As a domain expert in physiology, I can explain the process of depolarization in muscles. Depolarization is a critical step in the excitation-contraction coupling process of muscle fibers. It occurs when the muscle cell membrane, or sarcolemma, becomes less polarized, meaning it allows a flow of positively charged ions into the cell, which changes the cell's electrical potential.

In skeletal muscles, depolarization is initiated by an action potential that travels down the motor neuron and reaches the neuromuscular junction. Here, the action potential triggers the release of the neurotransmitter acetylcholine (ACh), which then binds to receptors on the muscle cell membrane. The binding of ACh causes the opening of sodium (Na+) ion channels in the sarcolemma, allowing Na+ ions to flow into the cell. This influx of positive charge makes the inside of the cell more positive relative to the outside, which is known as depolarization.

The depolarization causes a conformational change in the proteins of the sarcolemma, which leads to the opening of voltage-gated calcium (Ca2+) channels in the sarcoplasmic reticulum (SR), a specialized form of endoplasmic reticulum found in muscle cells. The release of Ca2+ from the SR is what then triggers the actual contraction of the muscle through a series of molecular events involving the proteins actin and myosin.

To summarize, the cause of depolarization in muscles is the influx of sodium ions through voltage-gated sodium channels in the sarcolemma, which is triggered by the binding of acetylcholine to its receptors following an action potential in the motor neuron.

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depolarization. the reduction of a membrane potential to a less negative value. It is caused by the influx of cations, such as sodium and calcium, through ion channels in the membrane. In many neurons and muscle cells, depolarization may lead to an electric impulse called an action potential.read more >>