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The sarcolemma, also called the myolemma, is the cell membrane of a muscle cell. The cells of all muscle types in the human body - skeletal, cardiac, and smooth muscle - all have sarcolemmas. The cell membrane, or plasma membrane, is an important structure of all cells. It is built of lipids, proteins, and carbohydrates, and serves to separate the interior of the cell from the outside environment, and to selectively allow substances into and out of the cell. The sarcolemma is a specialized type of cell membrane that addresses the unique needs of muscle cells.
The sarcolemma is distinguished from other cell membranes in that it has two layers. The layer closest to the inside of the cell is the same as other cell membranes, while the outer layer consists of polysaccharides with thin collagen fibers. In skeletal muscles, the outer layer of the sarcolemma fuses with tendon fibers at the end of each muscle fiber, providing attachment of the muscles to the skeleton by way of the tendon.
In addition to the functions of protection and selective permeability common to all cell membranes, the sarcolemma additionally serves to receive and conduct stimuli essential to muscle contraction. In skeletal and cardiac muscles, the sarcolemma forms tubules deep inside the muscle cell known as transverse tubules, or T-tubules, that allow chemical changes in polarity to quickly penetrate the interior of the cell. These changes in polarity, called the action potential, are what drive muscular contractions.
In some forms of the hereditary muscle disease, muscular dystrophy, progressive muscle weakness is caused by an improperly functioning sarcolemma. In certain forms of the disease, the patient lacks the protein dystrophin, which exists in very small amounts in the skeletal muscle tissue of healthy individuals. This deficiency causes the cell membrane to be loose from the cytoskeleton, the intracellular structure that maintains the shape of the cell. Consequently, too many calcium channels open in the sarcolemma during muscle contraction, causing excess calcium ions to flood the cell. The surplus of calcium ions activates the enzyme calpain, which digests proteins, causing the muscle to become weaker.
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