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The sarcolemma is the delicate plasma membrane that surrounds each striated muscle fibre. It is composed of a cell, or plasma, membrane which presents an extracellular matrix of collagen fibrils and polysaccharides that make contact with the basal lamina. The sarcolemma is the site where calcium enters and leaves the cell through a distribution of ion channels, transporters, and pumps. It also contains tunnel-like invaginations into the sarcoplasm, which are known as transverse tubules.
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What You'll Learn
Sarcolemma is the membrane that surrounds each muscle fibre
The sarcolemma is the membrane that surrounds each muscle fibre. It is composed of a cell, or plasma, membrane which presents an extracellular matrix of collagen fibrils and polysaccharides that make contact with the basal lamina. The sarcolemma is a complex multilayered structure. The outermost layer is a fine network of fibrils, which, at the ends of the muscle, extend into the tendons and form the structural link with them. The next layer is a foundation, or basement, membrane. The innermost layer is a membrane that surrounds the muscle fibre.
Each myofibril with its repetitive sequence of sarcomeres is surrounded by the cytoplasm of the fibre, the sarcoplasm. This contains several organelles, including mitochondria, the sarcoplasmic reticulum and T tubule membrane systems, Golgi apparatus and a cytoskeleton of microtubules, intermediate filaments and microfilaments of actin, as well as glycogen, free ribosomes, lipid droplets and lipofuscin. The T-tubules are invaginations of the sarcolemma that form a permeability barrier between the cytosol and the extracellular space.
The sarcolemma is the site where calcium enters and leaves the cell through a distribution of ion channels, transporters, and pumps. It also supports the flow of current as it relates to the flow of specific ions (ion-specific channels).
Muscle fibres are composed of myofibrils, which are made up of contractile proteins actin and myosin.
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Sarcolemma is composed of a cell membrane
The sarcolemma is the cell membrane that surrounds a muscle fibre. It is a delicate, extensible membrane that covers each striated muscle fibre. It is composed of a cell, or plasma, membrane which presents an extracellular matrix of collagen fibrils and polysaccharides that make contact with the basal lamina.
The sarcolemma is also known as the myolemma and is similar to a typical plasma membrane but has specialised functions for the muscle cell. It is composed of a lipid bilayer and a thin outer coat of polysaccharide material (glycocalyx) that contacts the basement membrane. The basement membrane contains numerous thin collagen fibrils and specialised proteins such as laminin that provide a scaffold to which the muscle fibre can adhere.
The sarcolemma is the site where calcium enters and leaves the cell through a distribution of ion channels, transporters, and pumps. It contains tunnel-like invaginations into the sarcoplasm, which are known as transverse tubules. Transmembrane proteins found within the sarcolemma facilitate a physical connection between the actin cytoskeleton of the muscle fibre and the extracellular basal lamina. Like other types of cell membranes, the sarcolemma forms a defined barrier between extracellular and intracellular environments.
The sarcolemma is covered by a glycocalyx, which is composed of glycosaminoglycans (GAGs), proteoglycans, and other glycoproteins that consist of acidic oligosaccharides with sialic acids at the terminal position. Proteins associated with the glycocalyx function as transmembrane proteins that help link the membrane to the cytoskeleton of the cell. This keeps the structure of the membrane secure and allows signal transduction between the intracellular and extracellular components. The sarcolemma can be excited electrically, leading to the activation of muscle fibres by signals from motor nerves.
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Sarcolemma is composed of collagen fibrils and polysaccharides
The sarcolemma is the cell membrane that surrounds each muscle fibre. It is composed of a plasma membrane and a thin outer coat of polysaccharide material (glycocalyx) that contacts the basement membrane. The basement membrane contains numerous thin collagen fibrils and specialised proteins such as laminin that provide a scaffold to which the muscle fibre can adhere.
The sarcolemma is the site where calcium enters and leaves the cell through a distribution of ion channels, transporters, and pumps. It also contains tunnel-like invaginations into the sarcoplasm, which are known as transverse tubules. The sarcoplasm contains many of the elements found in the cytoplasm of other tissues, including mitochondria, which are located primarily in the intermyofibrillary space near the Z line and adjacent to the A bands.
The sarcolemma is described as having two layers. The first is the plasma membrane, which is a structure of similar biochemical composition to the general plasma membrane found in eukaryotic cells. The second layer is the glycocalyx, which is in contact with the basement membrane. The basement membrane is rich in collagen fibrils and proteins that allow the muscle fibres to adhere to it. The cytoskeleton of the muscle cell, which consists of a large amount of the protein actin, is connected to the basement membrane through transmembrane proteins in the plasma membrane.
The sarcolemma can be excited electrically, leading to the activation of muscle fibres by signals from motor nerves. It contains ion-conducting pathways and channels through which sodium, potassium, calcium, and chloride flow selectively and non-selectively. The sarcolemma forms a defined barrier between extracellular and intracellular environments.
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Sarcolemma is a complex multilayered structure
The sarcolemma is a complex multilayered structure that surrounds each muscle fibre. It is composed of a cell, or plasma, membrane which presents an extracellular matrix of collagen fibrils and polysaccharides that make contact with the basal lamina. The outermost layer of the sarcolemma is a fine network of fibrils, which, at the ends of the muscle, extend into the tendons and form the structural link with them. The next layer is a foundation, or basement, membrane. The innermost layer is a membrane that surrounds the muscle fibre.
The sarcolemma is a delicate membrane that is extensible, meaning it can be stretched. It is composed of a cell, or plasma, membrane which presents an extracellular matrix of collagen fibrils and polysaccharides that make contact with the basal lamina. The sarcolemma also contains tunnel-like invaginations into the sarcoplasm, which are known as transverse tubules. These tubules form a permeability barrier between the cytosol and the extracellular space.
The sarcolemma is the site where calcium enters and leaves the cell through a distribution of ion channels, transporters, and pumps. It also supports the flow of current as it relates to the flow of specific ions. The sarcolemma forms a defined barrier between extracellular and intracellular environments, in the same way as other types of cell membranes.
The sarcolemma surrounds the sarcoplasm, which is the muscle cell cytoplasm. The sarcoplasm contains many of the elements found in the cytoplasm of other tissues, including mitochondria, which are located primarily in the intermyofibrillary space near the Z line and adjacent to the A bands. It also contains numerous unbound glycogen granules and small fat droplets. The sarcoplasm also contains the contractile proteins actin and myosin, which make up the myofibrils. These are the smallest units of contraction in living muscle.
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Sarcolemma is the site where calcium enters and leaves the cell
The sarcolemma is a delicate plasma membrane that surrounds each muscle fibre. It is composed of a cell or plasma membrane that presents an extracellular matrix of collagen fibrils and polysaccharides that make contact with the basal lamina. The sarcolemma is the site where calcium enters and leaves the cell through a distribution of ion channels, transporters and pumps.
The T-tubules are invaginations of the sarcolemma that form a permeability barrier between the cytosol and the extracellular space. T-tubules are also a site for calcium entry into the cell and calcium removal. This is important as it means that calcium levels within the cell can be tightly controlled in a small area.
The sodium-calcium exchanger passively removes one calcium ion from the cell in exchange for three sodium ions. As a passive process, it can therefore allow calcium to flow into or out of the cell depending on the combination of the relative concentrations of these ions and the voltage across the cell membrane.
In cardiac muscle cells, as the action potential passes down the T-tubules, it activates L-type calcium channels in the T-tubular membrane. Activation of the L-type calcium channel allows calcium to pass into the cell. T-tubules contain a higher concentration of L-type calcium channels than the rest of the sarcolemma and therefore the majority of the calcium that enters the cell occurs via T-tubules. This calcium binds to and activates a receptor, known as a ryanodine receptor, located on the cell's own internal calcium store, the sarcoplasmic reticulum. Activation of the ryanodine receptor causes calcium to be released from the sarcoplasmic reticulum, causing the muscle cell to contract.
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Frequently asked questions
A sarcolemma is a delicate plasma membrane that surrounds each striated muscle fibre.
The sarcolemma is composed of a cell, or plasma, membrane which presents an extracellular matrix of collagen fibrils and polysaccharides that make contact with the basal lamina.
The sarcolemma forms a defined barrier between extracellular and intracellular environments. It is also the site where calcium enters and leaves the cell through a distribution of ion channels, transporters, and pumps.
The sarcolemma surrounds the sarcoplasm (muscle cell cytoplasm) and separates it from the extracellular surroundings. The sarcoplasm contains many of the elements found in the cytoplasm of other tissues, including mitochondria, which are located primarily in the intermyofibrillary space near the Z line and adjacent to the A bands.
Each myofibril is surrounded by the sarcoplasm, which is in turn surrounded by the sarcolemma. The sarcolemma also contains tunnel-like invaginations into the sarcoplasm, which are known as transverse tubules.
