The "undershoot" phase of after-hyperpolarization is due to:

a. slow opening of voltage-gated sodium channels

b. sustained opening of voltage-gated potassium channels

c. rapid opening of voltage-gated calcium channels

d. slow restorative actions of the sodium-potassium ATPase

e. ions that move away from their open ion channels

b. sustained opening of voltage-gated potassium channels

Explanation:

Voltage-gated potassium ion channels are closed during resting potential. Sustained opening of these channels after inactivation of voltage gated sodium ion channels leads to net diffusion of potassium ions out of the neuron. Since the positively charged ions are diffused out of the neurons, the inside of the neuron becomes more negative than the resting potential. This stage is called the undershoot.