Action Potential: What Is It And What Are Its Phases?
What we think, what we feel, what we do… all of this depends largely on our Nervous System, thanks to which we can manage each of the processes that occur in our body and receive, process and work with the information that it and the medium they provide us.
The operation of this system is based on the transmission of bioelectric pulses through the different neural networks that we have. This transmission involves a series of very important processes, one of the main ones being known as action potential
Action potential: basic definition and characteristics
It is understood as an action potential the wave or electrical discharge that arises from the set of changes that the neuronal membrane undergoes due to electrical variations and the relationship between the external and internal environment of the neuron.
It is a single electrical wave that It will be transmitted through the cell membrane until it reaches the end of the axon, causing the emission of neurotransmitters or ions to the membrane of the postsynaptic neuron, generating another action potential in it that in the long run will end up bringing some type of order or information to some area of the organism. Its initiation occurs in the axon cone, close to the soma, where a large number of sodium channels can be observed.
The action potential has the particularity of following the so-called law of all or nothing. That is, either it occurs or it does not occur, there being no intermediate possibilities. Despite this, whether or not the potential appears can be influenced by the existence of excitatory or inhibitory potentials that facilitate or hinder it.
All action potentials will have the same charge, and can only vary their quantity: whether a message is more or less intense (for example, the perception of pain when faced with a puncture or a stab will be different) will not generate changes in the intensity of the signal, but it will only cause action potentials to be carried out more frequently.
In addition to this and in relation to the above, it is also worth commenting on the fact that it is not possible to add action potentials, since They have a short refractory period in which that part of the neuron cannot initiate another potential.
Finally, it highlights the fact that the action potential is produced at a specific point of the neuron and must be produced along each of the points of the neuron that follow it, and the electrical signal cannot return back.
Phases of the action potential
The action potential occurs through a series of phases, ranging from the initial rest situation to the sending of the electrical signal and finally the return to the initial state.
1. Resting potential
This first step assumes a basal state in which no alterations have yet occurred that lead to the action potential. This is a moment in which the membrane is at -70mV, its base electric charge During this moment some small depolarizations and electrical variations can reach the membrane, but they are not enough to trigger the action potential.
2. Depolarization
This second phase (or first of the potential itself), the stimulation generates an electrical change of sufficient excitatory intensity to occur in the membrane of the neuron (which must at least generate a change up to -65mV and in some neurons up to – 40mV) to cause the sodium channels of the axon cone to open, in such a way that sodium ions (positively charged) enter en masse.
In turn, the sodium/potassium pumps (which normally keep the inside of the cell stable by exchanging three sodium ions for two potassium ions in such a way that more positive ions are expelled than those that enter) stop working. This will generate a change in the charge of the membrane, so that it reaches 30mV. This change is what is known as depolarization.
After this, the potassium channels begin to open of the membrane, which, since it is also a positive ion and is entering massively, will be repelled and will begin to leave the cell. This will cause depolarization to slow down, as positive ions are lost. That is why at most the electrical charge will be 40 mV. The sodium channels begin to close, and will be inactivated for a short period of time (which prevents additive depolarizations). A wave has been generated that cannot be reversed.
3. Repolarization
As the sodium channels have closed, it is no longer able to enter the neuron, at the same time that the fact that the potassium channels remain open means that it continues to be expelled. This is why the potential and the membrane become increasingly negative.
4. Hyperpolarization
As more and more potassium leaves, the electrical charge on the membrane becomes more and more negative to the point of hyperpolarization: they reach a level of negative charge that even exceeds that of rest. At this moment the potassium channels close, and the sodium channels are activated again (without opening). This causes the electrical charge to stop decreasing and technically there could be a new potential, however the fact that it suffers hyperpolarization means that the amount of charge that would be necessary for an action potential is much greater than usual. The sodium/potassium pump is also reactivated.
The reactivation of the sodium/potassium pump causes positive charge to gradually enter the cell, something that will ultimately cause it to return to its basal state, the resting potential (-70mV).
6. The action potential and the release of neurotransmitters
This complex bioelectric process will occur from the axon cone to the end of the axon, in such a way that the electrical signal will advance to the terminal buttons. These buttons have calcium channels that open when the potential reaches them, something that causes vesicles containing neurotransmitters to emit their contents and expel it into the synaptic space. Thus, it is the action potential that causes neurotransmitters to be released, being the main source of transmission of nervous information in our body.
