The basic functional unit of the skeletal muscle.
The skeletal muscle fiber is a multinuclear cell enveloped by a basal lamina and special muscle cell membrane called the sarcolemma. The sarcolemma acts as a physical barrier against the external environment and facilitates signaling to the fiber. Inside the sarcolemma, each fiber is comprised of parallel aligned myofibrils and the surrounding sarcoplasm, which is muscle cell cytoplasm. Myofibrils are assembled of repeated structures called sarcomeres – the smallest functional unit of muscle fiber. Each sarcomere is formed of actin (called thin) and myosin (called thick) filaments arranged in a precise order and of protein complexes which support the filament structures. The striated appearance of the myofibril is achieved by the combination of actin filaments that form the light band (I band) and myosin filaments that form the dark band (A band). The border between two neighboring sarcomeres is called the Z-line (also known as the Z-disc or Z-band).