Muscular Tissue

Muscular tissue is the tissue specialised for producing movement through contraction. Its cells, called muscle fibres because of their elongated shape, contain two contractile proteins, actin and myosin, arranged so that they can slide past one another and shorten the fibre when stimulated. Based on the fine structure, nucleation and control of these fibres, three distinct types of muscle tissue are recognised in the vertebrate body, and each is matched to a particular functional role.

Striated muscle, also called skeletal muscle because it is chiefly attached to and moves the bony skeleton, is made of long, cylindrical, unbranched fibres. Each fibre contains many nuclei (it is multinucleate), positioned at the periphery just beneath the cell membrane, and under the microscope the fibre shows alternating light and dark cross-bands — the striations that give this muscle type its name, produced by the regular, repeating arrangement of actin and myosin filaments within the fibre. Because contraction of skeletal muscle is normally initiated by a deliberate nerve impulse from the brain, it is described as voluntary muscle, and it is this muscle type that powers locomotion, posture and all consciously controlled movement.

Unstriated muscle, also called smooth muscle, has a very different fibre shape: each fibre is spindle-shaped, tapering to a point at both ends, and contains only a single nucleus positioned centrally. As its name indicates, smooth muscle shows no visible cross-striations, because its actin and myosin filaments are arranged less regularly than in skeletal muscle. Smooth muscle fibres line the walls of hollow internal organs — the alimentary canal, blood vessels, the urinary bladder and the iris of the eye among them — where their contraction is involuntary, driven by the autonomic nervous system and hormones rather than by conscious will, producing the slow, sustained movements such as peristalsis that move food along the gut or that regulate the diameter of a blood vessel. …