Structure of skeletal muscle

Skeletal muscles are composed of bundles of muscle fibers, which are long, cylindrical cells. Each muscle fiber contains myofibrils, which are the contractile units of the muscle. Myofibrils are made up of repeating units called sarcomeres, which contain the proteins actin and myosin. These proteins slide past each other to cause muscle contraction.

The muscle fibers are surrounded by connective tissue sheaths: the epimysium surrounds the entire muscle, the perimysium surrounds bundles of muscle fibers called fascicles, and the endomysium surrounds individual muscle fibers.