The sarcolemma (''sarco'' (from ''sarx'') from Greek; flesh, and ''lemma'' from Greek; sheath), also called the myolemma, is the
cell membrane
The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...
surrounding a
skeletal muscle fibre or a
cardiomyocyte.
It consists of a
lipid bilayer and a thin outer coat of polysaccharide material (
glycocalyx) that contacts the
basement membrane. The basement membrane contains numerous thin
collagen
Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...
fibrils and specialized proteins such as
laminin that provide a scaffold to which the muscle fibre can adhere. Through transmembrane proteins in the plasma membrane, the actin skeleton inside the cell is connected to the basement membrane and the cell's exterior. At each end of the muscle fibre, the surface layer of the sarcolemma fuses with a tendon fibre, and the tendon fibres, in turn, collect into bundles to form the muscle tendons that adhere to bones.
The sarcolemma generally maintains the same function in muscle cells as the plasma membrane does in other
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
cells.
It acts as a barrier between the extracellular and intracellular compartments, defining the individual muscle fibre from its surroundings. The lipid nature of the membrane allows it to separate the fluids of the intra- and extracellular compartments, since it is only selectively permeable to water through
aquaporin
Aquaporins, also called water channels, are channel proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells, mainly facilitating transport of water between cells. The cell membranes of ...
channels. As in other cells, this allows for the compositions of the compartments to be controlled by selective transport through the membrane. Membrane proteins, such as
ion pumps, may create ion gradients with the consumption of
ATP, that may later be used to drive transport of other substances through the membrane (
co-transport
In cellular biology, active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellula ...
) or generate electrical impulses such as
action potentials.
A special feature of the sarcolemma is that it
invaginates into the
sarcoplasm of the muscle cell, forming membranous tubules radially and longitudinally within the fiber called
T-tubules or transverse tubules. On either side of the transverse tubules are
terminal cisternal enlargements of the
sarcoplasmic reticulum (termed
endoplasmic reticulum
The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryote, eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for ...
in nonmuscle cells). A transverse tubule surrounded by two SR cisternae are known as a triad, and the contact between these structures is located at the junction of the A and I bands.
References
{{Authority control
Membrane biology