What immediate ionic event triggers the rapid depolarisation phase of a neuronal action potential once the threshold potential is reached?
Answer and explanation
Correct answer: D. Rapid opening of voltage-gated sodium channels causing an influx of Na+ into the cytosol
When the axon hillock membrane reaches the threshold potential (approximately -55 mV), activation gates of voltage-gated Na+ channels rapidly open. Sodium ions rush into the cytosol down their steep electrochemical gradient, producing the swift upstroke (depolarization) of the action potential toward roughly +30 mV.
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