20.5: Skeletal Muscle Anatomy
Skeletal muscle is the most abundant type of muscle in the body. Tendons are the connective tissue that attaches skeletal muscle to bones. Skeletal muscles pull on tendons, which in turn pull on bones to carry out voluntary movements.
Skeletal muscles are surrounded by a layer of connective tissue called epimysium, which helps protect the muscle. Beneath the epimysium, an additional layer of connective tissue, called perimysium, surrounds and groups together subunits of skeletal muscle called fasciculi.
Each fascicle is a bundle of skeletal muscle cells, or myocytes, which are often called skeletal muscle fibers due to their size and cylindrical appearance. Between the muscle fibers is an additional layer of connective tissue called endomysium.
The muscle fiber membrane is called the sarcolemma. Each muscle fiber is made up of multiple rod-like chains called myofibrils, which extend across the length of the muscle fiber and contract. Myofibrils contain subunits called sarcomeres, which are made up of actin and myosin in thin and thick filaments, respectively.
Actin contains myosin-binding sites that allow thin and thick filaments to connect, forming cross bridges. For a muscle to contract, accessory proteins that cover myosin-binding sites on thin filaments must be displaced to enable the formation of cross bridges. During muscle contraction, cross bridges are repeatedly broken and formed at binding sites further along the actin.