An action potential is a message in the form of an electrical impulse. This being a rapid change in membrane potential.
When a stimulus reaches the threshold at the axon hillock, an action potential begins.
An action potential relies on many protein channels: The Potassium leak channel and Sodium pump maintain the resting potential when no stimulus is inflicted. The voltage gated sodium channel and the voltage gated potassium channel are involved in the progression of an action potential.
The action potential progression can be separated into a several steps;
- Voltage channels are closed and the Potassium link channel and the sodium pump maintain the resting membrane potential of -70mV.
- Neurone becomes stimulated. The voltage gated Sodium channels begin to open and the membrane potential begins to slowly depolarises and sodium enters the cell down it's concentration gradient. All the voltage gated Sodium channels open when the membrane potential reaches aroung -55mV and there's a huge inflow of Sodium, causing a sharp rise in voltage. As the potential nears +30mV, the rate of depolarisation slows down as the voltage gated Soduim channels become saturated.
- Voltage gated potassium channels open, and potassium leaves the cell down it's concentration gradient. The depolarisation of the cell stops and repolatisation can occur through these voltage gated Potssium channnels.
- Voltage gated sodium channels are completely deactivated and potassium floods out through the coltage gated Potassium channels,
- Volatage gated potassium channels are slow to close, and therefore hyperpolarisation occurs. This is where the membrane potential drops below the resting potential of -70mV as potassium continues to leave.
- Once the voltage gated Potassium channels close, the resting state can be re-established through the Potassium leak channel and Sodium pump.
The action potential travels along the neurone's axon via current loops in order to reach the axon terminal