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

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

June 12, 2024

What we think, what we feel, what we do ... all of this depends 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 is 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 processes of great importance, being one of the main the one known as action potential .

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

Action potential: basic definition and characteristics

It is understood as action potential the wave or electrical discharge that arises from the set to the set of changes suffered by the neuronal membrane due to the electrical variations and the relationship between the external and internal environment of the neuron.

It is a unique electric 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 eventually will end up bringing some kind of order or information to some area of ​​the organism. Its onset occurs in the axonic cone, close to the soma, where a large number of sodium channels can be observed.

The action potential has the peculiarity of following the so-called law of all or nothing. That is, either it occurs or does not occur, there being no intermediate possibilities. Despite this, whether or not the potential can be influenced by the existence of excitatory or inhibitory potentials that facilitate or hinder it.

All the action potentials are going to have the same load, and their quantity can only vary: that a message is more or less intense (for example the perception of pain before a puncture or a stab will be different) will not generate changes in the intensity of the signal, but will only cause action potentials to be realized more 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 they have a brief refractory period in which that part of the neuron can not initiate another potential.

Finally, it highlights the fact that the action potential occurs at a specific point of the neuron and has to occur along each of the points of this that follow, not being able to return the electrical signal back.

  • You may be interested: "What are axons of neurons?"

Phases of action potential

The action potential occurs throughout a series of phases, which go from the initial rest situation to the sending of the electrical signal and finally the return to the initial state.

1. Potential for rest

This first step assumes a basal state in which alterations that lead to the action potential have not yet occurred. It is a moment in which the membrane is at -70mV, its base electric charge . During this time, 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 that occurs in the membrane of the neuron an electrical change of sufficient excitatory intensity (which should at least generate a change 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 way.

In turn, sodium / potassium pumps (which normally keep the stable cell interior expelling by exchanging three sodium ions for two of potassium in such a way that more positive ions are expelled from those that enter) stop working. This will generate a change in the load 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 up of the membrane, which also being a positive ion and entering these massively will be repelled and will start to leave the cell. This will cause depolarization to slow down, as positive ions are lost. That is why at most the electric 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 can not go back.

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

3. Repolarization

Once the sodium channels have been closed, it stops being able to enter the neuron , at the same time that the fact that the potassium channels remain open generates that this continues to be expelled. That is why the potential and the membrane become increasingly negative.

4. Hyperpolarization

As more and more potassium comes out, the electrical charge of the membrane it becomes more and more negative to the point of hyperpolarizing : 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 reactivated (without opening). This causes the electric charge to stop falling and technically there could be a new potential, but nevertheless the fact that it suffers a hyperpolarization means that the amount of charge that would be necessary for an action potential is much higher than usual. The sodium / potassium pump is also reactivated.

5. Rest potential

The reactivation of the sodium / potassium pump generates little by little positive charge entering the cell, something that will finally generate a 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 axonic cone to the end of the axon, in such a way that the electrical signal will progress to the terminal buttons. These buttons have calcium channels that open when the potential reaches them, something that causes the vesicles containing neurotransmitters to emit their contents and they expel him 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 body.

Bibliographic references

  • Gómez, M .; Espejo-Saavedra, J.M .; Taravillo, B. (2012). Psychobiology CEDE Manual of Preparation PIR, 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.

013 A Review of the Action Potential (June 2024).

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