CHANGES IN MUSCLE SIZE

1. What is the difference between hyperplasia and hypertrophy?

2. How do skeletal muscle fibers hypertrophy (grow in size) after birth?

3. Is regeneration of skeletal, cardiac, and smooth-muscle fibers possible?

4.

Muscle is the most adaptable tissue in the body of an animal. Individual muscle cells of skeletal, cardiac, and smooth muscle increase in size as a normal response to chronic mechanical stress, as with regular exercise. Similar stress in skeletal and smooth muscle causes division of muscle cells through mitosis to produce new cells. A decrease in size can occur in skeletal, cardiac, and smooth muscle in response to disease.

Hypertrophy and Hyperplasia

An increase in individual muscle-fiber size is referred to as hypertrophy. It is common in skeletal, cardiac, and smooth-muscle cells. Postnatal growth of skeletal-muscle fibers is not accomplished by an increase in.the number of muscle fibers but rather by the addition of myofibrils to the periphery and of sarcomeres to the tendinous ends. An increase in the number of muscle fibers is called hyperplasia. Regeneration of skeletal-muscle fibers is possible from so-called satellite cells, but this requires an intact endomysium for successful repair. An increase in cardiac-muscle size is similar to that of skeletal muscle in that it involves hypertrophy but not hyperplasia. Regeneration of cardiac­muscle fibers does not occur because there is no counterpart to the satellite cells of skeletal muscle. If myocardial cells die, they are replaced by fibrous, noncontractile scar tissue. Smooth-muscle organs can increase their size by hypertrophy and by hyperplasia, so smooth muscle has considerable regenerative ability.

Atrophy

A decrease in the size of a muscle is referred to as atrophy. When a body part has been immobilized for a period of time, the muscles become smaller (referred to as disuse atrophy). Loss of the nerve supply to a muscle results in denervation atrophy. This was formerly a common condition in harnessed draft horses. The presence of the collar presses on the suprascapular nerve that supplies the two major muscle masses of the shoulder blade (see Chapter 7). The resulting denervation causes the muscles,of the shoulder to atrophy, resulting in a condition known as sweeny (also called shoulder slip).

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