Action Potentials Quiz

See The Action Potential. Option C is correct. An action potential is a brief, rapid change in the electrical potential across the membrane of an excitable cell such as a neurone, and it is the signal that nerve cells use to communicate. At rest the inside of the neurone is negative (around -70 mV) and there is far more sodium outside the cell than inside. If a stimulus depolarises the membrane (makes it less negative) to the threshold, around -55 mV, voltage-gated sodium channels (channels that open in response to a change in voltage) open. Sodium then floods into the cell down its concentration gradient, and this sodium influx drives rapid depolarisation, swinging the membrane potential up towards positive values. After the peak the sodium channels become inactivated (they close and cannot reopen for a while) and voltage-gated potassium channels open, so potassium leaves the cell and the membrane repolarises (returns towards its negative resting value). Often it overshoots slightly into hyperpolarisation, a brief period when it is even more negative than at rest.

• Trigger: depolarisation to threshold, about -55 mV, opens voltage-gated sodium channels.
• Depolarisation: sodium floods in down its concentration gradient.
• Repolarisation: sodium channels inactivate and potassium channels open, so potassium leaves.