Chapter 5 - Skeletal Muscle: Structure, Chemistry, and Function

Abstract

This chapter describes the structure, chemistry, and function of skeletal muscle. Individual skeletal muscles are well formed by the 10th week of gestation, and in the next 6 weeks, myotubes proliferate by fusion with specific neighboring cells until the ultimate number of muscle fibers has been formed in a given muscle. Division by mitosis does not occur in striated muscle cells. As myotubes mature, their myofibrils increase in number and size until they are densely packed, and the nuclei migrate to their familiar location beneath the sarcolemma at which time they are termed myofibers. Maturation of the sarcotubular system and initiation of motor innervation occur concurrently. A skeletal muscle is composed of a number of discrete bundles of muscle fibers termed fascicles, each sheathed in connective tissue. Each muscle fiber, in turn, is composed of a number of myofibrils. The myofibrils are composed of myofilaments arranged in units called sarcomeres. There are two types of myofilaments: (1) thin filaments containing actin, troponin, and tropomyosin, and (2) thick filaments containing myosin. The arrangement of myofibrils into sarcomeres is responsible for the striated appearance of skeletal muscle under the light microscope.