Final answer:
When an action potential reaches an axon terminal, the sodium ion channels that had opened to depolarize the cell start to close, and potassium ion channels open, allowing potassium ions to leave the cell, which repolarizes the membrane to a negative internal charge.
Step-by-step explanation:
When an action potential reaches an axon terminal, several events unfold to continue the transmission of the nerve impulse. The correct answer to the question is that the interior of the cell becomes positively charged with respect to the exterior. Initially, sodium ion channels open to allow sodium ions (Na+) to rush into the neuron, causing the interior of the cell to depolarize.
This influx of positive ions reverses the charge across the membrane. As the action potential peaks, these Na+ channels start to close, and potassium ion channels (K+) open. This allows potassium ions to exit the cell, which repolarizes the membrane and restores the negative internal charge relative to the outside of the cell. Lastly, the Na+/K+ transporter helps to reestablish the resting membrane potential.
Therefore, it is during the repolarization phase that K+ channels open, leading to the inside of the cell becoming negatively charged again.