muscle

Contractile animal tissue that produces locomotion and power and maintains the movement of body substances. Muscle contains very specialized animal cells – long cells – that can contract to between one-half and one-third of their relaxed length.

Muscle tissue is sometimes found in large amounts, forming muscles, that are organs. Muscle tissue enables movement. It may move the whole body, or part of it, or some material along a tube within it. Muscles can only do this work by contracting. This explains why muscles are usually found in pairs (antagonistic pairs) where the work done in the contraction of one causes the stretching of the other.

Muscle is categorized into three main groups: striped (or striated) muscles are activated by motor nerves under voluntary control – their ends are usually attached via tendons to bones; involuntary or smooth muscles are controlled by motor nerves of the autonomic nervous system, and are located in the gut, blood vessels, iris, and various ducts; cardiac muscle occurs only in the heart, and is also controlled by the autonomic nervous system.