Mastering A&P Neurophysiology Practice Test 2025 - Free Neurophysiology Practice Questions and Study Guide

When a threshold stimulus is reached, the first significant change that occurs is the opening of voltage-gated sodium channels. This response is crucial as it marks the initiation of an action potential in a neuron. When the threshold is met, the increase in membrane potential causes these channels to undergo a conformational change. As a result, sodium ions (Na+) flood into the cell due to both the concentration gradient and the electrochemical gradient, leading to depolarization.

This influx of sodium ions significantly alters the membrane potential, shifting it toward a more positive value. It is this rapid change in membrane potential that sets the stage for the subsequent phases of the action potential, including repolarization where potassium channels open and potassium ions exit the cell, and the eventual return to resting membrane potential. Therefore, the opening of voltage-gated Na+ channels following a threshold stimulus is a critical first step in the process of generating an action potential.