muscle

muscle Tissue that has the ability to contract, enabling movement. There are three basic types: voluntary muscle (or skeletal muscle), involuntary muscle (or smooth muscle), and cardiac muscle. Voluntary muscle is the largest tissue component of the human body, comprising c.40% by weight. It attaches by tendons to the bones of the skeleton, and is characterized by cross-markings known as striations; it typically contains many nuclei per cell. Most voluntary muscles require conscious effort for contraction. A muscle whose contraction causes a limb or a part of the body to straighten (extend) is called an extensor. A muscle whose contraction causes a limb or part of the body to bend is called a flexor. Involuntary muscle lines the digestive tract, blood vessels and many other organs. It is not striated and typically has only one nucleus per cell; it is not under conscious control. Cardiac muscle is found only in the heart. It differs from the other types of muscle in that it beats rhythmically and does not need stimulation by a nerve impulse to contract. Cardiac muscle has some striations (but not as many as in voluntary muscle) and has only one nucleus per cell.