Anatomy & Physiology

Anatomy & Physiology Study Guide

©2018 Achieve Page 106 , which is the cytoplasm of the muscle fiber. The sarcolemma, like all plasma membranes, has a distinctive transmembrane potential due to the distribution of positive and negative charges across the membrane. A sudden change in the transmembrane potential in a muscle fiber is the first step towards contraction. To illicit a proper response, all regions of the cell must contract simultaneously. The skeletal muscle fiber is very large, thus the signal to contract must be distributed quickly throughout the interior of the cell. Transverse tubules conduct this signal. Transverse tubules , or T tubules, are narrow tubes that are extended into the sarcoplasm. T tubules are filled with extracellular fluid and form passageways through the muscle fiber. Electrical impulses conducted by the sarcolemma travel along the T tubules into the cell interior. These impulses, or action potentials, are the triggers for muscle fiber contraction. Myofibrils Each skeletal muscle fiber contains hundreds to thousands of myofibrils. Myofibrils are structures encircled by T tubules, and these structures consist of bundles of protein filaments called myofilaments . Thin filaments are composed of four proteins: F-actin, nebulin, tropomyosin, and troponin. Thick filaments are composed of roughly 300 myosin molecules. When myofibrils shorten, skeletal muscle fibers contract. Myofibrils are anchored to the inner surface of the sarcolemma at each end of the muscle fiber. In turn, the outer surface of the sarcolemma is attached to collagen fibers of 368 Skeletal Muscle Composition Each muscle has three layers of connective tissue: an epimysium, a perimysium, and an endomysium. The epimysium surrounds the entire muscle with a dense layer of collagen fibers that separates the muscle from surrounding tissues and organs. The connective tissue fibers of the perimysium divide the skeletal muscle into bundles of muscle fibers called fascicles . Within a fascicle, the delicate connective tissue of the endomysium surrounds the individual skeletal muscle fibers, and loosely interconnects adjacent muscle fibers. This flexible, elastic connective tissue layer contains capillary networks that supply blood to the muscle fibers, myosatellite cells , which are embryonic stem cells that function in the repair of damaged muscle tissue, and nerve fibers that control the muscle. The perimysium and endomysium are interwoven into one another. At each end of the muscle, the collagen fibers of the epimysium, perimysium, and endomysium come together to form either a bundle known as a tendon or a broad sheet called an aponeurosis. Tendons and aponeuroses usually attach skeletal muscles to bones and extend fibers into the bone matrix, providing a firm attachment. As a result, any contraction of the muscle will exert a pull on the attached bone (or bones). Skeletal muscle fibers differ significantly from any other cell. First, skeletal muscle fibers are much larger than other cells. Second, skeletal muscle fibers are multinucleate; each contains hundreds of nuclei. The genes in these nuclei control the production of enzymes and structural proteins required for normal muscle contraction, and the more copies of these genes, the faster these proteins can be produced. The Sarcolemma and Transverse Tubules The sarcolemma , or plasma membrane of a muscle fiber, surrounds the sarcoplasm