Question

During phase 3 of a contractile cell action potential, ________.

A) only sodium permeability is increased
B) only calcium permeability is increased
C) only potassium permeability is increased
D) sodium and calcium permeability are both increased
E) sodium and potassium permeability are both increased

Answer 1

During phase 3 of a contractile cell action potential, only potassium permeability is increased, leading to repolarization of the cell membrane.

Step-by-step explanation:

During phase 3 of a contractile cell action potential, only potassium permeability is increased. This phase, known as repolarization, involves the outflux of K+ ions from the cell, which helps restore the membrane potential after the depolarization that occurs during the action potential. While sodium and calcium ions play critical roles in the depolarization phases of the action potential, it is the opening of potassium channels and the resulting efflux of K+ that characterizes phase 3. The sodium-potassium pump also contributes to restoring the resting potential by transporting Na+ out of the cell and K+ into the cell, though it does this actively using energy from ATP and operates continuously, not just during the action potential phases.