Structure and function of muscle

The motor unit—the final common pathway for all voluntary muscle activity—is composed of an anterior horn cell, its peripheral axon, the axon terminal branches, the associated neuromuscular junctions, and the muscle fibres innervated.

Muscle cells—these are multinucleate units with unique structures adapted for response to metabolic, nervous, and autocrine signals. Their key elements being (1) sarcolemma—complex structured proteins maintain the integrity of the muscle fibre membrane, which contains specialized regions (motor endplates) by which innervating nerves interact at synapses; (2) contractile components—biochemical interactions between actin and myosin filaments are initiated by calcium ions released from the sarcoplasmic reticulum; contraction is powered by chemical energy released by the hydrolysis of ATP, in globular regions of myosin, after they form crosslinks with actin.

Different types of motor units—there are two biochemical variants (1) type 1—rich in mitochondria and specialized for oxidative metabolism of fat; (2) type 2—larger fibres with abundant glycogen that generate energy by glycosis and are critical for short-lived muscle contraction. All muscles contain populations of both fibre types, but differ in their proportions and functions.

Clinical perspective—knowledge of the underlying molecular cell biology, neurophysiology, and biochemical energetics of muscle provides a useful basis for understanding the symptoms, signs, and pathogenesis of clinical disorders affecting the muscles. Mutations in sarcolemmal proteins, such as dystrophin, cause diseases with widespread affects on skeletal muscle function, the heart, and survival.