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 this and the environment to us provide.

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 processes of great importance, being one of the main known as action potential.

• Related article: "Parts of the Nervous System: anatomical structures and functions"

Action potential: basic definition and characteristics

It is understood as an action potential the wave or electrical discharge that arises from the set to 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 in it another action potential that in the long run will end up bringing some kind of order or information to some area of ​​the organism. Its onset occurs in the axonal 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 to say, it either 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 their quantity can only vary: that a message is more or less intense (for example, the perception of pain in the face of a puncture or stab will be different) will not cause changes in signal intensity, but will only cause more action potentials to be realized frequently.

In addition to this and in relation to the above, it is also worth mentioning the fact that it is not possible to add action potentials, since 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 in the neuron and has to go occurring along each of the points of this that follow, not being able to return the electrical signal behind.

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Phases of the action potential

The action potential occurs over a series of phases, ranging from 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 there have not yet been any changes that lead to the action potential. This is a time when the membrane is at -70mV, its base electrical charge. During this time some small depolarizations and electrical variations may 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 (which should at least generate a change down to -65mV and in some neurons up to -40mV) to generate that the sodium channels of the axon cone open, in such a way that the sodium ions (positively charged) enter in a massive.

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

After that, the potassium channels begin to open. of the membrane, which, since it is also a positive ion and is entering these en masse, 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 become closed, and will be inactivated for a short period of time (which prevents summative depolarizations). A wave has been generated that cannot go back.

• Related article: "What is neuronal depolarization and how does it work?"

3. Repolarization

As the sodium channels have closed, it stops being able to enter the neuron, at the same time that the fact that the potassium channels remain open causes it to continue to be expelled. That is why the potential and the membrane become more and more negative.

4. Hyperpolarization

As more and more potassium comes out, the electrical charge on the membrane becomes increasingly negative to the point of hyperpolarization: they reach a level of negative charge that even exceeds that of rest. At this time, the potassium channels are closed, and the sodium channels are activated again (without opening). This means that the electric charge stops falling and that technically there could be a new potential, more however the fact that undergoes hyperpolarization makes the amount of charge that would be necessary for an action potential is much greater than the habitual. The sodium / potassium pump is also reactivated.

5. Resting potential

The reactivation of the sodium / potassium pump causes that little by little positive charge enters inside of the cell, something that will finally generate that it returns to its basal state, the resting potential (-70mV).

6. The action potential and neurotransmitter release

This complex bioelectric process will be produced 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 potential reaches them, something that causes vesicles containing neurotransmitters to emit their content and expel it into the synaptic space. Thus, it is the action potential that generates the neurotransmitters to be released, being the main source of transmission of nervous information in our body.

Bibliographic references

• Gómez, M.; Espejo-Saavedra, J.M.; Taravillo, B. (2012). Psychobiology. CEDE PIR Preparation Manual, 12. CEDE: Madrid
• Guyton, C.A. & Hall, J.E. (2012) Treaty of 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.