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 at full speed. The generation of pulses is based on the entry and exit of different ions and substances within the neuron membrane.
Thus, this input and output causes the conditions and 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 the neurons in circumstances prior to this, that is, 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 nervous impulse in another neuron. through another depolarization.
The human brain is functioning constantly 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 resting state that ends up being altered to generate a message.
Under normal circumstances, In a resting state, 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 outside 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 passable by potassium.
Although due to the diffusional force (or tendency of a fluid to distribute itself uniformly, balancing its concentration) and due to the electrostatic pressure or attraction between ions of opposite charge, the internal and external medium should be equal, this permeability greatly makes it difficult, the entry of positive ions being very gradual and limited
Besides, 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 potassium ions 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 it comes to transmitting information to other neurons, a change that, as mentioned, begins with the phenomenon known as depolarization.
The 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 end up traveling 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 phenomenon itself to which this concept refers is the sudden large increase in electrical charge inside the neuronal membrane This increase is due to the constant number 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 that runs through the neuron and travels to an area far from where it was initiated, expressing its effect. into a nerve terminal located next to a synaptic space and becomes extinct.
The role of sodium and potassium pumps
The process begins in the axon of the neurons, the area in which it is located a high number of voltage-sensitive sodium receptors Although they are normally closed, in a resting state, if electrical stimulation occurs that exceeds a certain excitation threshold (going from -70mV to between -65mV and -40mV) these receptors 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 quantities. At once, the sodium/potassium pump is inactivated, so positive ions are not removed
Over time, as the inside of the cell becomes more and more positive, other channels are opened, this time potassium, which also has a positive charge. Due to the repulsion between electric charges of the same sign, potassium ends up coming out. In this way, the increase in positive charge is stopped, 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 depolarization to an end. Furthermore, for a time they will remain inactive, avoiding new depolarizations. The change in polarity produced will be transferred along the axon, in the form of an action potential to transmit the information to the next neuron.
The depolarization It ends when sodium ions stop entering and the sodium channels finally close However, the potassium channels that opened due to the potassium’s escape from the incoming positive charge remain open, and potassium is constantly expelled.
Thus, over time a return to the original state will occur, resulting in repolarization, and even a point known as hyperpolarization will be reached in that due to the continued 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 entire process again.
It is a readjustment system that allows us to return to the initial situation despite the changes experienced by the neuron (and 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 functioning of the nervous system.
