A body tissue composed of sheets or bundles of cells that contract to produce movement or increase tension. Muscle cells contain filaments made of the proteins actin and myosin, which lie parallel to each other. When a muscle is signaled to contract, the actin and myosin filaments slide past each other in an overlapping pattern.

♦ Skeletal muscle effects voluntary movement and is made up of bundles of elongated cells (muscle fibers), each of which contains many nuclei.

♦ Smooth muscle provides the contractile force for the internal organs and is controlled by the autonomic nervous system. Smooth muscle cells are spindle-shaped and each contains a single nucleus.