Sarcolemma

The sarcolemma, also called the plasmalemma, is the cell membrane that encloses a muscle fiber. It defines the boundary between the cytoplasm of the muscle cell and the surrounding extracellular space, maintains ion gradients, and participates in signal transduction. In both skeletal and cardiac muscle, the sarcolemma is highly specialized to support rapid electrical signaling and contraction.

A defining feature of the sarcolemma is its network of transverse (T) tubules, invaginations that extend into the cell interior. In skeletal muscle, T-tubules typically form triads with two terminal cisternae of the sarcoplasmic reticulum, enabling efficient coupling of membrane depolarization to calcium release. In cardiac muscle, the T-tubule system forms primarily dyads with a single SR cisterna. These arrangements ensure that an action potential quickly reaches the internal cytoplasm to trigger contraction.

The sarcolemma houses a variety of proteins, including voltage-gated ion channels, pumps, and receptors that regulate membrane potential and calcium handling. It contains components of the neuromuscular junction, such as acetylcholine receptors at the motor endplate, and the postsynaptic folds of the endplate amplify synaptic signaling. The membrane also connects to the extracellular matrix through costameres and the dystrophin-glycoprotein complex, aiding force transmission from the contractile apparatus to surrounding tissue.

Clinical relevance arises when the sarcolemma or its associated proteins malfunction. Defects in dystrophin or related complexes can compromise membrane integrity, contributing to muscular dystrophies and impaired force transmission. The sarcolemma remains a central element in excitation-contraction coupling and muscle physiology.