Summary

In this chapter we have considered the remarkable malleability of striated muscle. We have seen that muscles respond to use or disuse, and that they respond in particular ways to specific kinds of use. Muscle fibers decrease in mass by atrophy and increase by hypertrophy. Depending on conditions of use, they express different isoforms of functional molecules, such as the myosin heavy chain of the thick filament, the Ca²⁺-ATPase pump of the sarcoplasmic reticulum (SR), or mitochondrial enzymes. The functions of muscle fibers are also influenced by their associations with capillaries (which proliferate around muscle fibers engaged in endurance exercise), motor neurons (which control the fiber type of an entire motor unit and also maintain a muscle fiber’s viability), and hormones and growth factors (which affect the balance of protein synthesis and degradation in the muscle fiber). Although muscle fibers change in response to specific activities, an individual’s genetic make-up also influences muscle function, for example by directing the proportions of different fiber types within a skeletal muscle or the degree of hypertrophy that muscle fibers can achieve. Because research on muscle plasticity is motivated to a large extent by interest in human health and performance, humans and small mammals are often used as research subjects. However, other animal models (including amphibians, nonavian reptiles, and large mammals) provide useful perspectives for understanding the plastic potential of muscle.