Answer
b. Chiefly a potassium (K+) current
Work Step by Step
At resting potential the inside of the cell is negative to the outside--the concentration of Na+ ions outside the cell is greater than the concentration inside. The concentrations are reversed for the K+ ions ( higher [K+] inside) . When a stimulus reaches threshold value, the Na+ channels open and Na+ ions rush in. This is the upswing. It is a brief event. Very soon after, the Na+ channels close,and the K+ channels open . As the K+ ions rush in, the cytosolic concentration of K+ once again becomes greater than [K+] in the ECF. The NA+/K+ ATPASE pump ejects the Na+ ions and brings K+ ions into the ICF. This is the repolarization process. It results in the restoration of the resting potential of the cell . The important component is the opening up of the K+ channels and the rushing in of the K+ ions. The resting potential of the cell is about-70mV. This is maintained because the cell pumps out more Na than it pumps in K+ at a ratio of 3:2. But that is not the while story ; there are also negative ions in the cell, For instance,there are Cl- anions. Also there area carboxyl anions of organic molecules that cannot cross the semipermeable cell membrane.
