Sodium ions (Na+) entering a neuron through open sodium-permeable channels depolarize it. The inward flow of positive charge makes the inside of the cell less negative.
What action depolarizes a neuron?
Opening sodium-permeable channels allows Na+ to move into the neuron down its electrochemical gradient. That inward movement of positive charge shifts the membrane potential in the positive direction, making the cell interior less negative. This is the key ionic action during the rising phase of a typical neuronal action potential.
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How sodium influx drives the action potential
If an initial change in voltage reaches the relevant threshold, voltage-gated sodium channels open. The resulting sodium influx depolarizes the membrane; that depolarization can open additional voltage-gated sodium channels, allowing still more sodium to enter. This positive-feedback process produces the rapid rising phase of the action potential. The University of Texas Medical School at Houston’s Neuroscience Online chapter on the action potential describes how increasing sodium permeability leads to further depolarization and the opening of more sodium channels.
How this differs from potassium movement
Potassium leaving the neuron through open potassium channels carries positive charge outward, tending to make the inside more negative. Along with sodium-channel inactivation, this potassium efflux contributes to repolarization—the return toward a more negative membrane potential—as the action potential progresses. So if the question asks which action depolarizes a neuron, choose sodium entering, not potassium leaving.
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Why the sodium-potassium pump is not the immediate answer
The sodium-potassium pump helps maintain the ion gradients that make sodium influx possible, but it is not the immediate cause of the action potential’s rising phase. For the direct depolarizing action, identify Na+ moving into the cell through open channels.
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