Three myocyte types, and a biceps with a quarter-million fibers
Vertebrates build muscle from three kinds of cell: skeletal, smooth and cardiac. Skeletal muscle cells are long threads formed when many precursor cells fuse, so each carries many nuclei, and a single biceps in a young man holds around 253,000 of these fibres.
All muscle cells, or myocytes, grow from embryonic precursors called myoblasts, steered by regulatory factors such as MyoD and myogenin. Skeletal fibres form when myoblasts merge into one long cell, each donating a nucleus, with the help of fusion proteins named myomaker and myomerger. Myoblasts that never fuse revert to satellite cells that wait beside the fibres and can be stirred into making new ones. Only skeletal muscle forms by fusion; heart cells keep a single central nucleus and lock onto their neighbours through intercalated discs.
Skeletal and heart muscle look striped because they are packed with myofibrils divided into sarcomeres. Thick filaments of myosin form the dark bands and thin actin filaments the light ones, with a giant elastic protein, titin, as a third type. Using ATP, the thick and thin filaments slide past each other and pull the Z discs together, shortening the whole fibre. Muscle anatomy has its own vocabulary: the cytoplasm is sarcoplasm, stocked with glycogen for energy and red myoglobin to hold oxygen; the membrane is the sarcolemma; and the calcium-filled sarcoplasmic reticulum wraps each myofibril.
Contraction begins at the neuromuscular junction, where a nerve releases acetylcholine and triggers an electrical signal that travels inward along transverse tubules. Each skeletal fibre is wired to just one motor neuron, and fibres come in slow-twitch and fast-twitch types.
Smooth muscle cells lack sarcomeres, so they are unstriped, but they still hold plenty of actin and myosin. These spindle-shaped cells, just 30 to 200 micrometres long, line the stomach, intestines, bladder, uterus and blood vessels, and they drive involuntary movements such as the waves of contraction that push food down the oesophagus. In the skin they raise hairs, and in the eye they adjust the lens.
Source: Muscle cell
