An action potential is an electrical signal that can be propagated within a cell. This leads to a temporary change in the resting potential of the respective cell. This complicated-looking process is nothing more than a kind of can telephone of the cell. The action potential in your nerve cells (neurons) is particularly important. But how exactly does it work?
Action potential – process
An action potential does not arise spontaneously but is the result of a series of cascades.
An action potential usually occurs in response to a stimulus. These can be physical stimuli, such as pressure, which act directly on specialized sensory cells. Much more frequently, however, nerve cells are prompted to form an action potential by chemical stimuli, i.e. neurotransmitters, which are released at synapses.
Generation of the action potential
That action potential a neuron is a temporary change in the resting potential that can only spread via the axon of the nerve cell and arises at its axon hillock.
At the beginning there is an irritation of the nerve cell body (soma) of the neuron. Chemical signals arrive there (usually from other neurons) which depolarize the soma.
One speaks of a depolarization of the membrane potential (resting potential), the PSP (postsynaptic potential) or more precisely the EPSP (excitatory postsynaptic potential).
As you may have heard before, there is also an IPSP (Inhibitory Postsynaptic Potential). This causes a hyperpolarization of the membrane potential of the soma. That is, the cell is no longer excitable.
When the EPSP reaches a certain threshold, called the threshold potential, an action potential is triggered at the neuron’s axon hillock. Before that, the so-called resting potential is present.
The duration and amplitude of the potential is always the same. With a strong stimulus, only the frequency of the action potentials is increased, but not the duration of the individual potential. Therefore, one can speak of an all-or-nothing principle here.
Action potentials can also arise in muscle cells. This happens when a stimulus to move a muscle arrives. The stimulus is transmitted through the cells of the muscle in the form of an action potential, which leads to the contraction of the muscle.
The pathway of the action potential
The action potential travels along the axon membrane to the synaptic terminal button. The incoming action potential causes neurotransmitters to be released into the synaptic cleft. As a result, an EPSP can occur again in the next neuron on the soma and ultimately lead to the transmission of stimuli.
Figure 1: Transmission of excitation in a neuron
Experimental triggers of an action potential
An action potential can also be triggered by external influences. It works by stimulating the axon with an electrical signal. Various experiments have shown that an action potential tends to run both in the direction of the terminal button and in the direction of the soma of the nerve cell. This was observed by stimulating an axon at its center.
One can therefore say that an action potential is only directed insofar as the action potential normally arises at the axon hillock and can therefore only run in the direction of the end button.
Course of an action potential according to the ion theory
An action potential is characterized by a charge reversal at the axon membrane. As the action potential propagates across the membrane, the selective permeability of the membrane changes. This creates the voltage curve typical of an action potential. This process is described by the ion theory.
The ion theory, also known as the Hodgkin-Huxley model after its discoverers, describes the development of the resting and action potentials on cell membranes. According to this theory, the resting potential is a consequence of the specific distribution of ions inside and outside a selectively permeable membrane. The action potential then occurs through the opening of ion channels.
In Figure 2 you can see the time course of the voltage of an action potential on a cell membrane.
Figure 2: Course of the action potential
1. The depolarization
One speaks of depolarization when a potential decreases. In this case, the charge difference between the inside and outside of the axonal membrane is reduced. As mentioned above, this occurs through an EPSP of the soma.
Now the resting potential, which is present on the axon without a stimulus, has a value of approximately -70 mV. It is important to know that sodium (Na+) and chloride ions (Cl-) are present on the outside of the axon membrane. Potassium ions (K+) and organic anions (A-) are found inside the membrane. The total charges inside and outside the cell are almost balanced. However, a constant ion imbalance is maintained by sodium-potassium pumping. The resting potential is largely determined by the equilibrium potential of the potassium ions.
When the ESPS arrives at the axon hillock, the membrane of the axon hillock is already slightly depolarized. If a threshold potential of around -55 mV is reached in this way, voltage-dependent sodium channels open and sodium ions flow into the nerve cell via the axon membrane. Due to the influx of positive charge, the membrane potential becomes more positive and is at its peak. peak) between +30 mV and +40 mV. A charge reversal has therefore taken place.
2. The repolarization
After the membrane potential has assumed a clearly positive value at its peak due to the depolarization, the permeability of the axon membrane for Na+ decreases again. The voltage-gated potassium ion channels now open. As a result, potassium ions flow outwards from the axon interior. As positively charged ions flow out of the axon, the membrane potential becomes more negative. This is called repolarization.
3. Hyperpolarization
The increased outflow of potassium ions means that the resting potential of -70 mV is often briefly undershot. It can even go as low as -100mV, so it gets even more negative. This is because the voltage-gated potassium channels react sluggishly to reaching the resting potential and only close again with a slight delay.
4. Restoring the resting potential
Now, after de- and repolarization, there is a state in which the sodium ions are on the inside of the axon and the potassium ions are on the outside. In order for the resting potential to be restored with the initial ion distribution, there is the sodium-potassium pump. Through these, the concentrations are created with potassium ions in the axon and sodium ions outside.
In the Sodium Potassium Pump is an energy-dependent transporter. With each pumping process, this pumps three sodium ions out and two potassium ions into the cell. This ensures an energy-dependent return to the resting potential.
5. The refractory period
You may have heard of the refractory period in connection with the action potential. This is a protection against overexcitation of a neuron.
as refractory period is the period of time after an action potential has expired, in which no new action potential can be triggered at the axon.
The absolute refractory period
The absolute refractory phase occurs immediately after the start of depolarization and is characterized by the fact that no action potential can be triggered. The strength of the incoming stimulus is irrelevant. No action potential can be generated because the sodium ion channels have not yet regenerated. The threshold for triggering a potential increases to infinity.
The relative refractory period
During the relative refractory phase, which occurs after repolarization, the cells can already be re-excited. However, stronger stimuli are required and the action potential is weaker overall. The threshold is approaching normal again.
In the heart muscle cells, the absolute refractory phase of up to 250 ms is significantly longer than that of normal skeletal muscle cells. There it is around 1 to 2 ms. This long refractory period is a protective mechanism to allow for targeted excitation of cardiac muscle cells. This enables a productive heartbeat that pumps blood through the body in a targeted manner.
Action Potential – The Most Important
- An action potential is an electrical signal that is used to transmit excitation in the nervous system
- Action potentials can be evoked naturally or experimentally
- The action potential is only triggered above a threshold value according to the all-or-nothing principle
- the order of the steps of the action potential are: depolarization – repolarization – hyperpolarization – resting potential
- Sodium ion channels and potassium ion channels on the axon membrane give rise to an action potential
- During the refractory period, a nerve cell cannot be excited or can only be excited with great difficulty