An action potential is a message in the form of an electrical impulse caused by a rapid change in a cell's membrane potential.
When a stimulus reaches the threshold at the axon hillock, an action potential begins.
An action potential relies on many protein channels. In a neurone, the Potassium leak channel and Sodium pump maintain the resting potential. The voltage gated sodium channel and the voltage gated potassium channel are involved in the progression of an action potential along the membrane.
The action potential progression can be separated into a several steps;
- Voltage channels are closed and the Potassium leak channel and the sodium pump maintain the resting membrane potential of -70mV.
- The 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 its concentration gradient. All the voltage gated Sodium channels open when the membrane potential reaches around -55mV and there's a large influx of Sodium, causing a sharp rise in voltage. As the potential nears +30mV, the rate of depolarisation slows down as the voltage gated Sodium channels become saturated and inactivate, preventing further sodium ions from entering the cell.
- Voltage gated potassium channels open, and potassium leaves the cell down its concentration gradient. The depolarisation of the cell stops and repolarisation can occur through these voltage gated Potssium channnels.
- Voltage gated sodium channels are completely deactivated and potassium floods out through the voltage gated potassium channels,
- Voltage 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.
An action potential is a transient, electrical signal, which is caused by a rapid change in resting membrane potential (-70mV). This occurs when the threshold potential (-55mV) is reached, this causes a rapid opening in the voltage gated sodium channels leading to a influx of sodium into the cell. The threshold potential also causes a slow opening of voltage gated potassium channels leading to the eflux of potassium out of the cell. This causes the cell to depolarise, meaning the inside of the cell is now positive compared to the outside.
The action potenial starts in the axon hilock as there is a high density of voltage gated sodium channels here, it is also where graded potentials need to reach the threshold potential to cause a action potential. If the do not reach the supratheshold level, then an Action Potenitial is not triggered and the graded potenital is known as subthreshold.
The higher the density of the myelin sheaths and higher the membrane resistance of the myelinated axon, the faster the axon potential can travel.
The point at which the membrane of an axon is depolarised causes a local circuit to be set up between the depolarised region and the region either side of it. This causes the resting at regions either side to become depolarised also. In this way the action potential sweeps along the axon.
The refractory period prevents the action potential from travelling backwards.