The sarcolemma is the cell membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...

 of a muscle cell (skeletal, cardiac, and smooth muscle). 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 configured to receive and conduct stimuli.

Alterations in the sarcolemma membrane stability and repair system can lead to muscular dystrophy
Muscular dystrophy
Muscular dystrophy is a group of muscle diseases that weaken the musculoskeletal system and hamper locomotion. Muscular dystrophies are characterized by progressive skeletal muscle weakness, defects in muscle proteins, and the death of muscle cells and tissue.In the 1860s, descriptions of boys who...

. The mechanism of one type of muscular dystrophy, for example, is lack of functional dystrophin
Dystrophin is a rod-shaped cytoplasmic protein, and a vital part of a protein complex that connects the cytoskeleton of a muscle fiber to the surrounding extracellular matrix through the cell membrane. This complex is variously known as the costamere or the dystrophin-associated protein complex...

. This means that the sarcolemma is not attached to the cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...

. Therefore during muscle contraction, the sarcolemma is not synchronized with the interior of the cell. The looseness of the sarcolemma permits membrane calcium channel
Calcium channel
A Calcium channel is an ion channel which displays selective permeability to calcium ions. It is sometimes synonymous as voltage-dependent calcium channel, although there are also ligand-gated calcium channels.-Comparison tables:...

s to open. The rise in internal calcium ions causes activation of the proteolytic enzyme calpain
A calpain is a protein belonging to the family of calcium-dependent, non-lysosomal cysteine proteases expressed ubiquitously in mammals and many other organisms. Calpains constitute the C2 family of protease clan CA in the MEROPS database...

, 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). Terminal cisternae
Terminal cisternae
Terminal cisternae are enlarged areas of the sarcoplasmic reticulum surrounding the transverse tubules. These discrete regions within the muscle cell store calcium and release it when an action potential courses down the transverse tubules, eliciting muscle contraction...

 is the enlargement of smooth endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...

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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