Topic:Conduction and transmission of impulse, mechanism of contraction, neuromuscular transmission, reflexes, control of equilibrium
Question: Discuss on the mechanism of excitation-contraction coupling in the skeletal muscle
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Excitation-contraction coupling is the process by which an action potential in a skeletal muscle fiber leads to muscle contraction. The mechanism of excitation-contraction coupling involves several steps, which are described below:
- Depolarization of the sarcolemma: The process of excitation-contraction coupling is initiated by an action potential that travels along the sarcolemma (the plasma membrane of the muscle fiber). The action potential causes a depolarization of the sarcolemma, which activates voltage-gated calcium channels (also known as dihydropyridine receptors) located in the membrane.
- Release of calcium ions from the sarcoplasmic reticulum: The activation of voltage-gated calcium channels triggers the release of calcium ions (Ca2+) from the sarcoplasmic reticulum (SR), which is a specialized type of endoplasmic reticulum that surrounds the myofibrils (the contractile elements of the muscle fiber). The release of Ca2+ from the SR is mediated by ryanodine receptors, which are calcium release channels located in the SR membrane.
- Binding of calcium ions to troponin: The release of Ca2+ from the SR increases the concentration of Ca2+ in the cytosol (the fluid inside the muscle fiber). The increased concentration of Ca2+ allows Ca2+ to bind to troponin, which is a regulatory protein associated with the thin filaments of the myofibrils. The binding of Ca2+ to troponin causes a conformational change in the troponin-tropomyosin complex, which exposes the binding sites for the cross-bridges of the thick filaments.
- Cross-bridge cycling: The exposure of the binding sites for the cross-bridges allows the cross-bridges to attach to the thin filaments and undergo a cyclical process of attachment, force generation, and detachment. This process is known as cross-bridge cycling, and it is responsible for generating the force that causes muscle contraction.
- Relaxation: The relaxation of the muscle fiber occurs when the concentration of Ca2+ in the cytosol decreases, which is mediated by calcium pumps and exchangers located in the SR and sarcolemma. The decrease in Ca2+ concentration causes the dissociation of Ca2+ from troponin, which allows the troponin-tropomyosin complex to return to its resting state and block the binding sites for the cross-bridges. This prevents further cross-bridge cycling and allows the muscle fiber to relax.
Because skeletal muscle contraction only occurs in response to neural stimulation and the subsequent release of intracellular calcium, myosin, and actin cannot always bind to each other so the cross-bridges cannot always swing. This prevents constant muscle contraction.
The process of muscle contraction is summarized below.
When an action potential is delivered by a motor neuron to a skeletal muscle fiber, the neuron releases acetylcholine (ACh) into the neuromuscular junction. ACh diffuses to a specialized area of the muscle cell, called the end plate. Muscle cell end plates are concentrated with receptors for ACh. ACh binds to the receptors causing the opening of sodium channels present in the muscle cell. With the opening of these channels, sodium ions rush into the cell, depolarizing (making the cell positively charged on the inside) and initiating an action potential. The action potential passes along the entire muscle fiber, depolarizing the fiber. Depolarization spreads into the fiber through small tubules, called transverse (T) tubules, which run along the juncture between the A and I bands. When the inside of the cell becomes positive, calcium ion is released from intracellular bags of calcium (called lateral sacs) that lie adjacent to the T tubules. The lateral sacs are outpouchings of a large intracellular calcium storage compartment: the sarcoplasmic reticulum causes high levels of intracellular calcium released from the sarcoplasmic reticulum to initiate muscle contraction.
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