muscle, the contractile tissue that effects the movement of and within the body. Muscle tissue in the higher animals is classified as striated, smooth, or cardiac, according to its structure and function. Striated, or skeletal, muscle forms the bulk of the body's muscle tissue and gives the body its general shape. It is called striated because it appears striped, in alternating bands of light and dark, when viewed under a microscope, and animals have conscious control over most of their striate muscles. Smooth muscle, which lines most of the hollow organs of the body, is not under voluntary control, but is regulated by the autonomic nervous system. Smooth muscle fibers are spindle-shaped, not striated, and generally are arranged in dense sheets. Smooth muscle lines the blood vessels, hair follicles, urinary tract, digestive tract, and genital tract. Its speed of contraction is slower than that of striated muscle, but it can remain contracted longer. Cardiac muscle is striated like skeletal muscle but, like smooth muscle, is controlled involuntarily. It is found only in the heart, where it forms that organ's thick walls. The contractions of cardiac muscle are stimulated by a special clump of muscle tissue located on the heart (the pacemaker), although the rate of contractions is subject to regulation by the autonomic nervous system.
Skeletal muscles are attached (with some exceptions, such as the muscles of the tongue and pharynx) to the skeleton by means of tendons, usually in pairs that pull in opposite directions, e.g., the biceps (flexor) and triceps (extensor) that move the forearm at the elbow. The means by which all types of muscles contract is thought to be generally the same, although muscles are classified as phasic, or fast twitch, and tonic, or slow twitch, to differentiate between the various lengths of time a muscle may require to move in response to stimulation. Striated muscle is usually considered phasic, while cardiac and smooth muscle are thought to be tonic.
Perhaps because its action is most varied, striated muscle has been studied most extensively. This type of muscle is composed of numerous cylindrically shaped bundles of cells, each enclosed in a sheath called the sarcolemma. Each muscle fiber contains several hundred to several thousand tightly packed strands called myofibrils that consist of alternating filaments of the protein substances actin and myosin. Actin and myosin interact before muscle contraction, forming the contractile material actomyosin.
The energy required for muscle contraction comes from the breakdown of adenosine triphosphate (ATP), a substance that is present in the cells and is formed during cellular respiration. A muscle fiber is stimulated to contract by electrical impulses from the nervous system. The point of contact between nerve and muscle is the neuromuscular junction, where the chemical substance acetylcholine is secreted, initiating the changes that cause the muscle to contract. During resting states, some of the fibers in the musculature are maintained in a state of partial contraction, known as muscle tone. This permits muscles to contract quickly when stimulated without having to overcome the inertia of total relaxation.