Ion Gradients Drive the Neuron Action Potential

A neuron at rest sits at about -70 millivolts inside compared to outside — we call this the resting membrane potential. It exists because the sodium-potassium pump (Na⁺/K⁺-ATPase) constantly moves 3 Na⁺ out for every 2 K⁺ in, keeping Na⁺ concentrated outside and K⁺ concentrated inside. Negatively charged proteins trapped inside add to the negative interior charge. When a stimulus arrives and the membrane voltage rises to around -55 mV — the threshold — something dramatic happens: voltage-gated sodium channels snap open all at once. Na⁺ rushes inward down its concentration and electrical gradient, driving the membrane voltage to about +30 mV in under a millisecond. This is depolarization, and it is all-or-none: either the threshold is reached and the full spike fires, or nothing happens at all. There is no such thing as a 'partial' action potential. At the peak, sodium channels automatically inactivate (they enter a 'plugged' state), and voltage-gated potassium channels — slower to open — finally swing wide. K⁺ rushes outward, pulling voltage back down. This is repolarization. K⁺ channels close a little late, so voltage briefly undershoots past -70 mV into hyperpolarization (~-80 mV). During this refractory period, a second action potential cannot fire (absolute refractory) or requires a stronger-than-normal stimulus (relative refractory). The Na⁺/K⁺ pump then restores the original ion balance, ready for the next signal. A single action potential propagates along the axon because the depolarized patch raises voltage in the neighboring membrane patch above threshold, creating a self-regenerating wave. Myelin sheaths speed this up by forcing the signal to jump between gaps called nodes of Ranvier (saltatory conduction).