This muscular plasma membrane plays a central role in the structure and function of skeletal muscle.
In addition to cleaning a cellular plasma membrane, the sarcolemma is directly involved in the synaptic transmission, the propagation of the action potential, and the excitation-contraction coupling.
In addition to these well-established physiological functions, the sarcolemma, the subsarcolemmal cytoskeleton, and the surrounding basal membrane (extracellular matrix) play an essential structural role in skeletal muscle.
The biological importance of the sarcolemma-cytoskeleton of the basement membrane in skeletal muscle is underlined by the number of inherited muscle diseases caused by mutations in components of the basement membrane.
Also, the cytoskeleton or protein complexes of the sarcolemma join the basement membrane with the cytoskeleton.
This specialized cellular membrane surrounds the cells of striated muscle fibers. Sometimes called myolemma, the sarcolemma is similar to a typical plasma membrane but has technical functions for the muscle cell.
The sarcolemma also contains an extracellular matrix consisting of several polysaccharides that allow the cell to anchor in the tissues that build and support the muscle fibers.
Typically, the sarcolemma connects the basement membrane that surrounds all connective tissues or other muscle cells, creating a solid fiber that can contract.
Each sarcolemma has a biochemical composition similar to a plasmalemma, another word for a cell membrane.
The structure of muscle cells is explicitly such that the tunnel-like extensions of the sarcolemma pass through the muscle cells from one side to the other, so they are said to be transverse. These extensions of the sarcolemma are called transverse tubules, often abbreviated simply as “tubules T.”
The functions of the skeletal muscle sarcolemma are still uncertain due to our lack of knowledge of its structure and detailed distribution over the entire surface of the muscle cell.
Most of our knowledge is derived from research with electronic microscopes. The sarcolemma is considered a semipermeable unitary membrane of approximately 100 A thick, directly enclosing the cell’s contents.
This membrane is probably bimolecular and lipoprotein in character and similar to what most animal cells thought to be reversed and many of their cytoplasmic organelles. Immediately external to the sarcolemma is a layer of approximately 500 A thick.
This is an extracellular deposit of material of moderate density (the so-called basement membrane1) that gradually decreases in thickness as it extends from the sarcolemma to the extracellular space and merges with the fundamental substance of the endomysial connective tissue.
Due to its extreme thinness, the sarcolemma is challenging to observe in its longitudinal section; on the other hand, its characteristic unitary membrane structure can generally be recognized in the cross-section, and it is known that there are occasional thickenings and invasions of the sarcolemma in some areas of the cell.
However, it is challenging to follow detailed variations in their course over large cell surface areas using thin-section techniques.
For structural investigations of this type, surface replication methods, such as the layer removal method developed by Reed and Rudall, are probably much more helpful since the various levels exposed in the tissue can be examined over large areas.
The first investigations that used the elimination of layers revealed the sarcolemma as a continuous elastic sheet, able to accommodate the structures it encloses since it often presents undulations corresponding to the striations of the myofibrils.
The sarcolemma plays an essential role in the process of muscle contraction, as well as types of muscle contractions.
When the muscle action potential (which is a form of electrical “training”) travels along the sarcolemma of the muscle fibers, then in the transverse tubules, the calcium ions (Ca2 +) are released into the sarcoplasm.
The sarcolemma has several unique characteristics that provide the muscle cells with the structure and the resources to function. The sarcolemma is very large compared to some cell membranes and must be constantly maintained to cover the many myofibrils that make up a muscle cell.
The sarcolemma of a cell adheres through extracellular connections to the cell next to it, which eventually leads to tendons that connect the muscles to the bones. By contracting against these levers, the muscle cells generate movement in the body.
Due to these contractions’ high energy demand, the sarcolemma is formed especially with channels that transport materials in and out of the cell.
These many channels also help restore the potential of the disturbed membrane when a motor neuron gives the muscle a signal to contract. In response to the signal sent by the neuron, the sarcolemma will generate an action potential over its entire length, signaling the proteins inside the cell to contract.
In addition to the specialized functions required by the high energy demand of muscle tissue, the sarcolemma functions as a normal cell membrane. It contains a series of integrated proteins that work in unison to control the cell’s contents.