Potential for action: what is it and what are its phases?

What we think, what we feel, what we do … it all 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 information that this and the environment provides us.

The functioning of this system is based on the transmission of bioelectric impulses through the various neural networks available to us. Transmission involves a series of processes of great importance, one of the main the so-called action potential.

    Potential for action: basic definition and characteristics

    It is understood as an action potential the electric wave or discharge that comes 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 electric wave which it will be transmitted through the cell membrane until it reaches the end of the axon, Causing the release of neurotransmitters or ions into the membrane of the postsynaptic neuron, generating another action potential which will eventually carry some sort of order or information to an area of ​​the body. Its appearance occurs in the axonal cone, near 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. In other words, it happens or does not happen, without intermediate possibilities. Despite this, whether the potential arises or not it can be influenced by the existence of excitatory or inhibitory potentials that facilitate or hinder it.

    All the action potentials will have the same charge, and will only vary their quantity: whether a message is more or less intense (for example the perception of pain before a blow or a blow was different) will not generate any changes. in signal strength, but will only realize action potentials more frequently.

    In addition to this and in connection with the above, it is also necessary to comment on the fact that it is not possible to add action potentials, because 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 occurs at a precise point of the neuron and must occur along each of the points that follow it, not being able to return the electrical signal.

      Phases of the action potential

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

      1. Resting potential

      This first step involves a baseline state in which no alterations have yet occurred that lead to the action potential. It’s a time when the membrane is at -70mV, its basic electrical charge. During this time, small electrical depolarizations and 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 an electrical change of sufficient excitatory intensity takes place in the membrane of the neuron (which must at least generate a change down to -65mV and in some neurons up to – 40mV) so as to generate that the sodium channels of the axon cone open, so that the sodium ions (positively charged) enter in mass.

      In turn, the sodium / potassium pumps (which normally keep the inside of the cell stable by expelling by exchanging 3 hours of sodium ions for two of potassium so that more positive ions are expelled than they enter) work. This will generate a change in the charge of the membrane, so that it reaches 30mV. This change is called depolarization.

      After that, the potassium channels start to open membrane, which is also a positive ion and penetrates massively into these, will be repelled and start to leave the cell. This will slow down depolarization, losing positive ions. This is why the electric charge will be at most 40 mV. The sodium channels close and will be inactivated for a short time (preventing summative depolarizations). A wave has been generated that cannot go back.

        3. Repolarization

        When the sodium channels are closed, it stops being able to enter the neuronAt the same time that the fact that the potassium channels remain open generates that it continues to be expelled. This is why the potential and the membrane become more and more negative.

        4.hypolarization

        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 which exceeds even that of rest. At this point, the potassium channels are closed and the sodium channels are activated again (without opening). This causes the electric charge to stop and technically there could be a new potential, more so, but the fact that it suffers from hyperpolarization means that the amount of charge that would be required for an action potential is much more. larger than the usual. The sodium / potassium pump is also reactivated.

        5. Resting potential

        The reactivation of the sodium / potassium pump generates a positive charge inside the cell, which will eventually return to its basal state, the resting potential (-70mV).

        6. The action potential and the release of neurotransmitters

        This complex bioelectric process took place from the axon cone to the end of the axon, so that the electrical signal advanced to the terminal buttons. These buttons have calcium channels that open when the potential reaches them, which causes the release of vesicles containing a neurotransmitter and expel it into the synaptic space. Thus, it is the action potential that generates the release of neurotransmitters, being the main source of transmission of nerve information in our body.

        bibliographical references

        • Gómez, M .; Espejo-Saavedra, JM; Taravillo, B. (2012). Psychobiology. CEDE PIR preparation manual, 12. CEDE: Madrid
        • Guyton, CA & Hall, JE (2012) Treatise on medical physiology. 12th edition. McGraw Hill.
        • Kandel, ER; Schwartz, JH and Jessell, TM (2001). Principles of neuroscience. Fourth edition. McGraw-Hill Inter-American. Madrid.

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