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Depolarization is the process by which electrical excitation of muscle is converted into force generation, also known as excitation-contraction coupling (ECC). ECC involves the propagation of action potentials from the neuromuscular junction along the length of the fibre and into a network of membrane invaginations in muscle known as transverse tubules (t-tubules). Depolarization of the resting membrane potential of skeletal muscle can cause failure of ECC in diseases such as periodic paralysis and ICU-acquired weakness.
| Characteristics | Values |
|---|---|
| Definition | Depolarization is an increase in membrane potential |
| Process | The excitation of skeletal muscle by motor neurons causes the opening of voltage-gated sodium channels. The opening of sodium channels causes depolarization of the skeletal muscle |
| Action | The channels are opened by depolarization of the nerve terminal membrane and selectively allow the passage of calcium ions |
| Result | The resting membrane potential is depolarized to a critical potential (Ecrit), a self-generating action potential follows, leading to muscle contraction |
| Failure | Depolarization of the resting membrane potential of skeletal muscle, when severe enough, causes failure of ECC in diseases such as periodic paralysis and intensive care unit (ICU)-acquired weakness |
| Prolonged depolarization | Nicotinic agonists can cause a prolonged depolarization state of the cells. They prevent the cells from undergoing repolarization. As a result, cells do not respond to the new stimuli |
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What You'll Learn
The excitation of skeletal muscle by motor neurons
Depolarization of the resting membrane potential of skeletal muscle can cause failure of excitation-contraction coupling (ECC) in diseases such as periodic paralysis and intensive care unit (ICU)-acquired weakness. It can also potentially contribute to fatigue during intense exercise. Studies of depolarization-induced failure of ECC in frog and mammalian skeletal muscle reveal that whole muscle force is generally stable or slightly increased with mild depolarization of the resting potential, followed by a steep decline with further depolarization.
Nicotinic agonists can cause a prolonged depolarization state of the cells by preventing them from undergoing repolarization. This results in the cells not responding to new stimuli.
The depolarization and repolarization sequence is accompanied by a flow of substantial current through the active cell membrane, creating a "dipole-current source" that is detectable at considerable distances. Small currents flow from this source through the aqueous medium containing the cell.
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The opening of sodium channels
During depolarization, the resting membrane potential of the skeletal muscle increases, leading to a self-generating action potential. This action potential then travels through the T-tubules, which are a network of membrane invaginations within the muscle cell. The propagation of the action potential along these T-tubules is an essential aspect of excitation-contraction coupling (ECC), which is the process by which electrical excitation of muscle is converted into force generation.
The T-tubules play a crucial role in coordinating the release of calcium ions from the sarcoplasmic reticulum, a specialized structure within the muscle cell that stores calcium. As the action potential travels through the T-tubules, it activates Cav1.1 channels, which in turn triggers the opening of ryanodine receptors. This sequence of events leads to the release of calcium ions from the sarcoplasmic reticulum into the cytoplasm of the muscle cell.
The increase in intracellular calcium concentration is the key trigger for muscle contraction. Calcium ions interact with contractile proteins within the muscle cell, causing them to undergo conformational changes that result in muscle shortening and force generation. This intricate process involves the precise coordination of ion channels, receptors, and intracellular signalling pathways, ensuring a controlled and efficient response to neural stimulation.
It is worth noting that disruptions in the depolarization process can have significant consequences. For example, severe depolarization of the resting membrane potential can lead to failure of ECC, contributing to conditions such as periodic paralysis and ICU-acquired weakness. Additionally, prolonged depolarization states induced by nicotinic agonists can prevent cells from undergoing repolarization, impairing their ability to respond to new stimuli. Understanding the delicate balance of ion channels and membrane potentials involved in depolarization is crucial for comprehending both normal muscle function and the pathophysiology of various muscle disorders.
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The depolarization of the resting membrane potential
Depolarization of the resting membrane potential of skeletal muscle occurs when the muscle is excited by motor neurons, causing the opening of voltage-gated sodium channels. This opening of sodium channels causes depolarization of the skeletal muscle. The action potential from the motor neuron travels through the T-tubules, activating Cav1.1 channels, which trigger the opening of ryanodine receptors. This allows the passage of calcium ions, which leads to muscle contraction.
Nicotinic agonists can cause a prolonged depolarization state of the cells by preventing them from undergoing repolarization. This results in cells that do not respond to new stimuli.
Overall, the depolarization of the resting membrane potential is a critical process in muscle contraction and can have significant implications for muscle function and health.
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The role of action potential changes
During ECC, action potentials propagate from the neuromuscular junction (NMJ) along the length of the muscle fibre and into a network of membrane invaginations called transverse tubules (t-tubules). The activation of Cav1.1 channels in the t-tubules triggers the opening of ryanodine receptors, leading to the release of Ca2+ ions from the sarcoplasmic reticulum (SR) and subsequent muscle contraction.
Depolarization of the resting membrane potential in skeletal muscle occurs when voltage-gated sodium channels open in response to excitation by motor neurons. This depolarization results in an increase in membrane potential, leading to a self-generating action potential that causes muscle contraction.
However, severe depolarization of the resting membrane potential can lead to failure of ECC, as observed in diseases such as periodic paralysis and ICU-acquired weakness. Additionally, mild depolarization of the resting potential can cause a slight increase in whole muscle force, while further depolarization results in a steep decline.
Overall, the role of action potential changes in depolarization is crucial for maintaining the proper functioning of skeletal muscles. Any disruptions to this process, such as failure of ECC or abnormal depolarization levels, can lead to muscle dysfunction and disease.
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The relationship between resting potential, action potential properties and conduction
Depolarization of the resting membrane potential of skeletal muscle can also be caused by nicotinic agonists, which can cause a prolonged depolarization state of the cells. This prevents the cells from undergoing repolarization, meaning they do not respond to new stimuli.
The process by which electrical excitation of muscle is converted into force generation is known as ECC. Successful ECC involves the propagation of action potentials from the neuromuscular junction (NMJ) along the length of the fibre, as well as into a network of membrane invaginations in muscle known as transverse tubules (t-tubules).
Depolarization in the t-tubules activates Cav1.1 channels, which triggers the opening of ryanodine receptors, Ca2+ exit from the sarcoplasmic reticulum (SR), and force production. This sequence, called depolarization and repolarization, is accompanied by a flow of substantial current through the active cell membrane.
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Frequently asked questions
Depolarization is the increase in membrane potential of the nerve terminal membrane.
Depolarization of the resting membrane potential of skeletal muscle is caused by the excitation of skeletal muscle by motor neurons. This causes the opening of voltage-gated sodium channels, which leads to depolarization of the skeletal muscle.
After depolarization, the channels are opened and selectively allow the passage of calcium ions. This leads to a self-generating action potential, which causes muscle contraction.
When depolarization is severe enough, it can cause failure of excitation-contraction coupling (ECC) in diseases such as periodic paralysis and intensive care unit (ICU)-acquired weakness. It can also contribute to fatigue during intense exercise.
