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The sarcolemma is the membrane of a muscle cell, also known as the myolemma. It is a thin, extensible, and transparent membrane that covers every striated muscle fibre. The sarcolemma is composed of a cell membrane (plasma membrane) and an outer coat made up of a thin layer of polysaccharide (glycocalyx) material with numerous thin collagen fibrils. The sarcolemma is also present in cardiac muscles, where it plays a vital role in excitation-contraction coupling (ECC) and maintaining the structural integrity of the heart.
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What You'll Learn
- The sarcolemma is the cell membrane of the muscle fibre
- It consists of a lipid bilayer and a thin outer coat of polysaccharide material
- It acts as a barrier between the extracellular and intracellular compartments
- The sarcolemma contains ion-conducting pathways and channels
- It is involved in regulating cardiomyocyte excitation-contraction
The sarcolemma is the cell membrane of the muscle fibre
The sarcolemma acts as a barrier between the extracellular and intracellular compartments, defining the individual muscle fibre from its surroundings. The lipid nature of the membrane allows it to separate the fluids of the intra- and extracellular compartments, as it is only selectively permeable to water through aquaporin channels. The sarcolemma is also involved in the flow of specific ions through ion-specific channels.
In the case of cardiac muscle, the sarcolemma is the cardiomyocyte plasma membrane containing transverse tubules (t-tubules). These t-tubules are highly branched invaginations of the cardiomyocyte sarcolemma that function in excitation-contraction coupling (ECC), action potential initiation and regulation, maintaining the resting membrane potential, and signal transduction. The cardiac sarcolemma is a complex structure that contains multiple channels, exchangers, and pumps that are necessary for normal ECC and myocyte contraction.
The sarcolemma is essential in receiving and conducting stimuli from motor nerves, which lead to the activation of muscle fibres. It contains ion-conducting pathways and channels through which ions like sodium, potassium, calcium, and chloride flow selectively and non-selectively. These membrane pathways can respond to specific molecules (ligands), transmitters, or voltage changes, and the sarcolemma uses natural regulatory processes to close these pathways.
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It consists of a lipid bilayer and a thin outer coat of polysaccharide material
The sarcolemma is the cell membrane surrounding a skeletal muscle fibre or a cardiomyocyte. It is a thin, transparent, extensible membrane covering every striated muscle fibre. The structure and design of the sarcolemma are essential in receiving and conducting stimuli.
The sarcolemma consists of a lipid bilayer and a thin outer coat of polysaccharide material (glycocalyx). The lipid nature of the membrane allows it to act as a barrier between the extracellular and intracellular compartments, defining the individual muscle fibre from its surroundings. The lipid membrane can separate the fluids of the intra- and extracellular compartments as it is only selectively permeable to water through aquaporin channels.
The glycocalyx is a coating covering the cell membrane. It is composed of glycosaminoglycans (GAGs), proteoglycans, and other glycoproteins that consist of acidic oligosaccharides with sialic acids at the terminal position. The basement membrane, which is in contact with the glycocalyx, contains numerous thin collagen fibrils and specialized proteins such as laminin that provide a scaffold to which the muscle fibre can adhere.
Through transmembrane proteins in the plasma membrane, the actin skeleton inside the cell is connected to the basement membrane and the cell's exterior. At each end of the muscle fibre, the surface layer of the sarcolemma fuses with a tendon fibre, and the tendon fibres, in turn, collect into bundles to form the muscle tendons that adhere to bones.
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It acts as a barrier between the extracellular and intracellular compartments
The sarcolemma is the cell membrane surrounding a skeletal muscle fibre or a cardiomyocyte. It acts as a barrier between the extracellular and intracellular compartments, defining the individual muscle fibre from its surroundings. The sarcolemma is a complex multilayered structure that consists of a cell membrane (plasma membrane) and an outer coat made up of a thin layer of polysaccharide (glycocalyx) material with numerous thin collagen fibrils. The outermost layer is a fine network of fibrils that, 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 sarcolemma generally maintains the same function in muscle cells as the plasma membrane does in other eukaryote cells. The lipid nature of the membrane allows it to separate the fluids of the intra- and extracellular compartments, since it is only selectively permeable to water through aquaporin channels. As in other cells, this allows for the compositions of the compartments to be controlled by selective transport through the membrane. Membrane proteins, such as ion pumps, may create ion gradients with the consumption of ATP, which may later be used to drive the transport of other substances through the membrane (co-transport) or generate electrical impulses such as action potentials.
The sarcolemma contains ion-conducting pathways and channels through which sodium, potassium, calcium, and chloride flow selectively and non-selectively. These membrane pathways can open in response to specific molecules (ligands), transmitters, or when changes in voltage occur. The sarcolemma uses naturally occurring regulatory processes to close these pathways. The sarcolemma is covered by a glycocalyx, which is a coating covering the cell membrane. It is composed of glycosaminoglycans (GAGs), proteoglycans, and other glycoproteins that consist of acidic oligosaccharides with sialic acids at the terminal position.
The cardiac sarcolemma is a complex structure that contains multiple channels, exchangers, and pumps that are necessary for normal E-C coupling and myocyte contraction. The sarcolemma of cardiac muscle cells contains voltage-gated calcium channels, specialized ion channels that skeletal muscle does not possess.
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The sarcolemma contains ion-conducting pathways and channels
The sarcolemma is the cell membrane surrounding a skeletal muscle fibre or a cardiomyocyte. It is composed of a lipid bilayer and a thin outer coat of polysaccharide material (glycocalyx) that contacts the basement membrane. The sarcolemma generally maintains the same function in muscle cells as the plasma membrane in other eukaryote cells.
The sarcolemma forms tubular invaginations, t-tubules, at regular intervals along its length. These t-tubules are highly branched invaginations of the cardiomyocyte sarcolemma that function in excitation-contraction coupling (ECC), action potential initiation and regulation, maintaining the resting membrane potential, and signal transduction. The T-tubule membrane portion of the sarcolemma is highly plastic and therefore provides stability during muscle contraction. Studies have also shown that t-tubules are involved in water balance and cell volume regulation, recovery from muscle fatigue, and the transport of molecules.
The t-tubules contain a subgroup of voltage-gated Ca2+ channels (VGCCs), termed dihydropyridine receptors (DHPRs), which connect the firing of action potentials (APs) with the activation of RyRs, thereby promoting SR Ca2+ release and muscle contraction. The SR Ca2+ release is finely tuned by Ca2+ release units: interacting clusters of DHPRs and RyRs (also known as couplons). These regions are enriched with other ion channels and second messengers, and they represent key signalling hubs for the regulation of muscle function.
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It is involved in regulating cardiomyocyte excitation-contraction
The sarcolemma is the cell membrane surrounding a skeletal muscle fibre or a cardiomyocyte. It is composed of a lipid bilayer and a thin outer coat of polysaccharide material (glycocalyx) that connects with the basement membrane. The sarcolemma is a vital component of cardiac muscle cells (cardiomyocytes) as it is responsible for regulating a variety of cellular processes.
The sarcolemma is essential for maintaining the structural integrity of cardiomyocytes and mediating cardiac contraction. It achieves this by forming microdomains such as the t-tubular network, caveolae, and the intercalated disc. The t-tubules, or transverse tubules, are highly branched invaginations of the sarcolemma that play a critical role in excitation-contraction coupling (ECC). They facilitate the initiation and regulation of action potentials, help maintain the resting membrane potential, and enable signal transduction.
During ECC, action potentials travel along the sarcolemma and into the t-tubules, leading to membrane depolarization. This process involves voltage-sensitive dihydropyridine (DHP) receptors on the t-tubules, which allow the influx of calcium into the cell through L-type calcium channels. This increase in intracellular calcium triggers the sarcoplasmic reticulum to release additional calcium via ryanodine receptors, a process known as calcium-induced calcium release.
The released calcium ions play a crucial role in the contraction process. They attach to troponin C, causing tropomyosin to detach from the myosin-binding sites on actin. This detachment allows actin and myosin to form cross-bridges, resulting in muscle contraction. The contraction persists as long as calcium remains attached to troponin. This intricate process, regulated by the sarcolemma, ensures the coordinated contraction of cardiomyocytes, enabling the heart to function effectively as a pump.
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Frequently asked questions
The sarcolemma is the cell membrane surrounding a skeletal muscle fibre or a cardiomyocyte. It is a thin, transparent, extensible plasma membrane consisting of a cell membrane (or plasma membrane) and an outer coat of polysaccharide material (glycocalyx).
The term 'sarcolemma' comes from the Greek 'sarx' meaning 'flesh' and Greek 'lemma' meaning 'sheath'.
The sarcolemma acts as a barrier between the extracellular and intracellular compartments, defining the individual muscle fibre from its surroundings. It is involved in receiving and conducting stimuli and plays a role in excitation-contraction coupling (ECC) and action potential initiation and regulation.
The sarcolemma has two layers. The first is the plasma membrane, which is similar in composition to the general plasma membrane found in eukaryotic cells. The second layer is the glycocalyx, a thin coat of polysaccharide material.
The sarcolemma is the cell membrane of the muscle fibre, while the endomysium is the connective tissue layer over the muscle fibre. It is important to distinguish between these two terms.
