What is neuronal depolarization and how does it work?

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 neurons to all velocity. The generation of pulses is based on the entry and exit of different ions and substances within the membrane of the neuron.

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

In order to understand depolarization, it is necessary to take into account the state of neurons in previous circumstances, that is, when the neuron is in a state of rest. 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 reach its destination, the areas adjacent to a synaptic space, to end up generating or not another nerve impulse in another neuron through another depolarization.

When the neuron does not act: state of rest

The human brain is working steadily throughout its life. Even during sleep, brain activity does not stop, the activity of certain brain locations is simply greatly reduced. However, neurons are not always emitting bioelectric pulses, but are in a state of rest that ends up altering to generate a message.

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

Although by the diffusional force (or tendency of a fluid to distribute itself evenly balancing its concentration) and by the pressure electrostatic or attraction between the ions of opposite charge the internal and external environment should be equal, said permeability makes it difficult in big measure, the entry of positive ions being very gradual and limited.

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

However, these circumstances will change when transmitting information to other neurons, a change that, as mentioned, 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, the which will end up traveling through the neuron to cause the transmission of information through the system highly strung. In fact, if we were to reduce all mental activity to a single event, depolarization would be a good candidate. to occupy that position, since without it there is no neural activity and therefore we would not even be able to keep up with lifetime.

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

The role of sodium and potassium pumps

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

Since the inside of the membrane is very negative, the positive sodium ions will be very attracted due to the electrostatic pressure, entering in large quantity. At once, the sodium / potassium pump is inactive, thus no positive ions are removed.

Over time, as the interior of the cell becomes more and more positive, other channels are opened, this time for potassium, which also has a positive charge. Due to the repulsion between electrical charges of the same sign, the potassium ends up going to the outside. In this way, the increase in positive charge is slowed down, until reaching a maximum of + 40mV inside the cell.

At this point the channels that started this process, the sodium channels, end up closing, bringing the depolarization to an end. In addition, for a time they will remain inactive, avoiding further depolarizations. The change in polarity produced will move along the axon, in the form of an action potential, to transmit the information to the next neuron.

And then?

Depolarization ends at the moment when sodium ions stop entering and finally the channels of this element are closed. However, the potassium channels that opened due to the flight of the incoming positive charge remain open, constantly expelling potassium.

Thus, over time there will be a return to the original state, having a repolarization, and even a point known as hyperpolarization will be reached in which, due to the continuous output of sodium, the load 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 a return to the initial situation despite the changes experienced by the neuron (and by its external environment) during the depolarization process. On the other hand, all this happens very quickly, in order to respond to the need for the nervous system to function.

Bibliographic references:

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

• Gómez, M. (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. Madrid. McGraw Hill.