ACTION POTENTIAL STEPS

Resting Potential

Neuron is at about −70 mV; sodium (Na⁺) is mostly outside the cell and potassium (K⁺) is mostly inside, maintained by the sodium-potassium pump.

Threshold

Membrane potential reaches about −55 mV; voltage-gated sodium channels open and an action potential is triggered.

Depolarization

Membrane potential rapidly rises from −55 mV to about +30 mV as sodium ions rush into the neuron.

Peak

Membrane potential is at approximately +30 mV; sodium channels close and potassium channels open.

Repolarization

Membrane potential falls from +30 mV back toward −70 mV as potassium ions leave the neuron.

Hyperpolarization (Undershoot)

Membrane potential briefly becomes more negative than resting, about −80 to −90 mV, because potassium channels stay open longer than needed.

Return to Resting Potential

Membrane potential returns to −70 mV as potassium channels close and the sodium-potassium pump restores ion balance.