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Every cell in the body maintains a membrane potential due to an uneven distribution of positive and negative charges across its plasma membrane. The membrane potential is measured in millivolts and quantifies the difference in charge across the membrane.
Like neurons, muscle cells are also regarded as excitable due to their capacity to change in response to stimuli, primarily due to voltage-gated ion channels embedded in their plasma membranes, which get activated by alterations in the cell's membrane potential.
In the resting state, a muscle cell maintains a negative internal charge called the resting membrane potential. The activity of the sodium-potassium pumps, which actively move potassium ions into the cell and sodium ions out of the cell, is instrumental in setting up the resting membrane potential. However, when a muscle cell receives a chemical signal at a neuromuscular junction, it triggers the opening of chemically-gated sodium channels. Sodium ions rush into the cell because of the concentration and electrical gradient, causing a localized depolarization — a scenario where the cell's inside becomes less negative.
In the event of this depolarization reaching a threshold, it opens voltage-gated sodium channels, resulting in a rapid influx of more sodium ions into the cell. This event generates a full-blown action potential, where the internal charge of the cell momentarily becomes positive, reaching a point called the overshoot.
Following this peak, voltage-gated potassium channels open to allow potassium ions to leave the cell, bringing about repolarization where the internal charge returns to its resting negative state. This entire event, from the initial depolarization to the subsequent repolarization, represents one complete action potential.
In conclusion, the excitability of muscle cells is a dynamic and complex process that carefully orchestrates ion movements across the cell membrane. This sequence enables the muscle cell to swiftly respond to signals, effectively transmit electrical impulses, and ultimately facilitate bodily movements.
