Biological Systems MCAT Practice Exam

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Prepare for the Biological Systems section of the MCAT. Review detailed explanations and interactive questions to understand complex systems. Enhance your exam readiness and aim for a high score!

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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
Inactivation of K+ channels

The correct answer is: 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.