14.6
Generation of Action Potential in Skeletal Muscles
Every cell in the body maintains a membrane potential due to an uneven distribution of positive and negative charges across its plasma membrane. The m…
The resting membrane potential of a muscle cell is the difference in electrical charge across its membrane at rest. It is typically around -85 mV.
At a neuromuscular junction, when the acetylcholine released from the axon terminals binds the nicotinic receptors on the motor end plate, it allows sodium ion influx into the muscle fiber.
This influx makes the membrane potential less negative, leading to local depolarization at the motor end plate.
If this potential change crosses a threshold of -50 to -55 mV, it opens voltage-gated sodium channels, triggering an action potential — a self-propagating electrical signal.
The action potential starts a depolarization wave by opening adjacent voltage-gated sodium ion channels, propagating the signal along the entire muscle fiber.
After the depolarization of the membrane reaches its highest point of about +40 mV, also known as the overshoot, the voltage-gated sodium channels shut down.
Simultaneously, the overshoot potential opens the voltage-gated potassium channels for the exit of potassium ions, dropping the electric charge of the membrane back to its resting potential. This phase is called repolarization.
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Q1: What is the resting membrane potential of a muscle cell?
The resting membrane potential is the electrical charge difference across a muscle cell's membrane at rest, typically around -85 mV. This negative internal charge is maintained by sodium-potassium pumps, which actively move potassium ions into the cell and sodium ions out. This uneven distribution of ions creates the electrical gradient necessary for muscle cell excitability and response to stimuli.
Q2: How does acetylcholine trigger depolarization at the neuromuscular junction?
When acetylcholine released from axon terminals binds to nicotinic receptors on the motor end plate, it opens chemically-gated sodium channels. Sodium ions rush into the muscle fiber due to concentration and electrical gradients, making the membrane potential less negative. This localized depolarization at the neuromuscular junction initiates the electrical signal cascade.
Q3: What happens when membrane potential crosses the threshold of -50 to -55 mV?
When depolarization reaches the threshold of -50 to -55 mV, voltage-gated sodium channels open, triggering a full action potential. This opening allows rapid sodium ion influx into the muscle fiber, creating a self-propagating electrical signal that spreads along the entire muscle membrane, enabling coordinated muscle cell activation.
Q4: What is the overshoot phase during an action potential?
The overshoot is the peak of the action potential when the membrane potential reaches approximately +40 mV, making the cell's internal charge momentarily positive. At this point, voltage-gated sodium channels close while voltage-gated potassium channels open simultaneously, allowing potassium ions to exit and initiating the repolarization phase.
Q5: How does repolarization restore the muscle cell's resting potential?
During repolarization, voltage-gated potassium channels open after the overshoot, allowing potassium ions to exit the cell. This outflow of positive charges returns the membrane potential back to its resting negative state of approximately -85 mV. Repolarization completes one full action potential cycle and prepares the muscle cell for subsequent stimulation.
Q6: Why are muscle cells considered excitable cells?
Muscle cells are excitable because they contain voltage-gated ion channels embedded in their plasma membranes that respond to changes in membrane potential. These channels enable rapid ion movements across the membrane, allowing muscle cells to generate and propagate action potentials in response to chemical signals, facilitating swift responses to neural stimulation and bodily movements.
Q7: How does the action potential propagate along the muscle fiber?
The action potential propagates as a self-sustaining depolarization wave. When voltage-gated sodium channels open in one region, sodium influx depolarizes adjacent membrane areas, opening neighboring voltage-gated sodium channels. This sequential channel activation creates a wave of depolarization that travels along the entire muscle fiber, ensuring coordinated electrical signal transmission through excitation contraction coupling in skeletal muscles.