Biological Systems MCAT Practice Exam 2025 – The Comprehensive All-in-One Guide for Exam Success!

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What initiates the depolarization phase during an action potential?

Opening of K+ channels

Closing of Na+ channels

Opening of voltage-gated Na+ channels

During an action potential, the depolarization phase is initiated by the opening of voltage-gated Na+ channels. When a neuron is stimulated, it reaches a threshold potential that causes these channels to open. As sodium ions (Na+) flood into the cell due to both the concentration gradient and the electrical gradient, the interior of the neuron becomes more positively charged compared to the outside. This rapid influx of sodium ions significantly depolarizes the membrane potential, moving it closer to the reversal potential for sodium.

The action potential is characterized by this swift depolarization, which is vital for the propagation of the nerve impulse along the neuron. Following this phase, the sodium channels will begin to inactivate, and potassium channels will open, leading to repolarization, but the initiation of the depolarization itself is certainly driven by the opening of the voltage-gated Na+ channels.

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Inactivation of K+ channels

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