Action Potential

An action potential is a brief reversal of the potential difference across the membrane of a neurone, from about −70 mV (inside negative) to about +40 mV (inside positive). It lasts about 2 to 3 ms and is caused by the opening and closing of voltage-gated ion channels.The sequence of events is shown on a graph of membrane potential against time:1. Resting state: the membrane is polarised at −70 mV.2. Stimulus: some sodium ion channels open and Na⁺ diffuses in, so the potential becomes less negative.3. Depolarisation: if the threshold (about −55 mV) is reached, many more voltage-gated Na⁺ channels open. Na⁺ floods in and the potential rises rapidly to about +40 mV.4. Repolarisation: the Na⁺ channels close and voltage-gated K⁺ channels open. K⁺ diffuses out and the potential falls.5. Hyperpolarisation: the K⁺ channels are slow to close, so the potential overshoots to about −80 mV.6. Return to resting potential: the K⁺ channels close and the sodium-potassium pump restores −70 mV.Each stage is covered in more detail in the next lessons. Two features apply to every action potential. It is all-or-nothing: every action potential reaches the same peak. And it is followed by a refractory period, during which another action potential cannot be generated straight away.