FIGURE 1 (Left) The attachment of a skeletal muscle. Contractions of the muscle result in movement of the arm, thus lifting the weight. (Center) Relaxation of cardiac muscle (top) allows the heart to fill with blood. Contraction of cardiac muscle (bottom) reduces the size of the ventricals, thus expelling blood into the pulmonary artery and the aorta. (Right) A small artery whose wall is comprised mostly of smooth muscle. Relaxation (top) and contraction (bottom) of the muscle increases and decreases, respectively, the diameter of the artery, thus altering the resistance to blood flow.

Many cells can change shape and/or move about. For example, intestinal and renal epithelial cells have microvilli that contract, perhaps to mix lumenal contents, and white blood cells can move between capillary endothelial cells and into tissues to combat invading organisms. However, for some cells, the ability to change shape is developed to a high degree. These cells are arranged such that their contractions result in the movement of attached structures (e.g., the skeleton) or in a change in shape of the structure they in part comprise (e.g., the heart, and arteries) (Fig. 1). These cells are muscle cells, and in humans such cells compose 50 to 70% of the lean body mass.

Muscle cells can be classified on the basis of structure, location, and function. All muscle cells can be divided into two groups—striated and smooth— based on their microscopic structure (see below). On the basis of location, striated muscle can be divided into three subgroups—skeletal, cardiac, and visceral— whereas smooth muscle can be divided into many subgroups (e.g., arterial, intestinal, uterine). Functional classifications of muscle cells are based on contractile behavior (e.g., fast twitch or slow twitch, tonic or phasic, unitary or multiunit) and biochemical activities (e.g., oxidative or glycolytic).

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