The rapid depolarisation phase of an action potential is caused by
- A. potassium ions rushing out of the axon
- B. chloride ions entering the axon
- C. sodium ions rushing into the axon through voltage gated channels
- D. the sodium potassium pump reversing direction
Explanation
When the membrane is depolarised to threshold, voltage gated sodium channels open and sodium floods in down both its concentration and its electrical gradient, driving the potential to about plus 40 millivolts. Those channels then close and potassium channels open, so potassium leaves and the membrane repolarises. The pump restores the original ion distribution afterwards but is far too slow to create the spike itself.
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