The Action Potential

The Action Potential

An action potential occurs when information is sent down the axon of a neuron and a change in membrane potential is activated.

action potential, A level, biology

Nerve and muscles cells are electrically excitable. This means that they’re able to change their membrane potential. They contain voltage-gated sodium and potassium channels which open and close in response to voltages moving across the membrane.

The resting potential in a human nerve cell is -70mV. When a stimulus occurs the membrane potential is reversed for about a millisecond. This is known as the action potential.

There are two main stages to the action potential:

  • depolarisation
  • repolarisation

If a neuron is stimulated past its threshold point (which is approximately -30 mV in humans) the sodium channels open. As the sodium flows into the axon this creates an area of positive charge, a process known as depolarisation.

The positive charge causes sodium channels in the vicinity to shut. As these close, potassium channels open and potassium flows out of the axon. This causes the charge to return to its resting potential, a process known as repolarisation.

Depolarisation and repolarisation occurs along the axon as a chain reaction, with each sodium influx causing depolarisation and each potassium outflow causing repolarisation. The sodium-potassium pump then restores the ion concentration to the resting potential.