Structure and Function of Skeletal Muscles
- Skeletal muscle is made up of millions of individual fibers.
- Whole muscles vary in size (2 cm to 60 cm) and shape (fusiform, pennate). They are encased in a three-part connective tissue framework. The fundamental concept of muscle function is the motor unit, defined as those muscle fibers innervated by a single motor nerve, its axon, and anterior horn cell.
- Satellite cells are dormant myoblasts; however, they can regenerate muscle when activated.
- Muscle fibers contain bundles of myofibrils arranged in parallel along the longitudinal axis and include the muscle membrane, myofibrils, sarcotubular system, aqueous sarcoplasm, and mitochondria. There two types of muscle fibers, type I and type II, determined by motor nerve innervation.
- Myofibrils and myofilaments contain the major muscle proteins, actin and myosin, which interact to form cross-bridges during muscle contraction. The nonprotein muscle constituents provide an energy source for contraction and regulate protein synthesis, enzyme systems, and membrane stabilization.
- Muscle contraction includes excitation, coupling, contraction, and relaxation.
- Muscle strength is graded by the all-or-nothing phenomenon and recruitment. Speed of contraction is affected by several factors: muscle fiber type, temperature, stretch, and weight of the load.
- There are two types of muscle contraction: isometric and isotonic. Muscle shortening occurs during contraction but can be seen also during pathologic and physiologic contracture.
- Skeletal muscle requires a constant supply of adenosine triphosphate (ATP) and phosphocreatine to fuel muscle contraction and for growth and repair. ATP and phosphocreatine can be generated aerobically or anaerobically.