Ultrastructure of the Skeletal Muscle Fibre

A whole skeletal muscle is a hierarchy of wrappings within wrappings. The entire muscle is enclosed by a connective-tissue sheath, the epimysium. Inside it, the muscle is divided into bundles called fascicles, each fascicle wrapped in its own sheath, the perimysium. Each fascicle is in turn made up of many individual muscle fibres (which are single, elongated, multinucleate cells), and each fibre is wrapped by a thin sheath called the endomysium.

The plasma membrane of a muscle fibre is called the sarcolemma, and the cytoplasm inside it is the sarcoplasm. The sarcoplasm is packed with mitochondria (sometimes called sarcosomes, supplying the large amount of ATP contraction needs), many nuclei lying just under the sarcolemma (since the fibre is a fused, syncytial cell), and an extensive internal membrane system, the sarcoplasmic reticulum, which stores and releases the calcium ions needed to trigger contraction. Running the length of t …