Sarcolemma
Overview
The sarcolemma is the fine transparent tubular sheath that envelops the fibers of skeletal muscles. It is a specialized cell membrane which surrounds striated muscle fiber cells. Sometimes called the myolemma, the sarcolemma is similar to a typical plasma membrane but has specialized functions for the muscle cell. The sarcolemma also contains an extracellular matrix consisting of various polysaccharides which allows for strong adhesion sites for muscle fibers.
Structure
The sarcolemma is composed of phospholipids, sphingolipids, and a high concentration of cholesterol. This composition allows the sarcolemma to maintain its fluidity and integrity, even during the intense mechanical stress that muscle fibers undergo during muscle contraction and relaxation. It also contains various proteins that are involved in structural support, signal transduction, transport, and cell adhesion.
Function
The primary function of the sarcolemma is to serve as a barrier between the intracellular and extracellular environments of the muscle fiber. It maintains the electrochemical gradient across the muscle cell, which is essential for muscle contraction. The sarcolemma also plays a crucial role in the excitation-contraction coupling, a process in which electrical signals are converted into mechanical action, leading to muscle contraction.
Role in Excitation-Contraction Coupling
The sarcolemma is involved in the initiation and propagation of action potentials along the muscle fiber. These action potentials are generated by the influx of sodium ions through voltage-gated sodium channels located in the sarcolemma. The action potential then travels along the sarcolemma and down the T-tubules, triggering the release of calcium ions from the sarcoplasmic reticulum, which ultimately leads to muscle contraction.
Role in Muscle Contraction and Relaxation
During muscle contraction, the sarcolemma undergoes significant changes. It depolarizes and repolarizes as action potentials pass along its surface. This depolarization and repolarization process is essential for the initiation and propagation of the action potential, which triggers muscle contraction. During muscle relaxation, the sarcolemma returns to its resting state, and the muscle fiber relaxes.
Role in Muscular Dystrophy
Mutations in the genes that encode for proteins located in the sarcolemma can lead to various forms of muscular dystrophy. For example, Duchenne muscular dystrophy and Becker muscular dystrophy are caused by mutations in the dystrophin gene, which encodes for a protein that helps stabilize the sarcolemma during muscle contraction and relaxation.