Skeletal Muscle Function and Metabolism

Skeletal muscle consists of bundles of long spindle shaped cells called muscle fibers that attach to bone by tendinous insertions. The blood vessels and nerves that nourish and control muscle function run in thin sheets of connective tissue that surround bundles of muscle fibers. Each nerve branch communicates with one muscle fiber at the motor end. The nerve and all the muscle fibers that it supplies are together termed a motor unit. Each time that a nerve is stimulated all of the muscle fibers under its control will contract. A muscle’s unique ability to contract is conferred by the highly organized parallel, overlapping arrangement of actin and myosin filaments. These repeating contractile units or sarcomeres extend from one end of the cell to another in the form of a myofibril. Each muscle fiber is packed with myofibrils that are arranged in register giving skeletal muscle a striated appearance under the microscope. Muscle contraction occurs when the overlapping actin and myosin filaments slide over each other, serving to shorten the length of the muscle cell from end to end and mechanically pulling the limb in the desired direction. The sliding of the filaments requires chemical energy in the form of ATP.