How is an action potential generated?

During rest, the outside of the neurone membrane is positively charged (relative to the inside). This is because approximately three sodium ions move out for every two potassium ions moving in through the sodium-potassium pump. When there is a stimulus, the sodium voltage-gated ion channels open. So, sodium ions diffuse into the axon and the membrane becomes more permeable to sodium ions if the threshold value of around -50mV is reached (positive feedback). This is known as depolarisation.An action potential of around +40mV is generated and after this, the sodium voltage-gated ion channels close. As a result, the potassium voltage-gated ion channels open and the potassium ions diffuse out causing repolarisation. There is usually an ‘overshoot’ as the potassium ions move out, where the potential difference is more negative, but the resting potential of around -65mV is established by the sodium-potassium pump.