Structure of a skeletal muscle fibre

Structure of a skeletal muscle fibre

A skeletal muscle fibre is a single, highly specialised cell — thin, elongated, and usually tapering at one or both ends. Several features distinguish it from a typical cell:

• Multiple nuclei lie just beneath the plasma membrane, an adaptation that lets a very large cell support protein synthesis along its whole length.
• The plasma membrane is called the sarcolemma, and the cytoplasm is called the sarcoplasm.
• The sarcoplasm is packed with glycosomes (granules of stored glycogen that release glucose during activity), myoglobin (an iron-containing, oxygen-storing red pigment similar to haemoglobin, which buffers the fibre's oxygen supply), and an extensive sarcoplasmic reticulum (the muscle-specific version of the endoplasmic reticulum, which stores and releases the calcium ions that trigger contraction).

Running the length of each myofibril is a repeating pattern of dark and light bands that gives skeletal muscle its striated appearance under the microscope: the dark A-bands (anisotropic) and light I-bands (isotropic) alternate regularly. Each A-band has a paler central region, the H-zone, itself bisected by a dark M-line. Each I-band is bisected by a dark Z-disc. …