Muscle tissue can be broken down into three types: skeletal muscle, smooth muscle, and cardiac muscle. The muscle tissue types are differentiated by their functions and the shapes of their individual cells, with each associated with a different type of motion. Skeletal muscle is associated with voluntary motion linked to the movements of the skeletal system. Smooth muscle is associated with involuntary motion governed by bodily functions or reflexes, while cardiac muscle is associated with pulmonary motion.
Examples of skeletal muscles include the biceps, quadriceps, and triceps — the type of muscle most commonly associated with the idea of physical musculature. These muscles are all connected in some way to the body's bone structure. They are comprised of long and short cylindrical fibers, which are in turn made of striated cells with nuclei at the edges of the cells. These muscles are under conscious control, or voluntary motion.
Skeletal tissue can further be broken down into white or red muscle tissue types. Each has different purposes and different chemical compositions, such as the myoglobin that gives red tissue its color. Other skeletal muscle tissue types include flexors, extensors, abductors, and adductors, which are categorized by the type of motion they control.
Smooth muscles, also called visceral muscles, make up various bodily organs. Most people don't think of the stomach, intestines, or veins as muscles, but the cells that make up these body parts are actually part of the involuntary motion system. Their actions — such as digestion, blood oxygenation, and waste removal — are not consciously controlled, but instead operate by their own unconscious physical cycles. The cells are different from skeletal muscles, with centered nuclei and no striations. Their shape resembles a spindle.
The last type of muscle tissue, cardiac muscle, can be found only in the walls of the heart. The cross-striated, single-nucleus cells form quadrangular muscles with interlaced branching fibers. Although their motion is involuntary, their specialized form and function separate cardiac muscles from other muscles with involuntary motion. It's because of cardiac muscle that the heart maintains its regular beat without conscious control.
The uniting factor in all three muscle tissue types is the ability to contract. All muscle motion, whether pulmonary contractions, arm flexion, or stomach churning, is performed by muscles clenching and relaxing. Muscles cells don't exert outward motion; instead they create motion by pulling and causing other body tissues or connective tissues to either draw inwards or fall outwards when the muscle cells relax.