What is neuronal depolarization and how does it work?

The function of our nervous system, which includes the brain, relies on the transmission of information.. This transmission is electrochemical in nature and depends on the generation of electrical impulses called action potentials, which are transmitted by neurons at full speed. The generation of pulses is based on the entry and exit of different ions and substances in the membrane of the neuron.

Thus, this input and this output vary the conditions and the electrical charge that the cell would normally vary, initiating a process which will result in the transmission of the message. One of the steps that allows this process of transmitting information is depolarization. This depolarization is the first step in the generation of an action potential, that is to say the emission of a message.

To be able to understand depolarization, it is necessary to take into account the state of the neurons in the previous circumstances, that is, when the neuron is at rest. It is in this phase that the mechanism of events begins which will end with the appearance of an electrical impulse which will cross the nerve cell until it reaches its destiny, the areas adjacent to a synaptic space, to end up generating or not another nervous impulse. in another neuron by another depolarization.

When the neuron does not act: state of rest

The human brain is constantly functioning throughout life. Even during sleep, brain activity does not stopSimply lowers the activity of certain locations in the brain. However, neurons do not always emit bioelectric impulses, but are in a state of rest which ends up being altered to generate a message.

Under normal circumstances, at rest the membrane of neurons has a concrete electric charge of -70 mV, Due to the presence of anions or negatively charged ions inside, in addition to potassium (although the latter has a positive charge). however, the exterior has a more positive charge due to the higher presence of sodium, Positively charged, with negatively charged chlorine. This state is maintained due to the permeability of the membrane, which at rest is only easily transferable to potassium.

Although by the force of diffusion (or the tendency of a fluid to distribute itself evenly while balancing its concentration) and by the electrostatic pressure or the attraction between the charged ions the internal and external medium must be equalized, this permeability makes it very difficult. being the entry of positive ions very gradual and limited.

Outraged, neurons have a mechanism that prevents the electrochemical balance from changing, the so-called sodium and potassium pump, Which regularly expels 3 sodium ions from the inside to let in two potassium ions from the outside. In this way, more positive ions are expelled than they could enter, which keeps the internal electrical charge stable.

However, these circumstances will change as information is transmitted to other neurons, a change which, as discussed, begins with the phenomenon known as depolarization.

depolarization

Depolarization is the part of the process that initiates the action potential. In other words, it is the part of the process that causes the release of an electrical signal, which will eventually travel through the neuron to cause the transmission of information through the nervous system. In fact, if we were to reduce all mental activity to a single event, depolarization would be a good candidate for this position, because without it there is no neural activity and therefore we could not even follow life.

The phenomenon itself to which this concept refers is the sudden and large increase in electrical charge inside the neuronal membrane. This increase is due to the constant of positively charged sodium ions inside the membrane of the neuron. From the moment this phase of depolarization occurs, what follows is a chain reaction through which an electrical impulse appears that passes through the neuron and travels to an area far from where it was initiated, the plasma. its effect on a nerve terminal located next to a synaptic. space and turns off.

The role of sodium and potassium pumps

The process begins in the axon of neurons, the area in which it is located a large amount of voltage-sensitive sodium receptors. Although they are normally closed, in a state of rest, if an electrical stimulation is presented which exceeds a certain threshold of excitation (when the -70mV appears to between -65mV and -40mV) these receptors arrive at s ‘to open.

Since the interior of the membrane is very negative, the positive sodium ions will be very attracted due to the electrostatic pressure, entering in large quantities. At the same time, the sodium / potassium pump is deactivated, so no positive ions are removed.

Over time, as the interior of the cell becomes more and more positive, other channels are opened up, this time potassium, which also has a positive charge. Due to the repulsion between electric charges of the same sign, eventually potassium comes out. This slows down the increase in positive charge, up to a maximum of + 40mV inside the cell.

At this point, the channels that initiate this process, those of sodium, eventually close, so depolarization ends. In addition, for a while, they will remain inactive, thus avoiding further depolarizations. The change in polarity produced moved along the axon, in the form of an action potential, To transmit information to the next neuron.

So what?

depolarization it ends when the sodium ions stop entering and the channels of this element are finally closed. However, potassium channels that have opened due to its leakage of the incoming positive charge remain open, constantly expelling potassium.

Thus, over time, it will return to its original state, with repolarization, and even it will reach a point known as hyperpolarization in which, due to the continuous flow of sodium, the load will be less than that of the quiescent state, which will cause the potassium channels to close and the sodium / potassium pump to reactivate. Once this is done, the membrane will be ready to start the whole process again.

It is a readjustment system which makes it possible to return to the initial situation despite the changes undergone by the neuron (and its external environment) during the depolarization process. On the other hand, all this happens very quickly, in order to meet the need for functioning of the nervous system.

Bibliographical references:

  • Gil, R. (2002). Neuropsychology. Barcelona, ​​Masson.
  • Gómez, M. (2012). Psychobiology. CEDE PIR preparation manual 12. CEDE: Madrid.
  • Guyton, CA and Hall, JE (2012) Treatise on medical physiology. 12th edition. McGraw Hill.
  • Kandel, ER; Schwartz, JH and Jessell, TM (2001). Principles of neuroscience. Madrid. McGraw Hill.

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