Sarcolema: Importance, Structure, Function, Characteristics and Investigations

This muscular plasma membrane plays a central role in the structure and function of skeletal muscle.

In addition to the functions of 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 subsarcolemma cytoskeleton, and the surrounding basal membrane (extracellular matrix) play an essential structural role in skeletal muscle.

Importance

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 of the cytoskeleton or protein complexes of the sarcolemma that join the basement membrane with the cytoskeleton.

Structure

This specialized cellular membrane that surrounds the cells of striated muscle fibers. Sometimes called 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 that consists 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 of the connective tissues or other muscle cells, creating a very strong fiber that can contract.

Each sarcolemma has a biochemical composition similar to that of a plasmalemma , which is another word for a cell membrane.

The structure of muscle cells specifically is 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, which are often abbreviated simply as “tubules T”.

Sarcolemma function

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 and the sarcolemma is considered a semipermeable unitary membrane of approximately 100 A thick, which directly encloses the contents of the cell.

This membrane is probably bimolecular and lipoprotein in character, and similar to what is thought to be reversed by most animal cells 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 density 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 difficult 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 certain regions of the cell.

However, using thin section techniques it is not easy to follow detailed variations in their course over large areas of the cell surface.

Research

For structural investigations of this type, surface replication methods, such as the layer removal method developed by Reed and Rudall, are probably much more useful, since the various levels exposed in the tissue can be examined over large areas.

The first investigations that used the method of 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 important 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.

characteristics

The sarcolemma has several unique characteristics that work by providing the muscle cells with both 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 a body.

Due to the high energy demand these contractions require, 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. The sarcolemma, in response to the signal sent by the neuron, will generate an action potential over its entire length, signaling the proteins inside the cell to contract.

Skeletal muscle

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 contents of the cell.

Like all cell membranes, the sarcolemma is formed from phospholipids, which affects the flow of water, ions and other molecules. The sarcolemma of different species can have many different proteins and compositions, reflecting the diverse evolutionary needs of the species over time.