Question

During the course of an action potential, the membrane potential rapidly rises and reaches a peak of about +40 mV before falling to about -75 mV. What causes the fall in membrane potential?
A) A decrease in Na+ permeability, and an increase in K+ permeability
B) A decrease in the permeability to both Na+ and K+
C) An increase in Na+ permeability, and a decrease in the K+ permeability
D) An increase in the permeability to both Na and K

Answer 1

Answer:

A)  A decrease in Na+ permeability, and an increase in K+ permeability.

Explanation:

When the potential action reaches a peak of about +40 mv Na+ channels are open and a high number of Na+ ions are entering the inside of the cell. Shortly after this happens the K+ channels will start to open their gates increasing the cell  K+ permeability  while the Na+ channels will start to close their gates, so the Na+ permeability will decrease. This happens  in order to valance the positive charge on the inside of the cell. Normally the inside has a negative charge while the outside has a positive one, as the inside is more positive due to the increase in Na+ permeability at the beginning of the potential action, K+ cations (which are in abundance in the cell) will have to go out through the K+ channels  so as to restore the charge valance, that means that there is an increase in K+ permeability.