Synonyms
nerve impulse, excitation potential, spike, excitation wave, action potential, electrical excitation
Definition
The action potential is a short change of the membrane potential of a cell from its rest potential. It is used to transmit electrical excitation and is therefore elementary for the transmission of stimuli.
Physiology
To understand the action potential, one must first become aware of the resting potential of a cell. Every excitable cell at rest has one. It is caused by the difference in charge between the inside and outside of the cell membrane and it depends on the cell in which height it is located.
Usually the values vary between -50 mV and -100 mV. Most nerve cells have a rest potential of -70mv, which means that in the rest state the inside of the cell membrane is negatively charged opposite the outside of the cell membrane. We now look at the development of an action potential using a nerve cell as an example.
Here, action potentials cause a rapid excitation conduction in the body over long distances. The cell has a resting membrane potential, which is maintained by the sodium–potassium pump. An excitation, triggered by a stimulus, reaches the cell.
Inflowing sodium ions make the inside of the cell more positive. If a certain threshold value is exceeded (in the case of the nerve cell approx. – 50mV) an action potential is triggered.
This works according to the “all or nothing principle”. This means that “a little action potential” does not exist, either it is created or not. The form of the action potential is always the same after the threshold value has been exceeded, regardless of the strength of the stimulus.
If the threshold value is exceeded, many sodium channels on the cell membrane open at once and from the outside many sodium ions flow into the cell interior at once. The cell becomes positive inside with up to approx. +20 to +30 mV.
This event is also called “spread” or “overshoot”. After the spread has reached its maximum, the sodium channels begin to close again. Potassium channels open, causing positively charged potassium ions to flow out of the cell and the inside of the cell becomes more negative again.
As a result of repolarisation, the rest potential is usually initially undershot and can reach values of up to – 90 mV, for example in a nerve cell with a rest potential of -70 mV. This is also called hyperpolarizing afterpotential. It is caused by the fact that the potassium channels close more slowly again and thus more positively charged potassium ions flow out of the cell.
The original ratio is then restored by the sodium-potassium pump, which transports three sodium ions out of the cell while expending energy and in return transports two potassium ions into the cell. Important for the action potential is the so-called refractory phase. It is caused by the fact that after the action potential has been triggered, the sodium channels are still inactive for a short time.
Thus, no further action potential can be triggered during the “absolute refractory time” and during the “relative refractory time” only conditionally a further action potential can be triggered. An action potential lasts about 1-2 milliseconds in nerve cells. In a heart muscle cell it can even last several hundred milliseconds.
