SARCOLEMMA: CHARACTERISTICS, STRUCTURE AND FUNCTION - BIOLOGY - 2025

The sarcolemma, also called myolemma, is the plasma membrane that makes up the muscle cells or fibers of the contractile tissues of animals. These fibers have the ability to contract in the face of specific electrical stimuli, that is, they can reduce their length, generating a mechanical force that allows the displacement of the joints, movement and ambulation of the animals.

Muscle cells are cells of great length (especially striated ones); they are nucleated cells that possess all the internal organelles characteristic of eukaryotic organisms: mitochondria, endoplasmic reticulum and Golgi complex, lysosomes, peroxisomes, etc.

Structural organization of a muscle fiber (Source: OpenStax via Wikimedia Commons)

However, unlike cells belonging to other tissues, the components of cells in muscle tissues are given specific names, which help distinguish them from other non-contractile cells.

Thus, its plasma membrane is known as sarcolemma, its cytosol as sarcoplasm, its endoplasmic reticulum as sarcoplasmic reticulum, and its mitochondria as sarcosomes.

Characteristics and structure

The sarcolemma, like all cell membranes, is a membrane composed of a lipid bilayer in which the lipids are organized in such a way that the hydrophilic portions “look” towards both surfaces of the same (intra- and extracellular) and the hydrophobic portions they are "faced" in the center.

It is approximately 100Ǻ thick and is a specialized membrane, as many of its characteristics are related to the functions of muscle cells.

In the immediate region to the outer periphery of the sarcolemma there is a much thicker layer (about 500Ǻ), which corresponds to an extracellular deposition of moderately dense materials.

These materials represent the basement membrane, whose density decreases as it moves away from the sarcolemma, approaches the extracellular space, and mixes with the ground substance of the surrounding connective tissue.

Sarcotubular system

The sarcolemma is an excitable membrane, which in many respects resembles the plasma membrane of neuronal cells, since it functions to conduct electrical impulses and has the ability to conduct an action potential.

In addition to covering them, this membrane extends into the striated muscle fibers in the form of projections or invaginations known as transverse tubules or T tubules, constituting what many authors recognize as a sarcotubular system, through which impulses propagate nervous into the fibers.

Sarcolemma, sarcoplasm and T-tubules (Source: Arcadian via Wikimedia Commons)

The T tubules of this system project transversely towards the binding sites of the bands A and I of the sarcomeres in the cells of the striated muscle, where they come into contact with the tubular system of the sarcoplasmic reticulum in the cytosol (sarcoplasm) of the same muscle fiber.

Since the contact between the sarcoplasmic reticulum and a T tubule occurs in such a way that the tubule is attached to each side with the reticulum membrane, this "structure" that is formed is known as a triad.

Thus, when a nerve impulse stimulates the sarcolemma on the cell surface, the depolarization of the membrane "travels" or spreads throughout its entire extension, including the T tubules in contact with the sarcoplasmic reticulum, which, in turn, is in closely related to contractile myofibrils (actin and myosin fibers).

The depolarization of the T tubules then causes the depolarization of the sarcoplasmic reticulum, which causes the release of calcium ions towards the myofilaments, activating their contraction.

Sarcolemmal proteins

As is true of all cell membranes, the sarcolemma is associated with various proteins, integral and peripheral, that provide it with many of its characteristic functional properties.

These proteins are known as sarcolemmal proteins and many of them contribute to the maintenance of the structural integrity of the muscle fibers, since they act against the physical forces of contraction that are exerted on the sarcolemma.

Some of these proteins anchor the internal structure of muscles to the basement membrane and the extracellular matrix. These include dystrophin, sarcoglycans, utrophin, dysferlin, caveolin, merosin, and intermediate filaments.

Since muscle cells have high energy demands, the sarcolemma is also equipped with a series of integral proteins in the form of channels that facilitate the transport of different types of molecules to and from the cell exterior, including carbohydrates, ions and others.

These channel-type proteins are essential for muscle contraction, because thanks to them a muscle fiber can return to its resting condition after depolarization induced by the impulse of the nerve fiber that innervates it.

Sarcolemma function

The sarcolemma works in the establishment of muscle cells, as well as the plasma membrane of any type of body cell. Therefore, this membrane performs important functions as a semi-permeable barrier to the passage of different types of molecules and as a structure for the maintenance of cellular integrity.

The extracellular matrix associated with sarcolemma has hundreds of polysaccharides that allow muscle cells to anchor themselves to the different components that make up and support muscle tissue, including other adjacent muscle fibers, favoring the simultaneous contraction of the same muscle.

Striated fiber muscle contraction

Each muscle fiber present in a given muscle is innervated by the branching of a specific motor neuron, which is what stimulates its contraction. The release of acetylcholine at the nerve synapse site between the neuron and the fiber sarcolemma generates a "current" that spreads and activates the sarcolemmal sodium channels.

Activation of these channels promotes the initiation of an action potential that begins at the site of the synapse and is distributed rapidly throughout the entire sarcolemma. In striated muscle fibers, this action potential, in turn, excites voltage-sensitive receptors in the triads formed between the T tubules and the sarcoplasmic reticulum.

These receptors activate calcium channels once they "feel" the presence of an action potential, allowing the release of small amounts of divalent calcium into the sarcoplasm (from the sarcoplasmic reticulum), increasing its intracellular concentration.

Calcium binds to special sites in the structure of a protein called troponin-C, eliminating the inhibitory effect on the myofibrils of another protein associated with it known as tropomyosin, stimulating contraction.

References

1. Bers, DM (1979). Isolation and characterization of cardiac sarcolemma. Biochimica et Biophysica Acta (BBA) -Biomembranes, 555 (1), 131-146.
2. Deisch, JK (2017). Muscle and Nerve Development in Health and Disease. In Swaiman's Pediatric Neurology (pp. 1029-1037). Elsevier.
3. Despopoulos, A., & Silbernagl, S. (2003). Color atlas of physiology. Thieme.
4. Kardong, KV (2002). Vertebrates: comparative anatomy, function, evolution (No. QL805 K35 2006). New York: McGraw-Hill.
5. Reed, R., Houston, TW, & Todd, PM (1966). Structure and function of the sarcolemma of skeletal muscle. Nature, 211 (5048), 534.