![]() These muscles can maintain continuous contraction and are useful in activities such as the maintenance of posture. They contain large amounts of mitochondria and myoglobin, an oxygen-storage molecule. Slow-twitch (type I) muscle fibers contract more slowly and rely on aerobic metabolism. Skeletal muscles are divided into two muscle fiber types. In humans, the fiber types cannot be distinguished based on gross examination, but require specific stains or treatments to differentiate the fibers. Most muscles contain a mixture of these extreme fiber types. Electron micrographs reveal the structural components of the sarcomere. Recall that action potentials in the T-tubule trigger the opening of calcium channels in the sarcoplasmic reticulum which increases the concentration of cytosolic calcium and triggers muscle contraction. The sarcoplasmic reticulum is the endoplasmic reticulum of skeletal muscle cells. The sarcoplasmic reticulum runs adjacent to the T-tubule. During contraction, the myosin motors pull on the actin filaments which bring the Z-disc closer to shorten the sarcomere and the overall muscle cell.Īlso note the T-tubule which is an invagination of the cell plasma and allows action potentials that start on the surface of the muscle cell to penetrate into the center of the cell. I-bands are lighter because they only contain actin filaments. A-bands are darker in electron micrographs and H&E-stained samples because they contain both actin filaments and myosin filaments. ![]() Myosin filaments sit in the center of the sarcomere and span the length of the A-band. ![]() The H-band does not contain actin filaments in a relaxed sarcomere. Z-disc bind the plus ends of actin filaments which extend toward the center of the sarcomere and end at the H-band. Sarcomeres are defined by Z-discs and composed of actin filaments and myosin filaments. Sarcomeres are the contractile units in both skeletal and cardiac muscle cells. Each muscle cell is enveloped by endomysium and groups are wrapped by perimysium. The perimysium is thicker and contains larger blood vessels (arterioles) that distribute blood to the capillaries in the endomysium. Note the presence of capillaries in the endomysium. H&E stain reveals striated pattern and connective tissue layers.Īt high magnifications, the endomysium is more apparent around each muscle cell. The cells are divided longitudinally into numerous sarcomeres that give skeletal muscle cells their striated appearance. The cytoplasm of skeletal muscle cells is organized into long cables called myofibrils that span the length of the cell. In longitudinal section, the striations identify skeletal muscle cells. In cross section, skeletal muscle cells appear as large cells with their nuclei located to one side. Skeletal muscle cells are easily identifiable in histological samples. Skeletal muscle consists of bundles of cells wrapped in connectives tissue layers. Blood vessels and nerves travel through the epimysium, perimysium and endomysium to reach the individual muscle fibers. The whole muscle is surrounded by a connective tissue layer called the epimysium. Finally, several fasicles are bundled to form the whole muscle. ![]() Muscle cells are grouped together into fasicles and surrounded by a connective tissue called perimysium that is rich in collagen fibers. Endomysium is similar in composition to basement membrane that underlies epithelia. Each muscle cell is surrounded by connective tissue called endomysium. Skeletal muscle is made up of elongated cylindrical multinucleate cells, also called muscle fibers, that span the length of the muscle. Muscles are multicellular contractile units. Structure and Function of Muscle and Nervous Tissue Skeletal Muscle
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