Muscle Tissue

Muscle tissue is built from many long, cylindrical fibres arranged in parallel arrays; each fibre is itself made of numerous fine myofibrils. In response to stimulation these fibres contract (shorten) and then relax (lengthen) back to their resting state in a coordinated way, and it is this contraction–relaxation cycle that produces essentially every active movement the body makes.

Three distinct muscle types exist. Skeletal muscle is closely attached to the skeleton; its fibres are striated (striped) because of the regular internal arrangement of contractile proteins, and in a typical muscle such as the biceps these striated fibres are bundled together in parallel, with a tough connective-tissue sheath enclosing several such bundles. Skeletal muscle is voluntary — its contraction can be consciously controlled.

Smooth muscle fibres are spindle-shaped (fusiform, tapering at both ends) and show no striations. They are held together by cell junctions and bundled within a connective-tissue sheath, and they form the walls of internal organs such as blood vessels, the stomach and the intestine. Unlike skeletal muscle, smooth muscle is involuntary: its activity cannot be directly and consciously controlled. …