Figure 1134

Velocity of the biceps muscle is amplified by the lever system of the arm, producing a greater velocity of the hand. The range of movement is also amplified (1 cm of shortening by the muscle produces 7 cm of movement by the hand).

Hypocalcemic Tetany Similar in symptoms to muscular cramping is hypocalcemic tetany, the involuntary tetanic contraction of skeletal muscles that occurs when the extracellular calcium concentration falls to about 40 percent of its normal value. This may seem surprising since we have seen that calcium is required for excitation-contraction coupling. However, recall that this calcium is sarcoplasmic-reticulum calcium, not extracellular calcium. The effect of changes in extracellular calcium is exerted not on the sarcoplasmic-reticulum calcium, but directly on the plasma membrane. Low extracellular calcium (hypocalcemia) increases the opening of sodium channels in excitable membranes, leading to membrane depolarization and the spontaneous firing of action potentials. It is this that causes the increased muscle contractions. The mechanisms controlling the extracellular concentration of calcium ions are discussed in Chapter 16.

Muscular Dystrophy This disease is one of the most frequently encountered genetic diseases, affecting one in every 4000 boys (but much less commonly in girls) born in America. Muscular dystrophy is associated with the progressive degeneration of skeletal- and cardiac-muscle fibers, weakening the muscles and leading ultimately to death from respiratory or cardiac failure. While exercise strengthens normal skeletal muscle, it weakens dystrophic muscle. The symptoms become evident at about 2 to 6 years of age, and most affected individuals do not survive much beyond the age of 20.

PART TWO Biological Control Systems

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

PART TWO Biological Control Systems

The recessive gene responsible for a major form of muscular dystrophy has been identified on the X chromosome, and muscular dystrophy is a sex-linked recessive disease. (As described in Chapter 19, girls have two X chromosomes and boys only one. Accordingly, a girl with one abnormal X chromosome and one normal one will not develop the disease. This is why the disease is so much more common in boys.) This gene codes for a protein known as dystrophin, which is either absent or present in a nonfunctional form in patients with the disease. Dystrophin is located on the inner surface of the plasma membrane in normal muscle. It resembles other known cytoskeletal proteins and may be involved in maintaining the structural integrity of the plasma membrane or of elements within the membrane, such as ion channels, in fibers subjected to repeated structural deformation during contraction. Preliminary attempts are being made to treat the disease by inserting the normal gene into dystrophic muscle cells.

Myasthenia Gravis Myasthenia gravis is characterized by muscle fatigue and weakness that progressively worsens as the muscle is used. It affects about 12,000 Americans. The symptoms result from a decrease in the number of ACh receptors on the motor end plate. The release of ACh from the nerve terminals is normal, but the magnitude of the end-plate potential is markedly reduced because of the decreased number of receptors. Even in normal muscle, the amount of ACh released with each action potential decreases with repetitive activity, and thus the magnitude of the resulting EPP falls. In normal muscle, however, the EPP remains well above the threshold necessary to initiate a muscle action potential. In contrast, after a few motor nerve impulses in a myasthenia gravis patient, the magnitude of the EPP falls below the threshold for initiating a muscle action potential. As described in Chapter 20, the destruction of the ACh receptors is brought about by the body's own defense mechanisms gone awry, specifically because of the formation of antibodies to the ACh-receptor proteins.

III. Actin-containing thin filaments are anchored to the Z lines at each end of a sarcomere, while their free ends partially overlap the myosin-containing thick filaments in the A band at the center of the sarcomere.

SECTION A SUMMARY

I. There are three types of muscle—skeletal, smooth, and cardiac. Skeletal muscle is attached to bones and moves and supports the skeleton. Smooth muscle surrounds hollow cavities and tubes. Cardiac muscle is the muscle of the heart.

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