What is neuronal depolarization and how does it work?

This is the basic mechanism that causes neurons to conduct electricity through the axon.

The functioning of our nervous system, which includes the brain, is based on the transmission of information.. This transmission is electrochemical in nature, and depends on the generation of electrical pulses known as action potentials, which are transmitted through the neurons at full speed. The generation of pulses is based on the input and output of different ions and substances within the neuron membrane.

Thus, this input and output causes the conditions and the electrical charge that the cell normally has to vary, initiating a process that culminates with the emission of the message. One of the steps that allows this process of information transmission is depolarization.. This depolarization is the first step in the generation of an action potential, i.e. the emission of a message.

In order to understand depolarization, it is necessary to take into account the state of neurons in circumstances prior to this, i.e. when the neuron is in a resting state. It is in this phase when the mechanism of events begins that will end in the appearance of an electrical impulse that will travel through the nerve cell until it reaches its destination, the areas adjacent to a synaptic space, to end up generating or not another nerve impulse in another neuron by means of another depolarization.

When the neuron does not act: resting state

The human brain is constantly functioning throughout its life. Even during sleep, brain activity does not cease.It is simply that the activity of certain brain locations is greatly reduced. However, neurons are not always emitting bioelectrical pulses, but are in a resting state that is eventually altered to generate a message.

Under normal circumstances, in a resting state, the membrane of neurons has a specific electrical charge of -70 mVdue to the presence of negatively charged anions or ions inside it, in addition to potassium (although this has a positive charge). However, the outside has a more positive charge due to the presence of anions or negatively charged ions, the outside has a more positive charge due to the greater presence of positively charged sodium, together with positively charged sodium and potassium.positively charged, along with negatively charged chlorine. This state is maintained due to the permeability of the membrane, which at rest is only easily penetrated by potassium.

Although the diffusional force (or tendency of a fluid to spread evenly, balancing its concentration) and the electrostatic pressure or attraction between ions of opposite charge should equalize the internal and external medium, this permeability makes it very difficult, the entry of positive ions is very gradual and limited..

In addition, neurons have a mechanism that prevents the entry of positive ions, neurons have a mechanism that prevents the electrochemical balance from changing, the so-called sodium-potassium pump, which regularly ejects three positive ions from the neuron.which regularly ejects three sodium ions from the inside to let in two potassium ions from the outside. In this way, more positive ions are expelled than could ever enter, keeping the internal electrical charge stable.

However, these circumstances will change when it comes to transmitting information to other neurons, a change that, as mentioned above, 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 an electrical signal to be released, which will eventually travel through the neuron to cause the transmission of information through the nervous system. In fact, if we had to reduce all mental activity to a single event, depolarization would be a good candidate for that position, since without it there is no neuronal activity and therefore we would not even be able to stay alive.

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

The role of sodium and potassium pumps

The process begins in the axon of the neurons, an area in which there are a large number of sodium voltage-sensitive sodium receptors are located in the axon of the neuron.. Although they are normally closed, in a resting state, if an electrical stimulation is presented that exceeds a certain threshold of excitation (going from -70mV to between -65mV and -40mV) these receptors open.

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

Over time, as the interior of the cell becomes increasingly positive, other channels are opened, this time for potassium, which is also positively charged. Due to the repulsion between electric charges of the same sign, the potassium ends up flowing out. In this way, the increase in positive charge is slowed down, until it reaches a maximum of +40mV inside the cell..

At this point the channels that initiated this process, the sodium channels, end up closing, so that depolarization comes to an end. In addition, they will remain inactive for a time, preventing further depolarization. The change in polarity produced will be transferred along the axon, in the form of an action potential, in order to transmit the information to the next one.to transmit the information to the next neuron.

And then?

Depolarization ends as soon as sodium ions stop entering and the sodium channels are closed.. However, the potassium channels that opened due to the escape of the incoming positive charge remain open, with potassium being constantly expelled.

Thus, over time there will be a return to the original state, with a repolarization, and even a point known as the a point known as hyperpolarization will be reached in which, due to the continuous outflow of sodium, the charge will be less than that of the resting state, which will cause the closure of the potassium channels and the reactivation of the sodium/potassium pump. Once this is done, the membrane will be ready to start the whole process again.

It is a readjustment system that allows to return to the initial situation in spite of the changes experienced by the neuron (and by its external environment) during the depolarization process. On the other hand, all this occurs very quickly, in order to respond to the need for the functioning of the nervous system.

Bibliographical references:

• Gil, R. (2002). Neuropsychology. Barcelona, Masson.

• Gómez, M. (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. Madrid. McGraw Hill.