Describe the changes at the cell membrane of a neuron during an action potential.

At rest the membrane potential is -70mV.

This is due to the  Na⁺/K⁺ pump, which actively transports 3 Na⁺ ions out of the cell for every 2 K⁺ ions in.

Therefore there is a net negative charge inside the cell.

Four stages:

Depolarization; Repolarization;Hyperpolarization; Afterhyperpolarization.

Depolarization - triggered by another neuron. The membrane becomes more leaky to Na⁺, so Na⁺ diffuses into the cell down an electrochemical gradient. This makes the potential difference across the membrane become more positive. Eventually a Threshold potential of -55mV is reached, which activates the voltage-gated Na⁺ channels. These open and allow a lot of Na⁺ to enter the neuron.

Repolarization - at +30mV the voltage-gated Na⁺ channels close and the voltage-gated K⁺ channels open. K⁺ diffuses out of the cell through the open channels, causing the membrane potential to become more negative again.

Hyperpolarization - or the 'overshoot' is when the membrane potential becomes more negative than the resting potential, due to prolonged K⁺ efflux.

Afterhyperpolarization - restoring the resting membrane potential. At this point the membrane potential is too negative AND the ions (Na⁺ and K⁺) are on the wrong side of the membrane. The Na⁺/K⁺ pump works to restore the ions to their correct sides and a new action potential cannot be generated.