NPTE Neurological Practice Test

Initiation of an action potential is driven by a rapid increase in Na+ permeability. When a neuron is depolarized to threshold, voltage-gated Na+ channels open and Na+ rushes into the cell. This inward current rapidly depolarizes the membrane toward the Na+ equilibrium potential, and because the depolarization opens more Na+ channels in a positive feedback loop, the rapid spike is produced. K+ channels do open as the spike develops and later contribute to repolarization by moving K+ outward, but the primary trigger for the spike is the Na+ influx. Movement of Ca2+ can contribute to depolarization in some neurons or contexts, and Cl- influx would tend to hyperpolarize, opposing initiation. A decrease in Na+ permeability with outward K+ flow would hinder the onset of the action potential.