Depolarisation is the change in the potential difference across a neurone membrane from its resting value (about −70 mV) towards zero and then above it, as the inside of the axon becomes less negative and then positive.
At rest, the voltage-gated sodium ion channels in the axon membrane are closed. Na+ is at a much higher concentration outside the axon, and the inside is negative, so there is a steep electrochemical gradient for Na+ to enter.When a stimulus arrives, for example at a receptor or as a local current from an adjacent part of the axon, it opens some Na+ channels. Na+ ions diffuse into the axon, making the inside less negative.If the potential reaches the threshold of about −55 mV, many more voltage-gated Na+ channels open. The more Na+ that enters, the more the membrane depolarises, and the more channels open. This is an example of positive feedback, and it causes the very rapid rise in potential. The inside of the axon becomes positive, reaching about +40 mV.If a stimulus is too weak to reach threshold, the few channels that opened close again and the membrane returns to the resting potential without an action potential. This is why action potentials are all-or-nothing.At the peak, the voltage-gated Na+ channels close (they become inactivated), which ends depolarisation. The Na+ that entered also spreads sideways inside the axon, forming local currents that depolarise the next region of membrane.
At rest, the voltage-gated sodium ion channels in the axon membrane are closed. Na+ is at a much higher concentration outside the axon, and the inside is negative, so there is a steep electrochemical gradient for Na+ to enter.When a stimulus arrives, for example at a receptor or as a local current from an adjacent part of the axon, it opens some Na+ channels. Na+ ions diffuse into the axon, making the inside less negative.If the potential reaches the threshold of about −55 mV, many more voltage-gated Na+ channels open. The more Na+ that enters, the more the membrane depolarises, and the more channels open. This is an example of positive feedback, and it causes the very rapid rise in potential. The inside of the axon becomes positive, reaching about +40 mV.If a stimulus is too weak to reach threshold, the few channels that opened close again and the membrane returns to the resting potential without an action potential. This is why action potentials are all-or-nothing.At the peak, the voltage-gated Na+ channels close (they become inactivated), which ends depolarisation. The Na+ that entered also spreads sideways inside the axon, forming local currents that depolarise the next region of membrane.