Structure

A single skeletal-muscle cell is known as a muscle fiber. Each muscle fiber is formed during development by the fusion of a number of undifferentiated, mononucleated cells, known as myoblasts, into a single cylindrical, multinucleated cell. Skeletal muscle differentiation is completed around the time of birth, and these differentiated fibers continue to increase in size during growth from infancy to adult stature, but no new fibers are formed from myoblasts. Adult skeletal-muscle fibers have diameters between 10 and 100 ^m and lengths that may extend up to 20 cm.

If skeletal-muscle fibers are destroyed after birth as a result of injury, they cannot be replaced by the division of other existing muscle fibers. New fibers can be formed, however, from undifferentiated cells

MUSCLE

known as satellite cells, which are located adjacent to the muscle fibers and undergo differentiation similar to that followed by embryonic myoblasts. This capacity for forming new skeletal-muscle fibers is considerable but will not restore a severely damaged muscle to full strength. Much of the compensation for a loss of muscle tissue occurs through an increase in the size of the remaining muscle fibers.

The term muscle refers to a number of muscle fibers bound together by connective tissue (Figure 11-1). The relationship between a single muscle fiber and a muscle is analogous to that between a single neuron and a nerve, which is composed of the axons of many neurons. Muscles are usually linked to bones by bundles of collagen fibers known as tendons, which are located at each end of the muscle.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Muscle CHAPTER ELEVEN

Muscle CHAPTER ELEVEN

Skeletal Muscle Organization Hierarchy

FIGURE 11-1

Organization of cylindrical skeletal-muscle fibers in a muscle that is attached to bones by tendons. %

FIGURE 11-1

Organization of cylindrical skeletal-muscle fibers in a muscle that is attached to bones by tendons. %

In some muscles, the individual fibers extend the entire length of the muscle, but in most, the fibers are shorter, often oriented at an angle to the longitudinal axis of the muscle. The transmission of force from muscle to bone is like a number of people pulling on a rope, each person corresponding to a single muscle fiber and the rope corresponding to the connective tissue and tendons.

Some tendons are very long, with the site of tendon attachment to bone far removed from the end of the muscle. For example, some of the muscles that move the fingers are in the forearm, as one can observe by wiggling one's fingers and feeling the movement of the muscles in the lower arm. These muscles are connected to the fingers by long tendons.

The most striking feature seen when observing skeletal- or cardiac-muscle fibers through a light microscope (Figure 11-2) is a series of light and dark bands perpendicular to the long axis of the fiber. Because of this characteristic banding, both types are known as striated muscle (Figure 11-3). Smooth-muscle cells do not show a banding pattern. The striated pattern in skeletal and cardiac fibers results from the arrangement of numerous thick and thin filaments in the cytoplasm into approximately cylindrical bundles (1 to 2 ^m in diameter) known as myofibrils (Figure 11-4). Most of the cytoplasm of a fiber is filled with myofibrils, each of which extends from one end of the fiber to the other and is linked to the tendons at the ends of the fiber.

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

II. Biological Control Systems

11. Muscle

PART TWO Biological Control Systems

© The McGraw-Hill Companies, 2001

- Multiple nuclei -

.ViUPlt vrftff' hu I

FIGURE 11-2

Skeletal-muscle fibers viewed through a light microscope. Each bracket at the left indicates one muscle fiber. Arrow indicates a blood vessel containing red blood cells.

From Edward K. Keith and Michael H. Ross, "Atlas of Descriptive Histology," Harper & Row, New York, 1968.

vrftff' hu I

FIGURE 11-2

Skeletal-muscle fibers viewed through a light microscope. Each bracket at the left indicates one muscle fiber. Arrow indicates a blood vessel containing red blood cells.

From Edward K. Keith and Michael H. Ross, "Atlas of Descriptive Histology," Harper & Row, New York, 1968.

The thick and thin filaments in each myofibril (Figures 11-4 and 11-5) are arranged in a repeating pattern along the length of the myofibril. One unit of this repeating pattern is known as a sarcomere (Greek, sarco, muscle; mere, small). The thick filaments are composed almost entirely of the contractile protein myosin. The thin filaments (which are about half the diameter of the thick filaments) contain the contractile protein actin, as well as to two other proteins— troponin and tropomyosin—that play important roles in regulating contraction, as we shall see.

The thick filaments are located in the middle of each sarcomere, where their orderly parallel arrangement produces a wide, dark band known as the A band (Figure 11-4). Each sarcomere contains two sets of thin filaments, one at each end. One end of each thin filament is anchored to a network of interconnecting proteins known as the Z line, whereas the other end overlaps a portion of the thick filaments. Two successive Z lines define the limits of one sarcomere. Thus, thin filaments from two adjacent sarcomeres are anchored to the two sides of each Z line.

A light band, known as the I band (Figure 11-4), lies between the ends of the A bands of two adjacent m ■ - -■ "i

Skeletal-muscle fiber

Cardiac-muscle fiber

Cardiac-muscle fiber

- Nucleus

Smooth-muscle fiber

FIGURE 11-3

The three types of muscle fibers. Note the differences in fiber diameter. (The single nucleus in the center of the cardiac-muscle fiber is not shown.) %

sarcomeres and contains those portions of the thin filaments that do not overlap the thick filaments. It is bisected by the Z line.

Two additional bands are present in the A-band region of each sarcomere (Figure 11-5). The H zone is a narrow light band in the center of the A band. It corresponds to the space between the opposing ends of the two sets of thin filaments in each sarcomere; hence, only thick filaments, specifically their central parts, are found in the H zone. A narrow, dark band in the center of the H zone is known as the M line and corresponds to proteins that link together the central region of the thick filaments. In addition, filaments composed of the protein titin extend from the Z line to the M line and are linked to both the M-line proteins and the thick filaments. Both the M-line linkage between thick filaments and the titin filaments act to maintain the regular array of thick filaments in the middle of each sar-comere. Thus, neither the thick nor the thin filaments are free-floating.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Muscle CHAPTER ELEVEN

Muscle CHAPTER ELEVEN

Muscle fiber

Muscle fiber

Z line Z line

Sarcomere

Z line Z line

Sarcomere

Z line Z line

Z line Z line

Sarcomere Structure And Function

Thick (myosin) filament Thin (actin) filament

FIGURE 11-4

Arrangement of filaments in a skeletal-muscle fiber that produces the striated banding pattern. % rrâ

Thick (myosin) filament Thin (actin) filament

FIGURE 11-4

Arrangement of filaments in a skeletal-muscle fiber that produces the striated banding pattern. % rrâ

A cross section through the A bands (Figure 11-6), shows the regular, almost crystalline, arrangement of overlapping thick and thin filaments. Each thick filament is surrounded by a hexagonal array of six thin filaments, and each thin filament is surrounded by a triangular arrangement of three thick filaments. Altogether there are twice as many thin as thick filaments in the region of filament overlap.

The space between overlapping thick and thin filaments is bridged by projections known as cross bridges. These are portions of myosin molecules that extend from the surface of the thick filaments toward the thin filaments (Figures 11-4 and 11-7). During muscle contraction, the cross bridges make contact with the thin filaments and exert force on them.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • Zuzanna
    How striated muscle is attached to tendons?
    7 years ago
  • nicole boyle
    Do skeletal muscle fibers continue from tendon to tendon?
    6 years ago
  • michal
    What are the levels of organization of skeletal muscle?
    5 years ago

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