Final answer:
During neuronal depolarization, sodium ions (Na+) rush into the neuron due to the opening of voltage-gated Na+ channels, which leads to a reversal of the inside-negative membrane potential, causing it to become positive. This influx of Na+ is followed by repolarization when potassium ions (K+) exit the cell, returning the membrane potential to its resting state.
Step-by-step explanation:
During the process of depolarization within a neuron, sodium ions (Na+) rush into the cell. This action is crucial for an action potential to occur, which is an all-or-nothing event. For depolarization to happen, the membrane must reach a certain threshold; once this happens, voltage-gated Na+ channels open, and Na+ enters the cell due to a higher concentration of Na+ outside compared to inside, following its concentration gradient.
This influx of Na+ changes the relative charge inside the neuron, causing it to switch from a resting membrane potential of approximately -70 mV to a positive value, typically around +30 mV. This sudden change from negative to positive inside the cell relative to the outside is what constitutes depolarization. Subsequently, as the membrane potential increases, voltage-gated potassium (K+) channels open, allowing K+ to leave the cell and initiate repolarization. Na+ channels become inactivated during this phase, preventing further Na+ from entering the cell. As K+ exits the cell, the membrane potential returns to its resting state.
These ion movements are critical for the propagation of action potentials down the axon towards synapses, ensuring that nerve signals can be transmitted efficiently through the neuron network. The action potential moves in one direction because the sodium channels behind it become inactivated and need to reach near-resting membrane potential before they can reset and be opened again.