Sarcolemma
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The sarcolemma is the cell membrane of a muscle cell. It consists of a true cell membrane, called the plasma membrane, and an outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils. At each end of the muscle fiber, this surface layer of the sarcolemma fuses with a tendon fiber, and the tendon fibers in turn collect into bundles to form the muscle tendons that then insert into bones. The membrane is designed to receive and conduct stimuli.
Alterations in the sarcolemma membrane stability and repair system can lead to muscular dystrophy. The mechanism of one type of muscular dystrophy, for example, is lack of functional dystrophin. This means that the sarcolemma is not attached to the cytoskeleton. Therefore during muscle contraction, the sarcolemma is not synchronized with the interior of the cell. The looseness of the sarcolemma permits membrane calcium channels to open. The rise in internal calcium ions causes activation of the proteolytic enzyme calpain, which digests proteins including contractile proteins, rendering the muscle much weaker.
The sarcolemma invaginates into the cytoplasm of the muscle cell, forming membranous tubules called transverse tubules (t-tubules). Sarcoplasmic reticulum is the enlargement of smooth endoplasmic reticulum found in muscle cells on either side of the transverse tubules. The triad of transverse tubules surrounded by two smooth ER cisternae transmit altered membrane permeability down the tubules. The nuclei lie just adjacent to the sarcolemma, at the periphery of the fiber.
This membrane is extensible and encloses the contractile substance of a muscle fiber.
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