to molecules, macromolecules, cells, tissues, organs, organ systems, and finally, the organism.
1.2 Anatomical Terms
Various terms are used to describe the location of body organs when the body is in the anatomical position (standing erect, with face forward, arms at the sides, and palms and toes directed forward).
A. The terms anterior/posterior, superior/inferior, medial/lateral, proximal/distal, superficial/deep, and central/peripheral describe the relative positions of body parts.
B. The body can be divided into axial and appendicular portions, each of which can be further subdivided into specific regions. For example, brachial refers to the arm, and pedal refers to the foot.
C. The body or its parts may be sectioned (cut) along certain planes. A sagittal (vertical) cut divides the body into right and left portions. A frontal (coronal) cut divides the body into anterior and posterior parts. A transverse (horizontal) cut is a cross section.
1.3. Body Cavities and Membranes
The human body has two major cavities: the posterior (dorsal) body cavity and the anterior (ventral) body cavity. Each is subdivided into smaller cavities, within which specific viscera are located. Specific serous membranes line body cavities and adhere to the organs within these cavities.
1.4 Organ Systems
The body has a number of organ systems. These systems have been characterized as follows:
A. Support, movement, and protection.
The integumentary system, which includes the skin, not only protects the body, but also has other functions. The skeletal system contains the bones, and the muscular system contains the three types of muscles. The primary function of the skeletal and muscular systems is support and movement, but they have other functions as well.
B. Integration and coordination.
The nervous system contains the brain, spinal cord, and nerves. Because the nervous system communicates with both the sense organs and the muscles, it allows us to respond to outside stimuli. The endocrine system consists of the hormonal glands. The nervous and endocrine systems coordinate and regulate the activities of the body’s other systems.
C. Maintenance of the body.
The cardiovascular system (heart and vessels), lymphatic system (lymphatic vessels and nodes, spleen, and thymus), respiratory
system (lungs and conducting tubes), digestive system (mouth, esophagus, stomach, small and large intestines, and associated organs), and urinary system (kidneys and bladder) all perform specific processing and transporting functions to maintain the normal conditions of the body.
D. Reproduction and development.
The reproductive system in males (testes, other glands, ducts, and penis) and in females (ovaries, uterine tubes, uterus, vagina, and external genitalia) carries out those functions that give humans the ability to reproduce.
1.5 Homeostasis
Homeostasis is the relative constancy of the body’s internal environment, which is composed of blood and the tissue fluid that bathes the cells.
A. Negative feedback mechanisms help maintain homeostasis. Positive feedback also occurs.
B. All of the body’s organ systems contribute to homeostasis. Some, including the respiratory, digestive, and urinary systems, remove and/or add substances to blood.
C. The nervous and endocrine systems regulate the activities of other systems. Negative feedback is a self-regulatory mechanism by which systems and conditions of the body are controlled.
SYNOPSIS OF CHAPTER 2
Cells differ in shape and function, but even so, a generalized cell can be described.
2.1 Cellular Organization
All human cells, despite varied shapes and sizes, have a plasma membrane and a central nucleus. The cytoplasm contains organelles and a cytoskeleton.
A. The plasma membrane, composed of phospholipid and protein molecules, regulates the entrance and exit of other molecules into and out of the cell.
B. The nucleus contains chromatin, which condenses into chromosomes just prior to cell division. Genes, composed of DNA, are on the chromosomes, and they code for the production of proteins in the cytoplasm. The nucleolus is involved in ribosome formation.
C. Ribosomes are small organelles where protein synthesis occurs. Ribosomes occur in the cytoplasm, both singly and in groups. Numerous ribosomes are attached to the endoplasmic reticulum.
D. The endomembrane system consists of the endoplasmic reticulum (ER), the Golgi apparatus, and the lysosomes and various transport vesicles.
E. The ER is involved in protein synthesis (rough ER) and various other processes such as lipid synthesis (smooth ER). Molecules produced or modified in the ER are eventually enclosed in vesicles that take them to the Golgi apparatus.
F. The Golgi apparatus processes and packages molecules, distributes them within the cell, and transports them out of the cell. It is also involved in secretion.
G. Lysosomes are produced by the Golgi apparatus, and their hydrolytic enzymes digest macromolecules from various sources. Mitochondria are the sites of cellular respiration, a process that uses nutrients and oxygen to provide ATP, the type of chemical energy needed by cells.
H. Mitochondria are involved in cellular respiration, a metabolic pathway that provides ATP molecules to cells.
I. Notable among the contents of the cytoskeleton are microtubules and actin filaments. The cytoskeleton maintains the shape of the cell and also directs the movement of cell parts.
J. Centrioles lie near the nucleus and may be involved in the production of the spindle during cell division and in the formation of cilia and flagella.
2.2 Crossing the Plasma Membrane
When substances enter and exit cells by diffusion, osmosis, or filtration, no carrier is required. Facilitated transport and active transport do require a carrier.
A. Some substances can simply diffuse across a plasma membrane. The diffusion of water is called osmosis. In an isotonic solution, cells neither gain nor lose water. In a hypotonic solution, cells swell. In a hypertonic solution, cells shrink.
B. During filtration, diffusion of small molecules out of a blood vessel is aided by blood pressure.
C. During facilitated transport, a carrier is required, but energy is not because the substance is moving from higher to lower concentration. Active transport, which requires a carrier and ATP energy, moves substances from lower to higher concentration.
D. Endocytosis (phagocytosis) involves the uptake of substances by a cell through vesicle formation. Exocytosis involves the release of substances from a cell as vesicles within the cell cytoplasm fuse with the plasma membrane.
2.3 The Cell Cycle
The cell cycle consists of interphase (G1 phase, S phase, G2 phase) and the mitotic stage, which includes mitosis and cytokinesis.
A. During interphase, DNA replication and protein synthesis take place. DNA serves as a template for its own replication: The DNA parental molecule unwinds and unzips, and new (daughter) strands form by complementary base pairing. Protein synthesis consists of transcription and translation. During transcription, DNA serves as a template for the formation of RNA. During translation, mRNA, rRNA, and tRNA are involved in polypeptide synthesis.
B. Mitosis consists of a number of phases, during which each newly formed cell receives a copy of each kind of chromosome. Later, the cytoplasm divides by furrowing. Mitosis occurs during growth and repair.
SYNOPSIS OF CHAPTER 3
3.1 Epithelial Tissue
A. Body tissues are categorized into four types: epithelial, connective, muscular, and nervous.
B. Epithelial tissue. This tissue is classified according to cell shape and number of layers. The cell shape may be squamous, cuboidal, or columnar. Simple tissues have one layer of cells, and stratified tissues have several layers.
3.2 Connective Tissue
A. In connective tissue, cells are separated by a matrix (organic ground substance plus fibers).
B. Fibrous connective tissue can be loose connective tissue, in which fibroblasts are separated by a jellylike ground substance, or dense connective tissue, which contains bundles of collagenous fibers. Adipose tissue is a type of loose connective tissue in which the fibroblasts enlarge and store fat.
C. Cartilage and bone are support tissues. Cartilage is more flexible than bone because the matrix is rich in protein, rather than the mineral salts found in bone.
D. Blood is a connective tissue in which the matrix is plasma.
3.3 Muscular Tissue
Muscular tissue contains actin and myosin protein filaments. These form a striated pattern in skeletal and cardiac muscle, but not in smooth muscle. Cardiac and smooth muscle are under involuntary control. Skeletal muscle is under voluntary control.
3.4 Nervous Tissue
Nervous tissue contains conducting cells called neurons. Neurons have processes called axons and dendrites. Outside the brain and spinal cord, long axons (fibers) are found in nerves.
3.5 Extracellular Junctions, Glands, and Membranes
A. In a tissue, cells can be joined by tight junctions, gap junctions, or adhesion junctions.
B. Glands are composed of epithelial tissue that produces and secretes a product, usually by exocytosis. Glands can be unicellular or multicellular. Multicellular exocrine glands have ducts and secrete onto surfaces; endocrine glands are ductless and secrete into the bloodstream.
C. Mucous membranes line the interior of organs and tubes that open to the outside. Serous membranes line the thoracic and abdominopelvic cavities, and cover the organs within these cavities. Synovial membranes line certain joint cavities. Meninges are membranes that cover the brain and spinal cord. The skin forms a cutaneous membrane.
SYNOPSIS OF THE LESSON IN INTEGUMENTARY SYSTEM
(Chapter 4)
4.1 Structure of the Skin
The skin has two regions: the epidermis and the dermis. The hypodermis lies below the skin.
A. The epidermis, the outer region of the skin, is made up of stratified squamous epithelium. New cells continually produced in the stratum basale of the epidermis are pushed outward and become the keratinized cells of the stratum corneum.
B. The dermis, which is composed of dense irregular connective tissue, lies beneath the epidermis. It contains collagenous and elastic fibers, blood vessels, and nerve fibers.
C. The hypodermis is made up of loose connective tissue and adipose tissue, which insulates the body from heat and cold.
4.2 Accessory Structures of the Skin
Accessory structures of the skin include hair, nails, and glands.
A. Both hair and nails are produced by the division of epidermal cells and consist of keratinized cells.
B. Sweat glands are numerous and present in all regions of the skin. Sweating helps lower the body temperature.
C. Sebaceous glands are associated with a hair follicle and secrete sebum, which lubricates the hair and skin.
D. Mammary glands located in the breasts produce milk after childbirth.
4.3 Disorders of the Skin
A. Skin cancer. Skin cancer, which is associated with ultraviolet radiation, occurs in three forms. Basal cell carcinoma and squamous cell carcinoma can usually be removed surgically. Melanoma is the most dangerous form of skin cancer.
B. Wound healing. The skin has regenerative powers and can grow back on its own if a wound is not too extensive.
C. Burns. The severity of a burn depends on its depth and extent. First-degree burns affect only the epidermis. Second-degree burns affect the entire epidermis and a portion of the dermis. Third-degree burns affect the entire epidermis and dermis. The “rule of nines” provides a means of estimating the extent of a burn injury.
4.4 Effects of Aging
Skin wrinkles with age because the epidermis is held less tightly, fibers in the dermis are fewer, and the hypodermis has less padding. The skin has fewer blood vessels, sweat glands, and hair follicles. Although pigment cells are fewer and the hair turns gray, pigmented blotches appear on the skin. Exposure to the sun results in many of the skin changes we associate with aging.
4.5 Homeostasis
A. Skin protects the body from physical trauma and bacterial invasion.
B. Skin helps regulate water loss and gain, which helps the urinary system. Also, sweat glands excrete some urea.
C. The skin produces a precursor molecule that is converted to vitamin D following exposure to UV radiation. A hormone derived from vitamin D helps regulate calcium and phosphorus metabolism involved in bone development.
D. The skin contains sensory receptors for touch, pressure, pain, hot, and cold, which help people to be aware of their surroundings. These receptors send information to the nervous system.
E. The skin helps regulate body temperature. When the body is too hot, surface blood vessels dilate, and the sweat glands are active. When the body is cold, surface blood vessels constrict, and the sweat glands are inactive.
F. Hyperthermia and hypothermia are two conditions that can result when the body’s temperature regulatory mechanism is overcome. With hyperthermia, the body temperature rises above normal, and with hypothermia, the body temperature falls below normal.
SYNOPSIS OF THE LESSON IN SKELETAL SYSTEM
(Chapter 5)
5.1 Skeleton: Overview
A. The skeleton supports and protects the body; produces red blood cells; serves as a storehouse for inorganic calcium and phosphate ions and fat; and permits flexible movement.
B. A long bone has a shaft (diaphysis) and two ends (epiphyses), which are covered by articular cartilage. The diaphysis contains a medullary cavity with yellow marrow and is bounded by compact bone. The epiphyses contain spongy bone with red bone marrow that produces red blood cells.
C. Bone is a living tissue. It develops, grows, remodels, and repairs itself. In all these processes, osteoclasts break down bone, and osteoblasts build bone.
D. Fractures are of various types, but repair requires four steps: (1) hematoma, (2) fibrocartilaginous callus, (3) bony callus, and (4) remodeling.
5.2 Axial Skeleton
The axial skeleton lies in the midline of the body and consists of the skull, the hyoid bone, the vertebral column, and the thoracic cage.
A. The skull is formed by the craniumn and the facial bones. The cranium includes the frontal bone, two parietal bones, one occipital bone, two temporal bones, one sphenoid bone, and one ethmoid bone. The facial bones include two maxillae,two palatine bones, two zygomatic bones, two lacrimal bones, two nasal bones, the vomer bone, two inferior nasal conchae, and the mandible.
B. The U-shaped hyoid bone is located in the neck. It anchors the tongue and does not articulate with any other bone.
C. The typical vertebra has a body, a vertebral arch surrounding the vertebral foramen, and a spinous process. The first two vertebrae are the atlas and axis. The vertebral column has four curvatures and contains the cervical, thoracic, lumbar, sacral, and coccygeal vertebrae, which are separated by intervertebral disks.
D. The rib cage contains the thoracic vertebrae, ribs and associated cartilages, and the sternum.
5.3 Appendicular Skeleton
The appendicular skeleton consists of the bones of the pectoral girdle, upper limbs, pelvic girdle, and lower limbs.
A. The pectoral (shoulder) girdle contains two clavicles and two scapulae.
B. The upper limb contains the humerus, the radius, the ulna, and the bones of the hand (the carpals, metacarpals, and phalanges).
C. The pelvic girdle contains two coxal bones, as well as the sacrum and coccyx. The female pelvis is generally wider and more shallow than the male pelvis.
D. The lower limb contains the femur, the patella, the tibia, the fibula, and the bones of the foot (the tarsals, metatarsals, and phalanges).
5.4 Joints (Articulations)
A. Joints are regions of articulation between bones. They are classified according to their degree of movement. Some joints are immovable, some are slightly movable, and some are freely movable (synovial). The different kinds of synovial joints are ball-and-socket, hinge, condyloid, pivot, gliding, and saddle.
B. Movements at joints are broadly classified as angular (flexion, extension, adduction, abduction); circular (circumduction, rotation, supination, and pronation); and special (inversion, eversion, elevation, and depression).
5.5 Effects of Aging
Two fairly common effects of aging on the skeletal system are arthritis and osteoporosis.
5.6 Homeostasis
A. The bones protect the internal organs: The rib cage protects the heart and lungs; the skull protects the brain; and the vertebrae protect the spinal cord. B. The bones assist all phases of respiration. The rib cage assists the breathing process, and red bone marrow produces the red blood cells that transport oxygen.
C. The bones store and release calcium. Calcium ions play a major role in muscle contraction and nerve conduction. Calcium ions also help regulate cellular metabolism.
D. The bones assist the lymphatic system and immunity. Red bone marrow produces not only the red blood cells but also the white blood cells.
E. The bones assist digestion. The jaws contain sockets for the teeth, which chew food, and a place of attachment for the muscles that move the jaws.
F. The skeleton is necessary for locomotion. Locomotion is efficient in human beings because they have a jointed skeleton for the attachment of muscles that move the bones.
SYNOPSIS OF THE LESSON IN MUSCULAR SYSTEM
(Chapter 6)
6.1 Functions and Types of Muscles
A. Muscular tissue is either smooth, cardiac, or skeletal. Skeletal muscles have tubular, multinucleated, and striated fibers that contract voluntarily.
B. Skeletal muscles support the body, make bones move, help maintain a constant body temperature, assist movement in cardiovascular and lymphatic vessels, and help protect internal organs and stabilize joints.
6.2 Microscopic Anatomy and Contraction of Skeletal Muscle
A. The sarcolemma, which extends into a muscle fiber, forms T tubules; the sarcoplasmic reticulum has calcium storage sites. The placement of actin and myosin in the contractile
myofibrils accounts for the striations of skeletal muscle fibers.
B. Skeletal muscle innervation occurs at neuromuscular junctions. Impulses travel down the tubules of the T system and cause the release of calcium from calcium storage sites. The presence of calcium and ATP in muscle cells prompts actin myofilaments to slide past myosin myofilaments, shortening the length of the sarcomere.
C. ATP, required for muscle contraction, can be generated by way of creatine phosphate breakdown and fermentation. Lactic acid from fermentation represents an oxygen deficit, because oxygen is required to metabolize this product. Cellular respiration, an aerobic process, is the best source of ATP.
6.3 Muscle Responses
A. In the laboratory, muscle fibers obey the all-or-none law, but whole muscles do not. The occurrence of a muscle twitch, summation, or tetanic contraction depends on the frequency with which a muscle is stimulated.
B. In the body, muscle fibers belong to motor units that obey the all-or-none law. The strength of muscle contraction depends on the recruitment of motor units. A muscle has tone because some fibers are always contracting.
6.4 Skeletal Muscles of the Body
A. When muscles cooperate to achieve movement, some act as prime movers, others as synergists, and still others as antagonists.
B. The skeletal muscles of the body are divided into those that move: the head and neck (see; the
trunk; the shoulder and arm; the forearm the hand and fingers; the thigh; the leg ; and the ankle and foot.
6.5 Effects of Aging
As we age, muscles become weaker, but exercise can help retain vigor.
6.6 Homeostasis
Smooth muscle contraction helps move the blood; cardiac muscle contraction pumps the blood. Skeletal muscle contraction produces heat and is needed for breathing.
CHAPTER 1
The Human Body
Anatomy and physiology is the study of the human body. Anatomy is concerned with the structure of a part. For example, the stomach is a J-shaped, pouchlike organ. The stomach wall has thick folds, which disappear as the stomach expands to increase its capacity. Physiology is concerned with the function of a part. For example, the stomach temporarily stores food, secretes digestive juices, and passes on partially digested food to the small intestine. Anatomy and physiology are closely connected in that the structure of an organ suits its function. For example, the stomach’s pouchlike shape and ability to expand are suitable to its function of storing food. In addition, the microscopic structure of the stomach wall is suitable to its secretion of digestive juices.
Organization of Body Parts
The structure of the body can be studied at different levels of organization (Fig. 1.1). First, all substances, including body parts, are composed of chemicals made up of submicroscopic particles called atoms. Atoms join to form molecules, which can in turn join to form macromolecules. For example, molecules called amino acids join to form a macromolecule called protein, which makes up the bulk of our muscles. Macromolecules are found in all cells, the basic units of all living things. Within cells are organelles, tiny structures that perform cellular functions. For example, the organelle called the nucleus is especially concerned with cell reproduction; another organelle, called the mitochondrion, supplies the cell with energy. Tissues are the next level of organization. A tissue is composed of similar types of cells and performs a specific function. An organ is composed of several types of tissues and performs a particular function within an organ system. For example, the stomach is an organ that is a part of the digestive system. It has a specific role in this system, whose overall function is to supply the body with the nutrients needed for growth and repair. The other systems of the body also have specific functions. All of the body systems together make up the organism— such as, a human being. Human beings are complex animals, but this complexity can be broken down and studied at ever simpler levels. Each simpler level is organized and constructed in a particular way.
1.2 Anatomical Terms
Certain terms are used to describe the location of body parts, regions of the body, and imaginary planes by which the body can be sectioned. You should become familiar with these terms before your study of anatomy and physiology begins. Anatomical terms are useful only if everyone has in mind the same position of the body and is using the same reference points. Therefore, we will assume that the body is in the anatomical position: standing erect, with face forward, arms at the sides, and palms and toes directed forward, as illustrated in Figure 1.1.
Directional Terms
Directional terms are used to describe the location of one body part in relation to another (Fig. 1.2): Anterior (ventral) means that a body part is located toward the front. The windpipe (trachea) is anterior to the esophagus. Posterior (dorsal) means that a body part is located toward the back. The heart is posterior to the rib cage. Superior means that a body part is located above another part, or toward the head. The face is superior to the neck. Inferior means that a body part is below another part, or toward the feet. The navel is inferior to the chin. Medial means that a body part is nearer than another part to an imaginary midline of the body. The bridge of the nose is medial to the eyes. Lateral means that a body part is farther away from the midline. The eyes are lateral to the nose. Proximal means that a body part is closer to the point of attachment or closer to the trunk. The elbow is proximal to the hand. Distal means that a body part is farther from the point of attachment or farther from the trunk or torso. The hand is distal to the elbow. Superficial (external) means that a body part is located near the surface. The skin is superficial to the muscles. Deep (internal) means that the body part is located away from the surface. The intestines are deep to the spine. Central means that a body part is situated at the center of the body or an organ. The central nervous system is located along the main axis of the body. Peripheral means that a body part is situated away from the center of the body or an organ. The peripheral nervous system is located outside the central nervous system.
Regions of the Body
The human body can be divided into axial and appendicular portions. The axial portion includes the head, neck, and trunk. The trunk can be divided into the thorax, abdomen, and pelvis. The pelvis is that part of the trunk associated with the hips. The appendicular portion of the human body includes the limbs—that is, the upper limbs and the lower limbs. The human body is further divided as shown in Figure 1.3. The labels in Figure 1.3 do not include the word “region.” It is understood that you will supply the word region in each case. The scientific name for each region is followed by the common name for that region. For example, the cephalic region is commonly called the head. Notice that the upper arm includes among other parts the brachial region (arm) and the antebrachial region (forearm), and the lower limb includes among other parts the femoral region (thigh) and the crural region (leg). In other words, contrary to common usage, the terms arm and leg refer to only a part of the upper limb and lower limb, respectively. Most likely, it will take practice to learn the terms in Figure 1.3. One way to practice might be to point to various regions of your own body and see if you can give the scientific name for that region. Check your answer against the figure.
Planes and Sections of the Body
To observe the structure of an internal body part, it is customary to section (cut) the body along a plane. A plane is an imaginary flat surface passing through the body. The body is customarily sectioned along the following planes (Fig. 1.4): A sagittal (median) plane extends lengthwise and divides the body into right and left portions. A midsagittal plane passes exactly through the midline of the body. The pelvic organs are often shown in midsagittal section (Fig. 1.4d). Sagittal cuts that are not along the midline are called parasagittal sections. A frontal (coronal)plane also extends lengthwise, but it is perpendicular to a sagittal plane and divides the body or an organ into anterior and posterior portions. The thoracic organs are often illustrated in frontal section (Fig. 1.4e). Atransverse(horizontal)plane is perpendicular to the body’s long axis and therefore divides the body horizontally to produce a cross section. A transverse cut divides the body or an organ into superior and inferior portions. Figure 1.4f is a transverse section of the head at the level of the eyes. The terms longitudinal sectionandcross sectionare often applied to body parts that have been removed and cut either lengthwise or straight across, respectively.
1.3 Body Cavities and Membranes
During embryonic development, the body is first divided into two internal cavities: the posterior (dorsal) body cavity and the anterior (ventral) body cavity. Each of these major cavities is then subdivided into smaller cavities. The cavities, as well as the organs in the cavities (called the viscera), are lined by membranes. Posterior (Dorsal) Body Cavity The posterior body cavity is subdivided into two parts: (1) The cranial cavity, enclosed by the bony cranium, contains the brain. (2) The vertebral canal, enclosed by vertebrae, contains the spinal cord (Fig. 1.5a) The posterior body cavity is lined by three membranous layers called the meninges. The most inner of the meninges is tightly bound to the surface of the brain and the spinal cord. The space between this layer and the next layer is filled with cerebrospinal fluid. Spinal meningitis, a serious condition, is an inflammation of the meninges usually caused by an infection. Anterior (Ventral) Body Cavity The large anterior body cavity is subdivided into the superior thoracic cavity and the inferior abdominopelvic cavity (Fig.1.5a).A muscular partition called the diaphragmseparates the two cavities. Membranes that line these cavities are called serous membranes because they secrete a fluid that has just about the same composition as serum, a component of blood. Serous fluid between the smooth serous membranes reduces friction as the viscera rub against each other or against the body wall. To understand the relationship between serous membranes and an organ, imagine a ball that is pushed in on one side by your fist. Your fist would be covered by one membrane (called a visceral membrane), and there would be a small space between this inner membrane and the outer membrane (called a parietal membrane):
Thoracic Cavity
The thoracic cavity is enclosed by the rib cage, and has three portions: the left, right, and medial portions. The medial portion, called the mediastinum, contains the heart, thymus gland, trachea, esophagus, and other structures (Fig. 1.5b).
The right and left portions of the thoracic cavity contain the lungs. The lungs are surrounded by a serous membrane called the pleura. The parietal pleuron lies next to the thoracic wall, and the visceral pleura adheres to a lung. In between the two pleura, the pleural cavity is filled with pleural fluid. Similarly, in the mediastinum, the heart is covered by the two-layered membrane called the pericardium. The visceral pericardium which adheres to the heart is separated from the parietal pericardium by a small space called the pericardial cavity (Fig. 1.5b). This small space contains pericardial fluid. Abdominopelvic Cavity The abdominopelvic cavity has two portions: the superior abdominal cavity and the inferior pelvic cavity. The stomach, liver, spleen, gallbladder, and most of the small and large intestines are in the abdominal cavity. The pelvic cavity contains the rectum, the urinary bladder, the internal reproductive organs, and the rest of the large intestine. Males have an external extension of the abdominal wall, called the scrotum, where the testes are found. Many of the organs of the abdominopelvic cavity are covered by the visceral peritoneum, while the wall of the abdominal cavity is lined with the parietal peritoneum. Peritoneal fluid fills the cavity between the visceral and parietal peritoneum. Peritonitis, another serious condition, is an inflammation of the peritoneum, again usually caused by an infection. Table 1.1 summarizes our discussion of body cavities and membranes. Clinically speaking, the abdominopelvic cavity is dividedinto four quadrants by running a transverse plane across the midsagittal plane at the point of the navel (Fig. 1.6a). Physicians commonly use these quadrants to identify the locations of patients’ symptoms. The four quadrants are: (1) right upper quadrant, (2) left upper quadrant, (3) right lower quadrant, and (4) left lower quadrant. Figure 1.6b shows the organs that lie within these four quadrants.
1.5 Homeostasis
Homeostasis is the relative constancy of the body’s internal environment. Because of homeostasis, even though external conditions may change dramatically, internal conditions stay within a narrow range. For example, regardless of how cold or hot it gets, the temperature of the body stays around 37°C (97° to 99°F). No matter how acidic your meal, the pH of your blood is usually about 7.4, and even if you eat a candy bar, the amount of sugar in your blood is just about 0.1%. It is important to realize that internal conditions are not absolutely constant; they tend to fluctuate above and below a particular value. Therefore, the internal state of the body is often described as one of dynamic equilibrium. If internal conditions change to any great degree, illness results. This
makes the study of homeostatic mechanisms medically important.
Negative Feedback
Negative feedback is the primary homeostatic mechanism that keeps a variable close to a particular value, or set point. A homeostatic mechanism has three components: a sensor, a regulatory center, and an effector (Fig. 1.7a). The sensor detects a change in the internal environment; the regulatory center activates the effector; the effector reverses the change and brings conditions back to normal again. Now, the sensor is no longer activated.
Mechanical Example
A home heating system illustrates how a negative feedback mechanism works (Fig. 1.7b). You set the thermostat at, say, 68°F. This is the set point. The thermostat contains a thermometer, a sensor that detects when the room temperature falls below the set point. The thermostat is also the regulatory center; it turns the furnace on. The furnace plays the role of the effector. The heat given off by the furnace raises the temperature of the room to 70°F. Now, the furnace turns off. Notice that a negative feedback mechanism prevents change in the same direction; the room does not get warmer and warmer because warmth inactivates the system. Human Example: Regulation of Blood Pressure Negative feedback mechanisms in the body function similarly to the mechanical model. For example, when blood pressure falls, sensory receptors signal a regulatory center in the brain (Fig. 1.7c). This center sends out nerve impulses to the arterial walls so that they constrict. Once the blood pressure rises, the system is inactivated.
Figure 1.7. Negative feedback. In each example, a sensor detects an internal environmental change and signals a regulatory center. The center activates an effector, which reverses this change. a. The general pattern. b. A mechanical example. c. A human example.
Human Example: Regulation of Body Temperature
The thermostat for body temperature is located in a part of the brain called the hypothalamus. When the body temperature falls below normal, the regulatory center directs (via nerve impulses) the blood vessels of the skin to constrict (Fig.1.8). This conserves heat. If body temperature falls even lower, the regulatory center sends nerve impulses to the skeletal muscles, and shivering occurs. Shivering generates heat, and gradually body temperature rises to 37°C. When the temperature rises to normal, the regulatory center is inactivated. When the body temperature is higher than normal, the regulatory center directs the blood vessels of the skin to dilate. This allows more blood to flow near the surface of the body, where heat can be lost to the environment. In addition, the nervous system activates the sweat glands, and the evaporation of sweat helps lower body temperature. Gradually, body temperature decreases to 37°C.
Positive Feedback
Positive feedback is a mechanism that brings about an ever greater change in the same direction. A positive feedback mechanism can be harmful, as when a fever causes metabolic changes that push the fever still higher. Death occurs at a body temperature of 45°C because cellular proteins denature at this temperature and metabolism stops. Still, positive feedback loops such as those involved in blood clotting, the stomach’s digestion of protein, and childbirth assist the body in completing a process that has a definite cutoff point. Consider that when a woman is giving birth, the head of the baby begins to press against the cervix, stimulating sensory receptors there. When nerve impulses reach the brain, the brain causes the pituitary gland to secrete the hormone oxytocin. Oxytocin travels inthe blood and causes the uterus to contract. As labor continues, the cervix is ever more stimulated, and uterine contractions become ever stronger until birth occurs.
Homeostasis and Body Systems
The internal environment of the body consists of blood and tissue fluid. Tissue fluid, which bathes all the cells of the body, is refreshed when molecules such as oxygen and nutrients move into tissue fluid from the blood, and when wastes move from tissue fluid into the blood (Fig. 1.9). Tissue fluid remains constant only as long as blood composition remains constant. As described in the Human Systems Work Together illustration on page 13, all systems of the body contribute toward maintaining homeostasis and therefore a relatively constant internal environment. The cardiovascular system conducts blood to and away from capillaries, where exchange occurs. The heart pumps the blood and thereby keeps it moving toward the capillaries. The formed elements also contribute to homeostasis. Red blood cells transport oxygen and participate in the transport of carbon dioxide. Platelets participate in the clotting process. The lymphatic system is accessory to the cardiovascular system. Lymphatic capillaries collect excess tissue fluid, and this is returned via lymphatic veins to the cardiovascular veins. Lymph nodes help purify lymph and keep it free of pathogens. This action is assisted by the white blood cells that are housed within lymph nodes. The respiratory system adds oxygen to and removes carbon dioxide from the blood. It also plays a role in regulating blood pH because removal of CO2 causes the pH to rise and helps prevent acidosis. The digestive system takes in and digests food, providing nutrient molecules that enter the blood and replace the nutrients that are constantly being used by the body cells. The liver, an organ that assists the digestive process by producing bile, also plays a significant role in regulating blood composition. Immediately after glucose enters the blood, any excess is removed by the liver and stored as glycogen. Later, the glycogen can be broken down to replace the glucose used by the body cells; in this way, the glucose composition of blood remains constant. The liver also removes toxic chemicals, such as ingested alcohol and other drugs. The liver makes urea, a nitrogenous end product of protein metabolism. Urea and other metabolic waste molecules are excreted by the kidneys, which are a part of the urinary system. Urine formation by the kidneys is extremely critical to the body, not only because it rids the body of unwanted substances, but also because urine formation offers an opportunity to carefully regulate blood volume, salt balance, and pH. The integumentary, skeletal, and muscular systems protect the internal organs we have been discussing. In addition, the integumentary system produces vitamin D, while the skeletal system stores minerals and produces the blood cells. The muscular system produces the heat that maintains the internal temperature. The nervous system and the endocrine system regulate the other systems of the body. They work together to control body systems so that homeostasis is maintained. We have already seen that in negative feedback mechanisms, sensory receptors send nerve impulses to regulatory centers in the brain, which then direct effectors to become active. Effectors can be muscles or glands. Muscles bring about an immediate change. Endocrine glands secrete hormones that bring about a slower, more lasting change that keeps the internal environment relatively stable.
Disease
Disease is present when homeostasis fails and the body (or part of the body) no longer functions properly. The effects may be limited or widespread. A local diseaseis more or less restricted to a specific part of the body. On the other hand, a systemic disease affects the entire body or involves several organ systems. Diseases may also be classified on the basis of their severity and duration. Acute diseases occur suddenly and generally last a short time. Chronic diseases tend to be less severe, develop slowly, and are long term. The medical profession has many ways of diagnosing disease including, as discussed in the Medical Focus on page 14, imaging internal body parts.
CHAPTER 2: CELL STRUCTURE AND FUNCTION
2.1 Cellular Organization
Every human cell has a plasma membrane, a nucleus, and cytoplasm. The plasma membrane, which surrounds the cell and keeps it intact, regulates what enters and exits a cell. The plasma membrane is a phospholipid bilayer that is said to be semipermeable because it allows certain molecules but not others to enter the cell. Proteins present in the plasma membrane play important roles in allowing substances to enter the cell. The nucleus is a large, centrally located structure that can often be seen with a light microscope. The nucleus contains the chromosomes and is the control center of the cell. It controls the metabolic functioning and structural characteristics of the cell. The nucleolus is a region inside the nucleus. The cytoplasm is the portion of the cell between the nucleus and the plasma membrane. The matrix of the cytoplasm is a semifluid medium that contains water and various types of molecules suspended or dissolved in the medium. The presence of proteins accounts for the semifluid nature of the matrix.
The cytoplasm contains various organelles (Table 2.1 and Fig. 2.1). Organelles are small, usually membranous structures that are best seen with an electron microscope. Each type of organelle has a specific function. For example, one type of organelle transports substances, and another type produces ATP for the cell. Because organelles are composed of membrane, we can say that membrane compartmentalizes the cell, keeping the various cellular activities separated from one another. Just as the rooms in your house have particular pieces of furniture that serve a particular purpose, organelles have a structure that suits their function. Cells also have a cytoskeleton, a network of interconnected filaments and microtubules in the cytoplasm. The name cytoskeleton is convenient in that it allows us to compare the cytoskeleton to our bones and muscles. Bones and muscles give us structure and produce movement. Similarly, the elements of the cytoskeleton maintain cell shape and allow the cell and its contents to move. Some cells move by using cilia and flagella, which are made up of microtubules.
The Plasma Membrane
Our cells are surrounded by an outer plasma membrane. The plasma membrane separates the inside of the cell, termed the cytoplasm, from the outside. Plasma membrane integrity is necessary to the life of the cell. The plasma membrane is a phospholipid bilayer with attached or embedded proteins. The phospholipid molecule has a polar head and nonpolar tails (Fig. 3.2a). Because the polar heads are charged, they are hydrophilic (water-loving) and face outward, where they are likely to encounter a watery environment. The nonpolar tails are hydrophobic (water-fearing) and face inward, where there is no water. When phospholipids are placed in water, they naturally form a spherical bilayer because of the chemical properties of the heads and the tails. At body temperature, the phospholipid bilayer is a liquid; it has the consistency of olive oil, and the proteins are able to change their positions by moving laterally. The fluid-mosaic model, a working description of membrane structure, suggests that the protein molecules have a changing pattern (form a mosaic) within the fluid phospholipid bilayer (Fig. 3.2b). Our plasma membranes also contain a substantial number of cholesterol molecules. These molecules lend stability to the phospholipid bilayer and prevent a drastic decrease in fluidity at low temperatures. Short chains of sugars are attached to the outer surfaces of some protein and lipid molecules (called glycoproteins and glycolipids, respectively). These carbohydrate chains, specific to each cell, mark the cell as belonging to a particular individual and account for such characteristics as blood type or why a patient’s system sometimes rejects an organ transplant. Some glycoproteins have a special configuration that allows them to act as a receptor for a chemical messenger such as a hormone. Some plasma membrane proteins form channels through which certain substances can enter cells, while others are carriers involved in the passage of molecules through the membrane.
The Nucleus
The nucleus is a prominent structure in human cells. The nucleus is of primary importance because it stores the genetic information that determines the characteristics of the body’s cells and their metabolic functioning. Every cell contains a copy of genetic information, but each cell type has certain genes turned on, and others turned off. Activated DNA, with messenger RNA (mRNA) acting as an intermediary, controls protein synthesis (see page 48). The proteins of a cell determine its structure and the functions it can perform. When you look at the nucleus, even in an electron micrograph, you cannot see DNA molecules, but you can see chromatin (Fig. 3.3). Chemical analysis shows that chromatin
contains DNA and much protein, as well as some RNA. Chromatin undergoes coiling into rodlike structures called chromosomes just before the cell divides. Chromatin is immersed in a semifluid medium called nucleoplasm. Most likely, too, when you look at an electron micrograph of a nucleus (Fig. 2.3), you will see one or more regions that look darker than the rest of the chromatin. These are nucleoli (sing., nucleolus) where another type of RNA, called ribosomal RNA (rRNA), is produced and where rRNA joins with proteins to form the subunits of ribosomes. (Ribosomes are small bodies in the cytoplasm that contain rRNA and proteins.) The nucleus is separated from the cytoplasm by a double membrane known as the nuclear envelope,which is continuous with the endoplasmic reticulum discussed on page 40. The nuclear envelope has nuclear pores of sufficient size to permit the passage of proteins into the nucleus and ribosomal subunits out of the nucleus.
Ribosomes
Ribosomes are composed of two subunits, one large and one small. Each subunit has its own mix of proteins and rRNA. Protein synthesis occurs at the ribosomes. Ribosomes are found free within the cytoplasm either singly or in groups called polyribosomes. Ribosomes are often attached to the endoplasmic reticulum, a membranous system of saccules and channels discussed next (Fig. 2.4). Proteins synthesized by cytoplasmic ribosomes are used inside the cell for various purposes. Those produced by ribosomes attached to endoplasmic reticulum may eventually be secreted from the cell.
Endomembrane System
The endomembrane system consists of the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, and vesicles (tiny membranous sacs) (Fig. 2.5). These components of the cell work together to produce and secrete a product.
The Endoplasmic Reticulum
The endoplasmic reticulum (ER), a complicated system of membranous channels and saccules (flattened vesicles), is physically continuous with the outer membrane of the nuclear envelope. Rough ER is studded with ribosomes on the side of the membrane that faces the cytoplasm. Here proteins are synthesized and enter the ER interior where processing and modification begin. Some of these proteins are incorporated into membrane, and some are for export. Smooth ER, which is continuous with rough ER, does not have attached ribosomes. Smooth ER synthesizes the phospholipids that occur in membranes and has various other functions, depending on the particular cell. In the testes, it produces testosterone, and in the liver it helps detoxify drugs. Regardless of any specialized function, ER also forms vesicles in which large molecules are transported to other parts of the cell. Often these vesicles are on their way to the plasma membrane or the Golgi apparatus.
The Golgi Apparatus
The Golgi apparatus is named for Camillo Golgi, who discovered its presence in cells in 1898. The Golgi apparatus consists of a stack of three to twenty slightly curved saccules whose appearance can be compared to a stack of pancakes (Fig. 2.5). In animal cells, one side of the stack (the inner face) is directed toward the ER, and the other side of the stack (the outer face) is directed toward the plasma membrane. Vesicles can frequently be seen at the edges of the saccules.
The Golgi apparatus receives protein and/or lipid-filled vesicles that bud from the ER. Some biologists believe that these fuse to form a saccule at the inner face and that this saccule remains a part of the Golgi apparatus until the molecules are repackaged in new vesicles at the outer face. Others believe that the vesicles from the ER proceed directly to the outer face of the Golgi apparatus, where processing and packaging occur within its saccules. The Golgi apparatus contains enzymes that modify proteins and lipids. For example, it can add a chain of sugars to proteins and lipids, thereby making them glycoproteins and glycolipids, which are molecules found in the plasma membrane. The vesicles that leave the Golgi apparatus move to other parts of the cell. Some vesicles proceed to the plasma membrane, where they discharge their contents. Because this is secretion, note that the Golgi apparatus is involved in processing, packaging, and secretion. Other vesicles that leave the Golgi apparatus are lysosomes.
Lysosomes
Lysosomes, membranous sacs produced by the Golgi apparatus, contain hydrolytic digestive enzymes. Sometimes macromolecules are brought into a cell by vesicle formation at the plasma membrane (Fig. 3.5). When a lysosome fuses with such a vesicle, its contents are digested by lysosomal enzymes into simpler subunits that then enter the cytoplasm. Even parts of a cell are digested by its own lysosomes (called autodigestion). Normal cell rejuvenation most likely takes place in this manner, but autodigestion is also important during development. For example, when a tadpole becomes a frog, lysosomes digest away the cells of the tail. The fingers of a human embryo are at first webbed, but they are freed from one another as a result of lysosomal action. Occasionally, a child is born with Tay-Sachs disease, a metabolic disorder involving a missing or inactive lysosomal enzyme. In these cases, the lysosomes fill to capacity with macromolecules that cannot be broken down. The cells become so full of these lysosomes that the child dies. Someday soon, it may be possible to provide the missing enzyme for these children.
Mitochondria
Although the size and shape of mitochondria (sing., mitochondrion) can vary, all are bounded by a double membrane. The inner membrane is folded to form little shelves called cristae, which project into the matrix, an inner space filled with a gel-like fluid (Fig. 2.6). Mitochondria are the site of ATP (adenosine triphosphate) production involving complex metabolic pathways. As you know, ATP molecules are the common carrier of energy in cells. A shorthand way to indicate the chemical transformation that involves mitochondria is as follows:
Mitochondria are often called the powerhouses of the cell: Just as a powerhouse burns fuel to produce electricity, the mitochondria convert the chemical energy of glucose products into the chemical energy of ATP molecules. In the process, mitochondria use up oxygen and give off carbon dioxide and water. The oxygen you breathe in enters cells and then mitochondria; the carbon dioxide you breathe out is released by mitochondria. Because oxygen is involved, we say that mitochondria carry on cellular respiration. The matrix of a mitochondrion contains enzymes for breaking down glucose products. ATP production then occurs at the cristae. The protein complexes that aid in the conversion of energy are located in an assembly-line fashion on these membranous shelves. Every cell uses a certain amount of ATP energy to synthesize molecules, but many cells use ATP to carry out their specialized functions. For example, muscle cells use ATP for muscle contraction, which produces movement, and nerve cells use it for the conduction of nerve impulses, which make us aware of our environment.
The Cytoskeleton
Several types of filamentous protein structures form a cytoskeleton that helps maintain the cell’s shape and either anchors the organelles or assists their movement as appropriate. The cytoskeleton includes microtubules, intermediate filaments, and actin filaments (see Fig. 2.1). Microtubules are hollow cylinders whose wall is madeup of 13 longitudinal rows of the globular protein tubulin. Remarkably, microtubules can assemble and disassemble. Microtubule assembly is regulated by the centrosome which lies near the nucleus. Microtubules radiate from the centrosome, helping to maintain the shape of the cell and acting as tracks along which organelles move. It is well known that during cell division, microtubules form spindle fibers, which assist the movement of chromosomes. Intermediate filaments differ in structure and function. Actin filaments are long, extremely thin fibers that usually occur in bundles or other groupings. Actin filaments have been isolated from various types of cells, especially those in which movement occurs. Microvilli, which project from certain cells and can shorten and extend, contain actin filaments. Actin filaments, like microtubules, can assemble and disassemble.
Centrioles
Centrioles are short cylinders with a 9 0 pattern of microtubules, meaning that there are nine outer microtubule triplets and no center microtubules (see Fig. 3.1). Each cell has a pair of centrioles in the centrosome near the nucleus. The members of each pair of centrioles are at right angles to one another. Before a cell divides, the centrioles duplicate, and the members of the new pair are also at right angles to one another. During cell division, the pairs of centrioles separate so that each daughter cell gets one centrosome. Centrioles may be involved in the formation of the spindle that functions during cell division. Their exact role in this process is uncertain, however. Centrioles also give rise to basal bodies that direct the formation of cilia and flagella.
Cilia and Flagella
Cilia and flagella (sing., cilium, flagellum) are projections of cells that can move either in an undulating fashion, like a whip, or stiffly, like an oar. Cilia are shorter than flagella (Fig. 3.7). Cells that have these organelles are capable of self-movement or moving material along the surface of the cell. For example, sperm cells, carrying genetic material to the egg, move by means of flagella. The cells that line our respiratory tract are ciliated. These cilia sweep debris trapped within mucus back up the throat, and this action helps keep the lungs clean. Each cilium and flagellum has a basal body at its base, which lies in the cytoplasm. Basal bodies, like centrioles, have a 9 0 pattern of microtubule triplets. They are believed to organize the structure of cilia and flagella even though cilia and
flagella have a 9 2 pattern of microtubules. In cilia and flagella, nine microtubule doublets surround two central microtubules. This arrangement is believed to be necessary to their ability to move.
2.2 Crossing the Plasma Membrane
The plasma membrane keeps a cell intact. It allows only certain molecules and ions to enter and exit the cytoplasm freely; therefore, the plasma membrane is said to be selectively permeable. Both passive and active methods are used to cross the plasma membrane (see Table 2.2).
Diffusion
Diffusion is the random movement of molecules from the area of higher concentration to the area of lower concentration until they are equally distributed. To illustrate diffusion, imagine putting a tablet of dye into water. The water eventually takes on the color of the dye as the dye molecules diffuse. The chemical and physical properties of the plasma membrane allow only a few types of molecules to enter and exit a cell simply by diffusion. Lipid-soluble molecules such as alcohols can diffuse through the membrane because lipids are the membrane’s main structural components. Gases can also diffuse through the lipid bilayer; this is the mechanism by which oxygen enters cells and carbon dioxide exits cells. As an example, consider the movement of oxygen from the alveoli (air sacs) of the lungs to the blood in the lung capillaries. After inhalation (breathing in), the concentration of oxygen in the alveoli is higher than that in the blood; therefore, oxygen diffuses into the blood. When molecules simply diffuse from higher to lower concentration across plasma membranes, no cellular energy
is involved.
Osmosis
Osmosis is the diffusion of water across a plasma membrane. It occurs whenever an unequal concentration of water exists on either side of a selectively permeable membrane. Normally, body fluids are isotonic to cells (Fig. 2.8a)—that is, there is an equal concentration of solutes (substances) and solvent (water) on both sides of the plasma membrane, and cells maintain their usual size and shape. Intravenous solutions medically administered usually have this tonicity. Tonicity is the degree to which a solution’s concentration of solute versus water causes water to move into or out of cells. Solutions (solute plus solvent) that cause cells to swell or even to burst due to an intake of water are said to be hypotonic solutions. If red blood cells are placed in a hypotonic solution, which has a higher concentration of water (lower concentration of solute) than do the cells, water enters the cells and they swell to bursting (Fig. 2.8b). The term lysis refers to disrupted cells; hemolysis, then, is disrupted red blood cells. Solutions that cause cells to shrink or to shrivel due to a loss of water are said to be hypertonic solutions. If red blood cells are placed in a hypertonic solution, which has a lower concentration of water (higher concentration of solute) than do the cells, water leaves the cells and they shrink (Fig. 2.8c). The term crenation refers to red blood cells in this condition. These changes have occurred due to osmotic pressure. Osmotic pressure is the force exerted on a selectively permeable membrane because water has moved from the area of higher concentration of water to the area of lower concentration (higher concentration of solute).
Filtration
Because capillary walls are only one cell thick, small molecules (e.g., water or small solutes) tend to passively diffuse across these walls, from areas of higher concentration to those of lower concentration. However, blood pressure aids matters by pushing water and dissolved solutes out of the capillary. This process is called filtration. Filtration is easily observed in the laboratory when a solution is poured past filter paper into a flask. Large substances stay behind, but small molecules and water pass through. Filtration of water and substances in the region of capillaries is largely responsible for the formation of tissue fluid, the fluid that surrounds the cells. Filtration is also at work in the kidneys when water and small molecules move from the blood to the inside of the kidney tubules.
Transport by Carriers
Most solutes do not simply diffuse across a plasma membrane; rather, they are transported by means of protein carriers within the membrane. During facilitated transport, a molecule (e.g., an amino acid or glucose) is transported across the plasma membrane from the side of higher concentration to the side of lower concentration. The cell does not need to expend energy for this type of transport because the molecules are moving down their concentration gradient. During active transport, a molecule is moving contrary to the normal direction—that is, from lower to higher concentration (Fig. 2.9). For example, iodine collects in the cells of the thyroid gland; sugar is completely absorbed from the gut by cells that line the digestive tract; and sodium (Na ) is sometimes almost completely withdrawn from urine by cells lining kidney tubules. Active transport requires a protein carrier and the use of cellular energy obtained from the breakdown of ATP. When ATP is broken down, energy is released, and in this case the energy is used by a carrier to carry out active transport. Therefore, it is not surprising that cells involved in active transport have a large number of mitochondria near the plasma membrane at which active transport is occurring. Proteins involved in active transport often are called pumps because just as a water pump uses energy to move water against the force of gravity, proteins use energy to move substances against their concentration gradients. One type of pump that is active in all cells but is especially associated with nerve and muscle cells moves sodium ions (Na ) to the outside of the cell and potassium ions (K ) to the inside of the cell. The passage of salt (NaCl) across a plasma membrane is of primary importance in cells. First, sodium ions are pumped across a membrane; then, chloride ions simply diffuse through channels that allow their passage. Chloride ion channels malfunction in persons with cystic fibrosis, and this leads to the symptoms of this inherited (genetic) disorder. Endocytosis and Exocytosis During endocytosis, commonly called phagocytosis, a portion of the plasma membrane invaginates to envelop a substance, and then the membrane pinches off to form an intracellular vesicle (see Fig. 2.1, top). Digestion may be required before molecules can cross a vesicle membrane to enter the cytoplasm. During exocytosis, a vesicle fuses with the plasma membrane as secretion occurs (see Fig. 2.1, bottom). This is the way insulin leaves insulin-secreting cells, for instance. Table 2.2 summarizes the various ways molecules cross the plasma membrane.
2.3 The Cell Cycle
The cell cycle is an orderly set of stages that take place between the time a cell divides and the time the resulting daughter cells also divide. The cell cycle is controlled by internal and external signals. A signal is a molecule that stimulates or inhibits a metabolic event. For example, growth factors are external signals received at the plasma membrane that cause a resting cell to undergo the cell cycle. When blood platelets release a growth factor, skin fibroblasts in the vicinity finish the cell cycle, thereby repairing an injury. Other signals ensure that the stages follow one another in the normal sequence and that each stage of the cell cycle is properly completed before the next stage begins. The cell cycle has a number of checkpoints, places where the cell cycle stops if all is not well. Any cell that did not successfully complete mitosis and is abnormal undergoes apoptosis at the restriction checkpoint. Apoptosis is often defined as programmed cell death because the cell progresses through a series of events that bring about its destruction. The cell rounds up and loses contact with its neighbors. The nucleus fragments, and the plasma membrane develops blisters. Finally, the cell fragments, and its bits and pieces are engulfed by white blood cells and/or neighboring cells. The enzymes that bring about apoptosis are ordinarily held in check by inhibitors, but are unleashed by either internal or external signals. Following a certain number of cell cycle revolutions, cells are apt to become specialized and no longer go through the cell cycle. Muscle cells and nerve cells typify specialized cells that rarely, if ever, go through the cell cycle. At the other extreme, some cells in the body, called stem cells, are always immature and go through the cell cycle repeatedly. There is a great deal of interest in stem cells today because it may be possible to control their future development into particular tissues and organs.
Cell Cycle Stages
The cell cycle has two major portions: interphase and the mitotic stage (Fig. 2.10).
Interphase
The cell in Figure 2.1 is in interphase because it is not dividing. During interphase, the cell carries on its regular activities, and it also gets ready to divide if it is going to complete the cell cycle. For these cells, interphase has three stages, called G1 phase, S phase, and G2 phase.
G1 Phase: Early microscopists named the phase before DNA replication G1, and they named the phase after DNA replication G2. G stood for “gap.” Now that we know how metabolically active the cell is, it is better to think of G as standing for “growth.” Protein synthesis is very much a part of these growth phases. During G1, a cell doubles its organelles (such as mitochondria and ribosomes) and accumulates materials that will be used for DNA synthesis.
S Phase: Following G1, the cell enters the S (for “synthesis”) phase. During the S phase, DNA replication occurs. At the beginning of the S phase, each chromosome is composed of one DNA double helix, which is equal to a chromatid. At the end of this phase, each chromosome has two identical DNA double helix molecules, and therefore is composed of two sister chromatids. Another way of expressing these events is to say that DNA replication has resulted in duplicated chromosomes.
G2 Phase: During this phase, the cell synthesizes proteins that will assist cell division, such as the protein found in microtubules. The role of microtubules in cell division is described later in this section. Also, chromatin condenses, and the chromosomes become visible.
Mitotic Stage
Following interphase, the cell enters the M (for mitotic) stage. This cell division stage includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm). During mitosis, daughter chromosomes are distributed to two daugh-
ter nuclei. When cytokinesis is complete, two daughter cells are present.
Events During Interphase
Two significant events during interphase are replication of DNA and protein synthesis.
Replication of DNA
During replication, an exact copy of a DNA helix is produced. The double-stranded structure of DNA aids replication because each strand serves as a template for the formation of a complementary strand. A template is most often a mold used to produce a shape opposite to itself. In this case, each old (parental) strand is a template for each new (daughter) strand. Figures 2.11 and 2.12 show how replication is carried out. Figure 2.12 uses the ladder configuration of DNA for easy viewing.
1. Before replication begins, the two strands that make up parental DNA are hydrogen-bonded to one another.
2. During replication, the old (parental) DNA strands unwind and “upzip” (i.e., the weak hydrogen bonds between the two strands break).
3. New complementary nucleotides, always present in the nucleus, pair with the nucleotides in the old strands. A pairs with T and C pairs with G. The enzyme DNA polymerase joins the new nucleotides forming new (daughter)c omplementary strands.
4. When replication is complete, the two double helix molecules are identical. Each strand of a double helix is equal to a chromatid, which means that at the completion of replication each chromosome is composed of two sister chromatids. They are called sister chromatidsbecause they are identical. The chromosome is called a duplicated chromosome. Cancer, which is characterized by rapidly dividing cells, is treated with chemotherapeutic drugs that stop replication and therefore cell division. Some chemotherapeutic drugs are analogs that have a similar, but not identical, structure to the four nucleotides in DNA. When these are mistakenly used by the cancer cells to synthesize DNA, replication stops, and the cells die off.
Figure 2.11. Overview of DNA replication. During replication, an old strand serves as a template for a new strand. The new double helix is composed of an old (parental) strand and a new (daughter) strand.
Figure 2.12. Ladder configuration and DNA replication. Use of the ladder configuration better illustrate show complementary nucleotides available in the cell pair with those of each old strand before they are joined together to form a daughter strand.
Protein Synthesis
DNA not only serves as a template for its own replication, but is also a template for RNA formation. Protein synthesis requires two steps, called transcription and translation. During transcription, an mRNA molecule is produced, and during translation, this mRNA specifies the order of amino acids in a particular polypeptide (Fig. 3.13). A gene (i.e., DNA) contains coded information for the sequence of amino acids in a particular polypeptide. The code is a triplet code: Every three bases in DNA (and therefore in mRNA) stands for a particular amino acid.
Transcription and Translation
During transcription, complementary RNA nucleotides from an RNA nucleotide pool in the nucleus pair with the DNA nucleotides of one strand. The RNA nucleotides are joined by an enzyme called RNA polymerase, and an mRNA molecule results. Therefore, when mRNA forms, it has a sequence of bases complementary to DNA. A sequence of three bases that are complementary to the DNA triplet code is a codon.
Translation requires several enzymes and two other types of RNA: transfer RNA and ribosomal RNA. Transfer RNA (tRNA) molecules bring amino acids to the ribosomes, which are composed of ribosomal RNA (rRNA) and protein. There is at least one tRNA molecule for each of the 20 amino acids found in proteins. The amino acid binds to one end of the molecule, and the entire complex is designated as tRNA–amino acid.
At the other end of each tRNA molecule is a specific anticodon, a group of three bases that is complementary to an mRNA codon. A tRNA molecule comes to the ribosome, where its anticodon pairs with an mRNA codon. For example, if the codon is ACC, then the anticodon is UGG and the amino acid is threonine. (The codes for each of the 20 amino acids are known.) Notice that the order of the codons of the mRNA determines the order that tRNA–amino acids come to a ribosome, and therefore the final sequence of amino acids in a polypeptide.
Events during the Mitotic Stage
The mitotic stage of the cell cycle consists of mitosis and cytokinesis. By the end of interphase (Fig. 3.14, upper left), the centrioles have doubled and the chromosomes are becoming visible. Each chromosome is duplicated—it is composed of two chromatids held together at a centromere. As an aid in describing the events of mitosis, the process is divided into four phases: prophase, metaphase, anaphase, and telophase (Fig.2.14). The parental cell is the cell that divides, and the daughter cells are the cells that result.
Prophase
Several events occur during prophasethat visibly indicate the cell is about to divide. The two pairs of centrioles outside the nucleus begin moving away from each other toward opposite ends of the nucleus. Spindle fibers appear between the separating centriole pairs, the nuclear envelope begins to fragment, and the nucleolus begins to disappear. The chromosomes are now fully visible. Although humans have 46 chromosomes, only four are shown in Figure 2.14 for ease in following the phases of mitosis. Spindle fibers attach to the centromeres as the chromosomes continue to shorten and thicken. During prophase, chromosomes are randomly placed in the nucleus.
Structure of the Spindle: At the end of prophase, a cell has a fully formed spindle. A spindle has poles, asters, and fibers. The asters are arrays of short microtubules that radiate from the poles, and the fibers are bundles of microtubules that stretch between the poles. Centrioles are located in centrosomes, which are believed to organize the spindle.
Metaphase
During metaphase, the nuclear envelope is fragmented, and the spindle occupies the region formerly occupied by the nucleus. The chromosomes are now at the equator (center) of the spindle. Metaphase is characterized by a fully formed spindle, and the chromosomes, each with two sister chromatids, are aligned at the equator (Fig. 3.15).
Anaphase
At the start of anaphase, the sister chromatids separate. Once separated, the chromatids are called chromosomes. Separation of the sister chromatids ensures that each cell receives a copy of each type of chromosome and thereby has a full complement of genes. During anaphase, the daughter chromosomes move to the poles of the spindle. Anaphase is characterized by the movement of chromosomes toward each pole. Function of the Spindle The spindle brings about chromosome movement. Two types of spindle fibers are involved in the movement of chromosomes during anaphase. One type extends from the poles to the equator of the spindle; there, they overlap. As mitosis proceeds, these fibers increase in length, and this helps push the chromosomes apart. The chromosomes themselves are attached to other spindle fibers that simply extend from their centromeres to the poles. These fibers get shorter and shorter as the chromosomes move toward the poles. Therefore, they pull the chromosomes apart.
Spindle fibers, as stated earlier, are composed of microtubules. Microtubules can assemble and disassemble by the addition or subtraction of tubulin (protein) subunits. This is what enables spindle fibers to lengthen and shorten, and it ultimately causes the movement of the chromosomes.
Telophase and Cytokinesis
Telophase begins when the chromosomes arrive at the poles. During telophase, the chromosomes become indistinct chromatin again. The spindle disappears as nucleoli appear, and nuclear envelope components reassemble in each cell. Telophase is characterized by the presence of two daughter nuclei.
Cytokinesis is division of the cytoplasm and organelles. In human cells, a slight indentation called a cleavage furrow passes around the circumference of the cell. Actin filaments form a contractile ring, and as the ring gets smaller and smaller, the cleavage furrow pinches the cell in half. As a result, each cell becomes enclosed by its own plasma membrane.
Importance of Mitosis
Because of mitosis, each cell in our body is genetically identical, meaning that it has the same number and kinds of chromosomes. Mitosis is important to the growth and repair of multicellular organisms. When a baby develops in the mother’s womb, mitosis occurs as a component of growth. As a wound heals, mitosis occurs, and the damage is repaired.
Chapter 3: Body Tissues and Membranes
3.1 Epithelial Tissue
A tissue is composed of specialized cells of one type that perform a common function in the body. There are four major types of tissues: (1) Epithelial tissue, also called epithelium, covers body surfaces and organs and lines body cavities; (2) connective tissue binds and supports body parts; (3) muscular tissue contracts; and (4) nervous tissue responds to stimuli and transmits impulses from one body part to another (Table 3.1). In epithelial tissue, the cells are tightly packed, with little space between them. Externally, this tissue protects the body from drying out, injury, and bacterial invasion. On internal surfaces, epithelial tissue protects, but it also may have an additional function. For example, in the respiratory tract, epithelial tissue sweeps up impurities by means of cilia. Along the digestive tract, it secretes mucus, which protects the lining from digestive enzymes. In kidney tubules, its absorptive function is enhanced by the presence of fine, cellular extensions called microvilli.
Epithelial cells readily divide to produce new cells that replace lost or damaged ones. Skin cells as well as those that line the stomach and intestines are continually being replaced. Surprisingly, then, epithelial tissue lacks blood vessels and must get its nutrients from underlying connective tissues. Because epithelial tissue covers surfaces and lines cavities, it always has a free surface. The other surface is attached to underlying tissue by a layer of carbohydrates and proteins called the basement membrane. Epithelial tissues are classified according to the shape of the cells and the number of cell layers. Simple epithelial tissue is composed of a single layer, and stratified epithelial tissue is composed of two or more layers. Squamous epithelium has flattened cells; cuboidal epithelium has cube-shaped cells; and columnar epithelium has elongated cells.
Squamous Epithelium
Simple squamous epithelium is composed of a single layer of flattened cells, and therefore its protective function is not as significant as that of other epithelial tissues (Fig. 3.1). It is found in areas where secretion, absorption, and filtration occur. For example, simple squamous epithelium lines the lungs where oxygen and carbon dioxide are exchanged, and it lines the walls of capillaries, where nutrients and wastes are exchanged. Stratified squamous epithelium has many cell layers and does play a protective role. While the deeper cells may be cuboidal or columnar, the outer layer is composed of squamous-shaped cells. The outer portion of skin is stratified squamous epithelium. New cells produced in a basal layer become reinforced by keratin, a protein that provides strength, as they move toward the skin’s surface. Aside from skin, stratified squamous epithelium is found lining the various orifices of the body.
Cuboidal Epithelium
Simple cuboidal epithelium (Fig. 3.2) consists of a single layer of cube-shaped cells attached to a basement membrane. This type of epithelium is frequently found in glands, such as salivary glands, the thyroid gland, and the pancreas, where its function is secretion. Simple cuboidal epithelium also covers the ovaries and lines most of the kidney tubules. In one part of the kidney tubule, it absorbs substances from the tubule, and in another part it secretes substances into the tubule. When the cells function in secretion, microvilli (tiny extensions from the cells) increase the surface area of cells. Also, the cuboidal epithelial cells contain many mitochondria, which supply the ATP needed for active transport. Stratified cuboidal epithelium is mostly found lining the larger ducts of certain glands, such as the mammary glands and the salivary glands. Often this tissue has only two layers.
Columnar Epithelium
Simple columnar epithelium (Fig. 3.3) has cells that are longer than they are wide. They are modified to perform particular functions. Some of these cells are goblet cells that secrete mucus onto the free surface of the epithelium. This tissue is well known for lining digestive organs, including the small intestine, where microvilli expand the surface area and aid in absorbing the products of digestion. Simple columnar epithelium also lines the uterine tubes. Here, many cilia project from the cells and propel the egg toward the uterus, or womb. Stratified columnar epithelium is not very common but does exist in parts of the pharynx and the male urethra.
Pseudostratified Columnar Epithelium
Pseudostratified columnar epithelium is so named because it appears to be layered; however, true layers do not exist because each cell touches the basement membrane. In particular, the irregular placement of the nuclei in comparison to columnar epithelium makes the tissue seem stratified Pseudostratified ciliated columnar epithelium (Fig. 3.4) lines parts of the reproductive tract as well as the air passages of the respiratory system, including the nasal cavities and the trachea (windpipe) and its branches. Mucus-secreting goblet cells are scattered among the ciliated epithelial cells. A surface covering of mucus traps foreign particles, and upward ciliary motion carries the mucus to the back of the throat, where it may be either swallowed or expectorated.
Transitional Epithelium
The term transitional epithelium implies changeability, and this tissue changes in response to tension. It forms the lining of the urinary bladder, the ureters, and part of the urethra organs that may need to stretch. When the walls of the bladder are relaxed, the transitional epithelium consists of several layers of cuboidal cells. When the bladder is distended with urine, the epithelium stretches, and the outer cells take on a squamous appearance. It’s interesting to observe that the cells in transitional epithelium of the bladder are physically able to slide in relation to one another while at the same time forming a barrier that prevents any part of urine from diffusing into the internal environment.
3.2 Connective Tissue
Connective tissue binds structures together, provides support and protection, fills spaces, produces blood cells, and stores fat. The body uses this stored fat for energy, insulation, and organ protection. As a rule, connective tissue cells are widely separated by an extracellular matrix composed of an organic ground substance that contains fibers and varies in consistency from solid to semifluid to fluid. Whereas the functional and physical properties of epithelial tissues are derived from its cells, connective tissue properties are largely derived from the characteristics of the matrix (Table 3.2). The fibers within the matrix are of three types. White fibers contain collagen, a substance that gives the fibers flexibility and strength. Yellow fibers contain elastin, which is not as strong as collagen but is more elastic. Reticular fibers are very thin, highly branched, collagenous fibers that form delicate supporting networks.
Fibrous Connective Tissue
Fibrous connective tissue includes loose connective tissue and dense connective tissue. The body’s membranes are composed of an epithelium and fibrous connective tissue.
Loose (areolar) connective tissue commonly lies between other tissues or between organs, binding them together. The cells of this tissue are mainly fibroblasts—large, star-shaped cells that produce extracellular fibers (Fig. 3.5). The cells are located some distance from one another because they are separated by a matrix with a jellylike ground substance that contains many white (collagenous) and yellow (elastic) fibers. The white fibers occur in bundles and are strong and flexible. The yellow fibers form a highly elastic network that returns to its original length after stretching. Adipose tissue (Fig. 3.6) is a type of loose connective tissue in which the fibroblasts enlarge and store fat, and there is limited extracellular matrix. Dense connective tissue (Fig. 3.7) has a matrix produced by fibroblasts that contains bundles of white collagenous fibers. In dense regular connective tissue, the bundles are parallel as in tendons (which connect muscles to bones) and ligaments (which connect bones to other bones at joints). In dense irregular connective tissue, the bundles run in different directions. This type of tissue is found in the inner portion of the skin. The fibroblasts of reticular connective tissues are called reticular cells, and the matrix contains only reticular fibers. This tissue, also called lymphatic tissue, is found in lymph nodes, the spleen, thymus, and red bone marrow. These organs are a part of the immune system because they store and/or produce white blood cells, particularly lymphocytes. All types of blood cells are produced in red bone marrow.
Cartilage
In cartilage, the cells (chondrocytes), which lie in small chambers called lacunae, are separated by a matrix that is solid yet flexible. Unfortunately, because this tissue lacks a direct blood supply, it heals very slowly. The three types of cartilage are classified according to the type of fiber in the matrix. Hyaline cartilage (Fig. 3.8) is the most common type of cartilage. The matrix, which contains only very fine collagenous fibers, has a glassy, white, opaque appearance. This type of cartilage is found in the nose, at the ends of the long bones and ribs, and in the supporting rings of the trachea. The fetal skeleton is also made of this type of cartilage, although the cartilage is later replaced by bone. Elastic cartilage has a matrix containing many elastic fibers, in addition to collagenous fibers. For this reason, elastic cartilage is more flexible than hyaline cartilage. Elastic cartilage is found, for example, in the framework of the outer ear. Fibrocartilage has a matrix containing strong collagenous fibers. This type of cartilage absorbs shock and reduces friction between joints. Fibrocartilage is found in structures that withstand tension and pressure, such as the pads between the vertebrae in the backbone and the wedges in the knee joint.
Bone
Bone is the most rigid of the connective tissues. It has an extremely hard matrix of mineral salts, notably calcium salts, deposited around protein fibers. The minerals give bone rigidity, and the protein fibers provide elasticity and strength, much as steel rods do in reinforced concrete. The outer portion of a long bone contains compact bone. Compact bone consists of many cylindrical-shaped units called an osteon, or Haversian system (Fig. 3.9). In an osteon, matrix is deposited in thin layers called lamellae that form a concentric pattern around tiny tubes called central canals. The canals contain nerve fibers and blood vessels. The blood vessels bring nutrients to bone cells (called osteocytes) that are located in lacunae between the lamellae. The nutrients can reach all of the cells because minute canals (canaliculi) containing thin extensions of the osteocytes connect the osteocytes with one another and with the central canals. The ends of a long bone contain spongy bone, which has an entirely different structure. Spongy bone contains numerous bony bars and plates called trabeculaeseparated by irregular spaces. Although lighter than compact bone, spongy bone is still designed for strength. Like braces used for support in buildings, the solid portions of spongy bone follow lines of stress. Blood cells are formed within red marrow found in spongy bone at the ends of certain long bones.
Blood
Blood (Fig. 4.10) is a connective tissue composed of cells suspended in a liquid matrix called plasma. Collectively, the blood cells are called formed elements. Blood cells are of two types: red blood cells (erythrocytes), which carry oxygen, and white blood cells (leukocytes), which aid in fighting infection. Also present are platelets, which are important to the initiation of blood clotting. Platelets are not complete cells; rather, they are fragments of giant cells found in the bone marrow. In red bone marrow, stem cells continually divide to produce new cells that mature into the different types of blood cells. Blood is unlike other types of connective tissue in that the extracellular matrix (plasma)is not made by the cells of the tissue. Plasma is a mixture of different types of molecules that enter blood at various organs.
3.3 Muscular Tissue
Muscular (contractile) tissue is composed of cells called muscle fibers (Table 4.3). Muscle fibers contain actin and myosin, which are protein filaments whose interaction accounts for movement. The three types of vertebrate muscles are skeletal, smooth, and cardiac.
Skeletal Muscle
Skeletal muscle, also called voluntary muscle (Fig. 3.11), is at tached by tendons to the bones of the skeleton. When skeletal muscle contracts, body parts such as arms and legs move. Contraction of skeletal muscle, which is under voluntary control, is forceful but of short duration. Skeletal muscle fibers are cylindrical and quite long—sometimes they run the length of the muscle. They arise during development when
several cells fuse, resulting in one fiber with multiple nuclei. The nuclei are located at the periphery of the cell, just inside the plasma membrane. The fibers have alternating light and dark bands that give them a striated (striped) appearance. These bands are due to the placement of actin filaments and myosin filaments in the fiber.
Smooth Muscle
Smooth (visceral) muscle is so named because the arrange ment of actin and myosin does not give the appearance of cross-striations. The spindle-shaped cells form layers in which the thick middle portion of one cell is opposite the thin ends of adjacent cells. Consequently, the nuclei form an irregular pattern in the tissue (Fig. 3.12). Smooth muscle is not under voluntary control and therefore is said to be involuntary. Smooth muscle is found in the walls of hollow viscera, such as the intestines, stomach, uterus, urinary bladder, and blood vessels. Smooth muscle contracts more slowly than skeletal muscle but can remain contracted for a longer time. Contractility is inherent in this type of muscle, and it contracts rhythmically on its own. Even so, its contraction can be modified by the nervous system. Smooth muscle of the small intestine contracts in waves, thereby moving food along its lumen (central cavity). When the smooth muscle of blood vessels contracts, blood vessels constrict, helping to regulate blood flow.
Cardiac Muscle
Cardiac muscle (Fig. 3.13) is found only in the walls of the heart. Its contraction pumps blood and accounts for the heartbeat. Cardiac muscle combines features of both smooth muscle and skeletal muscle. Like skeletal muscle, it has striations, but the contraction of the heart is involuntary for the most part. Also like skeletal muscle, its contractions are strong, but like smooth muscle, the contraction of the heart is inherent and rhythmical. Also, its contraction can be modified by the nervous system. Even though cardiac muscle fibers are striated, the cells differ from skeletal muscle fibers in that they have a single, centrally placed nucleus. The cells are branched and seemingly fused one with the other, and the heart appears to be composed of one large, interconnecting mass of muscle cells. Actually, cardiac muscle cells are separate and individual, but they are bound end-to-end at intercalated disks, areas where folded plasma membranes between two cells contain adhesion junctions and gap junctions. These permit extremely rapid spread of contractile stimuli so that the fibers contract almost simultaneously.
3.4 Nervous Tissue
Nervous tissue, found in the brain and spinal cord, contain specialized cells called neurons that conduct nerve impulses. A neuron (Fig. 4.14) has three parts: (1) A dendrite collects signals that may result in a nerve impulse; (2) the cell body contains the nucleus and most of the cytoplasm of the neuron; and (3) the axon conducts nerve impulses. Long axons are called fibers. Outside the brain and spinal cord, fibers are bound together by connective tissue to form nerves. Nerves conduct impulses from sense organs to the spinal cord and brain, where the phenomenon called sensation occurs. They also conduct nerve impulses away from the spinal cord and brain to the muscles, causing the muscles to contract. In addition to neurons, nervous tissue contains neuroglia.
Neuroglia
Neuroglia are cells that outnumber neurons nine to one and take up more than half the volume of the brain. The primary function of neuroglia is to support and nourish neurons. For example, types of neuroglia found in the brain are microglia, astrocytes, and oligodendrocytes. Microglia, in addition to supporting neurons, engulf bacterial and cellular debris. Astrocytes provide nutrients to neurons and produce a hormone known as glia-derived growth factor, which someday might be used as a cure for Parkinson disease and other diseases caused by neuron degeneration. Oligodendrocytes form myelin, a protective layer of fatty insulation. Schwann cells are the type of neuroglia that encircles all long nerve fibers located outside the brain or spinal cord. Each Schwann cell encircles only a small section of a nerve fiber. The gaps between Schwann cells are called nodes of Ranvier. Collectively, the Schwann cells provide nerve fibers with a myelin sheath interrupted by the nodes. The myelin sheath speeds conduction because the nerve impulse jumps from node to node. Because the myelin sheath is white, all nerve fibers appear white.
3.5 Extracellular Junctions,
Glands, and Membranes Extracellular Junctions. The cells of a tissue can function in a coordinated manner when the plasma membranes of adjoining cells interact. The junctions that occur between cells help cells function as a tissue. A tight junction forms an impermeable barrier because adjacent plasma membrane proteins actually join, producing a zipperlike fastening (Fig. 3.15a). In the small intestine, gastric juices stay out of the body, and in the kidneys, the urine stays within kidney tubules because epithelial cells are joined by tight junctions. A gap junction forms when two adjacent plasma membrane channels join (Fig. 3.15b). This lends strength, but it also allows ions, sugars, and small molecules to pass between the two cells. Gap junctions in heart and smooth muscle ensure synchronized contraction. In an adhesion junction (desmosome), the adjacent plasma membranes do not touch but are held together by extracellular filaments firmly attached to cytoplasmic plaques, composed of dense protein material (Fig. 3.15c).
Membranes
Membranes line the internal spaces of organs and tubes that open to the outside, and they also line the body cavities.
Mucous Membranes
Mucous membranes line the interior walls of the organs and tubes that open to the outside of the body, such as those of the digestive, respiratory, urinary, and reproductive systems. These membranes consist of an epithelium overlying a layer of loose connective tissue. The epithelium contains goblet cells that secrete mucus. The mucus secreted by mucous membranes ordinarily protects interior walls from invasion by bacteria and viruses; for example, more mucus is secreted when a person has a cold, resulting in a “runny nose.” In addition, mucus usually protects the walls of the stomach and small intestine from digestive juices, but this protection breaks down when a person develops an ulcer.
Serous Membranes
Serous membranes line cavities, including the thoracic and abdominopelvic cavities, and cover internal organs such as the intestines. The term parietal refers to the wall of the body cavity, while the term visceral pertains to the internal organs. Therefore, parietal membranes line the interior of the thoracic and abdominopelvic cavities, and visceral membranes cover the organs. Serous membranes consist of a layer of simple squamous epithelium overlying a layer of loose connective tissue. They secrete a watery fluid that keeps the membranes lubricated. Serous membranes support the internal organs and tend to compartmentalize the large thoracic and abdominopelvic cavities. This helps hinder the spread of any infection.
In the thorax, the pleura are serous membranes that form a double layer around the lungs. The parietal pleura lines the inside of the thoracic wall, while the visceral pleura adheres to the surface of the lungs. Similarly a double-layered serous membrane is a part of the pericardium, a covering for the heart.
The peritoneum is the serous membranes within the abdomen. The parietal peritoneum lines the abdominopelvic wall, and the visceral peritoneum covers the organs. In between the organs, the visceral peritoneum comes together to form a double-layered mesentery that supports these organs.
Synovial Membranes
Synovial membranes line freely movable joint cavities and are composed of connective tissues. They secrete synovial fluid into the joint cavity; this fluid lubricates the ends of the bones so that they can move freely. In rheumatoid arthritis,the synovial membrane becomes inflamed and grows thicker. Fibrous tissue then invades the joint and may eventually become bony so that the bones of the joint are no longer capable of moving.
Meninges
The meninges are membranes found within the posterior cavity (see Fig. 1.5). They are composed only of connective tissue and serve as a protective covering for the brain and spinal cord. Meningitis is a life-threatening infection of the meninges.
CutaneousMembrane
The cutaneous membrane, or skin, forms the outer covering of the body. It consists of an outer portion of keratinized stratified squamous epithelium attached to a thick underlying layer of dense irregular connective tissue.
INTEGUMENTARY SYSTEM
Structure of the Skin
The skin covers the entire surface of the human body. In an adult, the skin has a surface area of about 1.8 square meters (20.83 square feet). The skin is sometimes called the cutaneous membrane or the integument. Because the skin has several accessory organs, it is also possible to speak of the integumentary system. The skin has two regions: the epidermis and the dermis. The hypodermis, a subcutaneous tissue, is found between the skin and any underlying structures, such as muscle. Usually, the hypodermis is only loosely attached to underlying muscle tissue, but where no muscles are present, the hypodermis attaches directly to bone. For example, there are flexion creases where the skin attaches directly to the joints of the fingers.
Epidermis
The epidermis is the outer and thinner region of the skin. It is made up of stratified squamous epithelium divided into several layers; the deepest layer is the stratum basale, and the most superficial layer is the stratum corneum.
Stratum Basale
The basal cells of the stratum basale lie just superior to the dermis and are constantly dividing and producing new cells that are pushed to the surface of the epidermis in two to four weeks. As the cells move away from the dermis, they get progressively farther away from the blood vessels in the dermis. Because these cells are not being supplied with nutrients and oxygen (the epidermis itself lacks blood vessels), they eventually die and are sloughed off. Langerhans cells are macrophages found deep in the epidermis. Macrophages are related to monocytes, white blood cells produced in red bone marrow. These cells phagocytize microbes and then travel to lymphatic organs, where they stimulate the immune system to react. Melanocytesare another type of specialized cell located in the deeper epidermis. Melanocytes produce melanin, the pigment primarily responsible for skin color. Since the number of melanocytes is about the same in all individuals, variation in skin color is due to the amount of melanin produced and its distribution. When skin is exposed to the sun, melanocytes produce more melanin to protect the skin from the damaging effects of the ultraviolet (UV) radiation in sunlight. The melanin is passed to other epidermal cells, and the result is tanning, or in some people, the formation of patches of melanin called freckles. A hereditary trait characterized by the lack of ability to produce melanin is known as albinism. Individuals with this disorder lack pigment not only in the skin, but also in the hair and eyes. Another pigment, called carotene, is present in epidermal cells and in the dermis and gives the skin of certain Asians its yellowish hue. The pinkish color of fair-skinned people is due to the pigment hemoglobin in the red blood cells in the capillaries of the dermis.
Stratum Corneum
As cells are pushed toward the surface of the skin, they become flat and hard, forming the tough, uppermost layer of the epidermis, the stratum corneum. Hardening is caused by keratinization, the cellular production of a fibrous, waterproof protein called keratin. Over much of the body, keratinization is minimal, but the palms of the hands and the soles of the feet normally have a particularly thick outer layer of dead, keratinized cells. The waterproof nature of keratin protects the body from water loss and water gain. The stratum corneum allows us to live in a desert or a tropical rain forest without damaging our inner cells. The stratum corneum also serves as a mechanical barrier against microbe invasion. This protective function of skin is assisted by the secretions of sebaceous glands, which weaken or kill bacteria on the skin.
Dermis
The dermis,a deeper and thicker region than the epidermis, is composed of dense irregular connective tissue. The upper layer of the dermis has fingerlike projections called dermal papillae. Dermal papillae project into and anchor the epidermis. In the overlying epidermis, dermal papillae cause ridges, resulting in spiral and concentric patterns commonly known as “fingerprints.” The function of the epidermal ridges is to increase friction and thus provide a better gripping surface. Because they are unique to each person, fingerprints and footprints can be used for identification purposes. The dermis contains collagenous and elastic fibers. The collagenousfibersare flexible but offer great resistance to overstretching; they prevent the skin from being torn. The elastic fibers stretch to allow movement of underlying muscles and joints, but they maintain normal skin tension. The dermis also contains blood vessels that nourish the skin. Blood rushes into these vessels when a person blushes; blood is reduced in them when a person turns cyanotic, or “blue.” Sometimes, blood flow to a particular area is restricted in bedridden patients, and consequently they develop decubitus ulcers (bedsores). These can be prevented by changing the patient’s position frequently and by massaging the skin to stimulate blood flow. There are also numerous sensory nerve fibers in the dermis that take nerve impulses to and from the accessory structures of the skin.
Hypodermis
Hypodermis, or subcutaneous tissue, lies below the dermis. From the names for this layer, we get the terms subcutaneous injection, performed with a hypodermic needle. The hypodermis is composed of loose connective tissue, including adipose (fat) tissue. Fat is an energy storage form that can be called upon when necessary to supply the body with molecules for cellular respiration. Adipose tissue also helps insulate the body. A well-developed hypodermis gives the body a rounded appearance and provides protective padding against external assaults. Excessive development of adipose tissue in the hypodermis layer results in obesity.
Accessory Structures of the Skin
Hair, nails, and glands are structures of epidermal origin, even though some parts of hair and glands are largely in the dermis.
Hair and Nails
Hair is found on all body parts except the palms, soles, lips, nipples, and portions of the external reproductive organs. Most of this hair is fine and downy, but the hair on the head includes stronger types as well. After puberty, when sex hormones are made in quantity, there is noticeable hair in the axillary and pelvic regions of both sexes. In the male, a beard develops, and other parts of the body may also become quite hairy. When women produce more male sex hormone than usual, they can develop hirsutism, a condition characterized by excessive body and facial hair. Hormonal injections and electrolysis to kill roots are possible treatments. Hairs project from complex structures called hair follicles. These hair follicles are formed from epidermal cells but are located in the dermis of the skin certain hair follicle cells continually divide, producing new cells that form a hair. At first, the cells are nourished by dermal blood vessels, but as the hair grows up and out of the follicle, they are pushed farther away from this source of nutrients, become keratinized, and die. The portion of a hair within the follicle is called the root, and the portion that extends beyond the skin is called the shaft. The life span of any particular hair is usually three to four months for an eyelash and three to four years for a scalp hair; then it is shed and regrows. In males, baldness occurs when the hair on the head fails to regrow. Alopecia, meaning hair loss, can have many causes. Male pattern baldness, or androgenic alopecia,is an inherited condition. Alopecia areatais characterized by the sudden onset of patchy hair loss. It is most common among children and young adults, and can affect either sex. Each hair has one or more oil, or sebaceous, glands, whose ducts empty into the follicle. A smooth muscle, the arrector pili, attaches to the follicle in such a way that contraction of the muscle causes the hair to stand on end. If a person has had a scare or is cold, “goose bumps” develop due to contraction of these muscles. Nails grow from special epithelial cells at the base of the nail in the region called the nail root. These cells become keratinized as they grow out over the nail bed. The visible portion of the nail is called the nail body. The cuticle is a fold of skin that hides the nail root. Ordinarily, nails grow only about 1 millimeter per week. The pink color of nails is due to the vascularized dermal tissue beneath the nail. The whitish color of the half-moon-shaped base, or lunula, results from the thicker germinal layer in this area.
Glands
The glands in the skin are groups of cells specialized to produce and secrete a substance into ducts.
SweatGlands
Sweat glands, or sudoriferous glands, are present in all regions of the skin. There can be as many as 90 glands per square centimeter on the leg, 400 glands per square centimeter on the palms and soles, and an even greater number on the fingertips. A sweat gland is tubular. The tubule is coiled, particularly at its origin within the dermis. These glands become active when a person is under stress. Apocrine glands open into hair follicles in the anal region, groin, and armpits. These glands begin to secrete at puberty, and a component of their secretion may act as a sex attractant. Eccrine glands open onto the surface of the skin. They become active when a person is hot, helping to lower body temperature as sweat evaporates. The sweat (perspiration) produced by these glands is mostly water, but it also contains salts and some urea, a waste substance. Therefore, sweat is a form of excretion. Ears contain modified sweat glands, called ceruminous glands, which produce cerumen, or earwax.
SebaceousGlands
Most sebaceous glands are associated with a hair follicle. These glands secrete an oily substance called sebum that flows into the follicle and then out onto the skin surface. This secretion lubricates the hair and skin, and helps waterproof them. Particularly on the face and back, the sebaceous glands may fail to discharge sebum, and the secretions collect, forming whiteheads or blackheads. If pus-inducing bacteria are also present, a boil or pimple may result. Acne vulgaris, the most common form of acne, is an inflammation of the sebaceous glands that most often occurs during adolescence. Hormonal changes during puberty cause the sebaceous glands to become more active at this time.
MammaryGlands
The mammary glands are located within the breasts. A female breast contains 15 to 25 lobes, which are divided into lobules. Each lobule contains many alveoli. When milk is secreted, the milk enters a duct that leads to the nipple. Cells within the alveoli produce milk only after childbirth in response to complex hormonal changes occurring at that time.
Disorders of the Skin
The skin is subject to many disorders, some of which are more annoying than life-threatening. For example, athlete’s foot is caused by a fungal infection that usually involves the skin of the toes and soles. Impetigois a highly contagious disease occurring most often in young children. It is caused by a bacterial infection that results in pustules that crust over. Psoriasis is a chronic condition, possibly hereditary, in which the skin develops pink or reddish patches covered by silvery scales due to overactive cell division. Eczema, an inflammation of the skin,is caused by sensitivity to various chemicals (e.g., soaps or detergents), to certain fabrics, or even to heat or dryness. Dandruff is a skin disorder not caused by a dry scalp, as is commonly thought, but by an accelerated rate of keratinization in certain areas of the scalp, producing flaking and itching. Urticaria, or hives, is an allergic reaction characterized by the appearance of reddish, elevated patches and often by itching.
Skin Cancer
Skin cancer is categorized as either melanoma or nonmelanoma. Nonmelanoma cancers, which include basal cell carcinoma and squamous cell carcinoma, are much less likely to metastasize than melanoma cancer. Basal cell carcinoma, the most common type of skin cancer, begins when ultraviolet (UV) radiation causes epidermal basal cells to form a tumor, while at the same time suppressing the immune system’s ability to detect the tumor. The signs of a tumor are varied. They include an open sore that will not heal; a recurring reddish patch; a smooth, circular growth with a raised edge; a shiny bump; or a pale mark. About 95% of patients are easily cured by surgical removal of the tumor, but recurrence is common. Squamous cell carcinoma (Fig. 5.6b) begins in the epidermis proper. While five times less common than basal cell carcinoma, it is more likely to spread to nearby organs, and death occurs in about 1% of cases. The signs of squamous cell carcinoma are the same as those for basal cell carcinoma, except that it may also show itself as a wart that bleeds and scabs. Melanoma , the type that is more likely to be malignant, starts in the melanocytes and has the appearance of an unusual mole. Unlike a normal mole, which is dark, circular, and confined, a melanoma mole looks like a spilled ink spot, and a single melanoma mole may display a variety of shades. A melanoma mole can also itch, hurt, or feel numb. The skin around the mole turns gray, white, or red. Melanoma is most common in fair-skinned persons, particularly if they have suffered occasional severe burns as children. Melanoma risk increases with the number of moles a person has. Most moles appear before the age of 14, and their appearance is linked to sun exposure. Melanoma rates have risen since the turn of the century, but the incidence has doubled in the last decade. In 2002, about 54,000 cases of melanoma were diagnosed in the United States. Raised growths on the skin, such as moles and warts, usually are not cancerous. Moles are due to an overgrowth of melanocytes, and warts are due to a viral infection.
Wound Healing
A wound that punctures a blood vessel will fill with blood. Chemicals released by damaged tissue cells will cause the blood to clot. The clot prevents pathogens and toxins from spreading to other tissues. The part of the clot exposed to air will dry and harden, gradually becoming a scab. White blood cells and fibroblasts move into the area. White blood cells help fight infection and fibroblasts are able to pull the margins of the wound together. Fibroblasts promote tissue regeneration: The basal layer of the epidermis begins to produce new cells at a faster than usual rate.
Burns
The epidermal injury known as a burn is usually caused by heat but can also be caused by radioactive, chemical, or electrical agents. Two factors affect burn severity: the depth of the burn and the extent of the burned area. A useful technique for estimating the extent of a burn, called the “rule of nines,” is often employed. In this method, the total body surface is divided into regions as follows: the head and neck, 9% of the total body surface; each upper limb, 9%; each lower limb, 18%; the front and back portions of the trunk, 18% each; and the perineum, which includes the anal and urogenital regions,1%. One way to classify burns is according to the depth of the burned area. In first-degree burns, only the epidermis is affected. The person experiences redness and pain, but no blisters or swelling. A classic example of a first-degree burn is a moderate sunburn. The pain subsides within 48–72 hours, and the injury heals without further complications or scarring. The damaged skin peels off in about a week. A second-degree burn extends through the entire epidermis and part of the dermis. The person experiences not only redness and pain, but also blistering in the region of the damaged tissue. The deeper the burn, the more prevalent the blisters, which enlarge during the hours after the injury. Unless they become infected, most second-degree burns heal without complications and with little scarring in 10–14 days. If the burn extends deep into the dermis, it heals more slowly over a period of 30–105 days. The healing epidermis is extremely fragile, and scarring is common. First- and second-degree burns are sometimes referred to as partial-thickness burns. Third-degree burns, or full-thickness burns, destroy the entire thickness of the skin. The surface of the wound is leathery and may be brown, tan, black, white, or red. The patient feels no pain because the pain receptors have been destroyed, as have blood vessels, sweat glands, sebaceous glands, and hair follicles. Fourth-degree burns involve tissues down to the bone. Obviously, the chances of a person surviving fourth-degree burns are not good unless a very limited area of the body is affected. The major concerns with severe burns are fluid loss, heat loss, and bacterial infection. Fluid loss is counteracted by intravenous administration of a balanced salt solution. Heat loss is minimized by placing the burn patient in a warm environment. Bacterial infection is treated by isolation and the application of an antibacterial dressing. As soon as possible, the damaged tissue is removed, and skin grafting is begun. The skin needed for grafting is usually taken from other parts of the patient’s body. This is called autografting, as opposed to heterografting, in which the graft is received from another person. Autografting is preferred because rejection rates are very low. However, if the burned area is quite extensive, it may be difficult to acquire enough skin for autografting. In that case, skin can be grown in the laboratory from only a few cells taken from the patient.
Effects of Aging
As aging occurs, the epidermis maintains its thickness, but the turnover of cells decreases. The dermis becomes thinner, the dermal papillae flatten, and the epidermis is held less tightly to the dermis so that the skin is looser. Adipose tissue in the hypodermis of the face and hands also decreases, which means that older people are more likely to feel cold. The fibers within the dermis change with age. The collagenous fibers become coarser, thicker, and farther apart; therefore, there is less collagen than before. Elastic fibers in the upper layer of the dermis are lost, and those in the lower dermis become thicker, less elastic, and disorganized. The skin wrinkles because (1) the epidermis is loose, (2) the fibers are fewer and those remaining are disorganized, and (3) the hypodermis has less padding. With aging, homeostatic adjustment to heat is limited due to less vasculature (fewer blood vessels) and fewer sweat glands. The number of hair follicles decreases, causing the hair on the scalp and extremities to thin. Because of a reduced number of sebaceous glands, the skin tends to crack. As a person ages, the number of melanocytes decreases. This causes the hair to turn gray and the skin to become paler. In contrast, some of the remaining pigment cells are larger, and pigmented blotches appear on the skin. Many of the changes that occur in the skin as a person ages appear to be due to sun damage. Ultraviolet radiation causes rough skin, mottled pigmentation, fine lines and wrinkles, deep furrows, benign skin growths, and the various types of skin cancer.
Bibliography
· Dr. Sylvia Mader: Understanding Human Anatomy and Physiology, 5th edition, The Mc Graw Hill Companies. 2004
· Colbert et al: Principles of Human Anatomy and Physiology,
· Elaine Marieb: Essentials of Human Anatomy and Physiology, 7th ed
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