Course of the action potential in nerve cells
The action potential leads to the transmission of an electrical excitation through a change in the membrane potential. Action potentials in cells are fundamental to any form of stimulus transmission and thus a necessary condition for life.
In order to be able to measure the action potential, two measuring electrodes are required: One measuring electrode is pierced into the nerve cell and the other is held from the outside to the cell. The following figure shows the electrically measured course of an action potential.
The phases of the action potential
1. rest potential: At rest potential the membrane potential is about -70mV
2. Exceeding the threshold potential: The dendrites absorb stimuli from surrounding nerve cells and forward them via the soma to the axon hillock. In order for an action potential to be triggered, a certain threshold (in our case -50 mV) must be exceeded at the axon hill. All excitations below +20 mV trigger no action potential and there is no transmission of the stimulus. It is the "all or nothing principle", either the threshold is exceeded and the action potential runs through the axon or the threshold is not exceeded and no reaction is triggered. Consequently, there are also no levels of reaction strength. The action potential always runs the same way.
3. Depolarization: If the threshold is exceeded, the action potential is via the axon: The Na+Channels open and from outside rush abruptly Na+Ions into the cell interior of the axon. (K+-Canals are closed during this time). It comes to Umpolarisierung, the so-called Overshoot. The intracellular space is now even positively charged.
4. repolarization: The Na+ Channels are starting to close again. K+ Channels open and ensure that potassium ions can diffuse out of the positively charged cell interior. This runs because of the voltage difference also relatively quickly, because the Zellduesser is negatively charged in comparison. Result: The electrical voltage in the cell interior sinks again.
5. Hyperpolarization: The K+-Canals close. Compared to Na+Channels are the K+Channels, however, much slower and it takes around 1-2ms until they are completely closed. In time, further K + ions are diffused outwards and the voltage drops below the actual resting potential (hyperpolarization).
After the Na+ Channels have closed again in the course of repolarization, a renewed action potential immediately thereafter is not possible. This period is also called the refractory time and lasts for about 2 ms.
1. rest potential: The sodium-potassium pumps then regulate the voltage back to about -70 mV, which is the original resting potential. The axon is ready for the next action potential.