Action potential: what is it and what are its phases?

We explain how the electrical impulses that run through our neurons work.

What we think, what we feel, what we do... all depend to a great extent 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 environment provide us with.

The functioning of this system is based on the transmission of bioelectrical pulses through the different neuronal networks available to us. This transmission involves a series of processes of great importance, one of the main ones being known as action potential.

Action potential: basic definition and characteristics

It is understood as 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. due to electrical variations and the relationship between the external and internal environment of the neuron.

It is a single electrical wave that is transmitted through the cell membrane until it reaches the end of the axon, causing the emission of neurotransmitters.This causes the emission of neurotransmitters or ions to the membrane of the postsynaptic neuron, generating in it another action potential that will eventually carry some kind of order or information to some area of the organism. Its onset 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 all-or-nothing law. That is to say, either it is produced or it is not produced, with no intermediate possibilities. Nevertheless, whether or not the action potential is produced or not, there are no intermediate possibilities. can be influenced by the existence of excitatory or inhibitory potentials that facilitate or hinder it. that facilitate or hinder it.

All action potentials will have the same charge, and can only vary in quantity: the fact that a message is more or less intense (for example, the perception of Pain after a puncture or a stab wound will be different) will not generate changes in the intensity of the signal, but will only cause action potentials to be performed more frequently.

In addition, 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. have a brief 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, not being able to return the electrical signal back.

Phases of the action potential

The action potential is produced in a series of phases, ranging from the initial resting situation to the from the initial resting state to the sending of the electrical signal and finally back to the initial state and finally the return to the initial state.

Resting potential

This first step involves a basal state in which no alterations leading to the action potential have yet occurred. It is a moment in which the membrane is at -70mV, its basal electric charge. During this moment some small depolarizations and electrical variations may reach the membrane, but they are not sufficient to trigger the action potential.

2. Depolarization

In this second phase (or first phase of the potential itself), the stimulation generates an electrical change of sufficient excitatory intensity in the membrane of the neuron (which should at least generate a change to -65mV and in some neurons to -40mV) to cause the sodium channels of the axon cone to open, so that sodium ions (positively charged) enter massively.

In turn, the sodium/potassium pumps (which normally keep the interior of the cell stable by exchanging three sodium ions for two potassium ions so that more positive ions are expelled than enter) cease to function. This will generate a change in the charge of the membrane, such that it reaches 30mV. This change is known as depolarization.

After this, the potassium channels begin to open. The positive ions are also a positive ion and are massively entering the membrane, so they will be repelled and will start to leave the cell. This will cause depolarization to slow down, as positive ions are lost. This is why the electrical charge will be at most 40 mV. The sodium channels become closed, and will be inactivated for a short period of time (which prevents summation depolarizations). A wave has been generated that cannot be reversed.

3. Repolarization

As the sodium channels have closed, sodium is no longer able to enter the neuron.The fact that the potassium channels remain open means that potassium continues to be expelled. This is why the potential and the membrane become increasingly negative.

4. Hyperpolarization

As more and more potassium is released, the electrical charge on the membrane becomes more and more negative to the point of becoming hyperpolarizedThe potassium channels close, and the sodium channels become active again (without opening). At this moment the potassium channels close, and the sodium channels become active again (without opening). This causes the electrical charge to stop dropping and technically there could be a new potential, but nevertheless the fact that it undergoes 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.

5. Resting potential

The reactivation of the sodium/potassium pump causes positive charge to gradually enter the cell, which will eventually cause it to return to its basal state, the resting potential (-70mV).

6. The action potential and the release of neurotransmitters

This complex bioelectrical process will be produced from the axonal 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, which causes the vesicles containing neurotransmitters to open. vesicles containing neurotransmitters emit their contents and expel them into the synaptic space. and expelled into the synaptic space. Thus, it is the action potential that generates the release of neurotransmitters, being the main source of transmission of nervous information in our organism.

Bibliographic references

• Gómez, M.; Espejo-Saavedra, J.M.; Taravillo, B. (2012). Psychobiology. Manual CEDE de Preparación PIR, 12. CEDE: Madrid.
• Guyton, C.A. & Hall, J.E. (2012) Treatise on medical physiology. 12th edition. McGraw Hill.
• Kandel, E.R.; Schwartz, J.H. & Jessell, T.M. (2001). Principles of neuroscience. Fourth edition. McGraw-Hill Interamericana. Madrid.