Sarkolemm

Sarkolemm, commonly called the sarcolemma, is the cell membrane that surrounds a muscle fiber (myofiber). It separates the cytoplasm of the muscle cell, the sarcoplasm, from the extracellular environment and provides the interface through which the muscle fiber interacts with nerves, blood vessels, and the surrounding connective tissue. The sarcolemma is a specialized phospholipid bilayer rich in ion channels, receptors, and pumps that support the rapid electrical excitability required for muscle contraction.

In skeletal and cardiac muscle, the sarcolemma forms extensive invaginations known as transverse tubules (T-tubules). Together with the terminal cisternae of the sarcoplasmic reticulum, these structures form the triad (in skeletal muscle) or diad (in cardiac muscle) that coordinates excitation-contraction coupling. The sarcolemma is connected to the cytoskeleton and extracellular matrix via the dystrophin-associated glycoprotein complex and costameres, helping to stabilize the cell during contraction.

Functions include maintaining ionic gradients, propagating action potentials, and participating in calcium handling that triggers contraction. It also plays a role in membrane repair and remodeling after injury through vesicle recruitment. Clinically, defects in the linkage between the sarcolemma and the cytoskeleton (for example in dystrophinopathies) compromise membrane integrity and contribute to muscle weakness.

In summary, the sarcolemma is the specialized muscle cell membrane essential for electrical signaling, mechanical stability, and calcium-mediated contraction of muscle tissue.