The human nervous system acts as a kind of coordinator in our body. It transmits commands from the brain to the muscles, organs, tissues and processes the signals coming from them. A nerve impulse is used as a kind of data carrier. What does he represent? At what speed does it work? These and a number of other questions can be answered in this article.
What is a nerve impulse?
This is the name of the wave of excitation that spreads through the fibers as a response to stimulation of neurons. Thanks to this mechanism, information is transmitted from various receptors to the central nervous system. And from it, in turn, to different organs (muscles and glands). But what is this process at the physiological level? The mechanism of transmission of a nerve impulse is that the membranes of neurons can change their electrochemical potential. And the process of interest to us takes place in the area of synapses. The speed of a nerve impulse can vary from 3 to 12 meters per second. We will talk more about it, as well as about the factors that influence it.
Research of structure and work
For the first time, the passage of a nerve impulse was demonstrated by Germanscientists E. Goering and G. Helmholtz on the example of a frog. At the same time, it was found that the bioelectric signal propagates at the previously indicated speed. In general, this is possible due to the special construction of nerve fibers. In some ways, they resemble an electrical cable. So, if we draw parallels with it, then the conductors are the axons, and the insulators are their myelin sheaths (they are the membrane of the Schwann cell, which is wound in several layers). Moreover, the speed of the nerve impulse depends primarily on the diameter of the fibers. The second most important is the quality of electrical insulation. By the way, the body uses myelin lipoprotein, which has the properties of a dielectric, as a material. Ceteris paribus, the larger its layer, the faster the nerve impulses will pass. Even at the moment it cannot be said that this system has been fully investigated. Much that relates to nerves and impulses is still a mystery and a subject of research.
Features of the structure and functioning
If we talk about the path of a nerve impulse, it should be noted that the myelin sheath does not cover the fiber along its entire length. The design features are such that the current situation can best be compared with the creation of insulating ceramic sleeves that are tightly strung on the rod of an electrical cable (although in this case on the axon). As a result, there are small uninsulated electrical areas from which the ion current can easily flow out ofaxon to the environment (or vice versa). This irritates the membrane. As a result, the generation of an action potential is caused in areas that are not isolated. This process is called the intercept of Ranvier. The presence of such a mechanism makes it possible to make the nerve impulse propagate much faster. Let's talk about this with examples. Thus, the speed of nerve impulse conduction in a thick myelinated fiber, the diameter of which fluctuates within 10-20 microns, is 70-120 meters per second. Whereas for those who have a suboptimal structure, this figure is 60 times less!
Where are they made?
Nerve impulses originate in neurons. The ability to create such "messages" is one of their main properties. The nerve impulse ensures the rapid propagation of the same type of signals along the axons over a long distance. Therefore, it is the most important means of the body for the exchange of information in it. Data on irritation are transmitted by changing the frequency of their repetition. A complex system of periodicals works here, which can count hundreds of nerve impulses in one second. According to a somewhat similar principle, although much more complicated, computer electronics work. So, when nerve impulses arise in neurons, they are encoded in a certain way, and only then are they transmitted. In this case, the information is grouped into special "packs", which have a different number and nature of the sequence. All this, put together, is the basis for the rhythmic electrical activity of our brain, which can be registered thanks toelectroencephalogram.
Cell types
Speaking about the sequence of the passage of a nerve impulse, one cannot ignore the nerve cells (neurons) through which the transmission of electrical signals occurs. So, thanks to them, different parts of our body exchange information. Depending on their structure and functionality, three types are distinguished:
- Receptor (sensitive). They encode and turn into nerve impulses all temperature, chemical, sound, mechanical and light stimuli.
- Insertion (also called conductor or closing). They serve to process and switch impulses. The greatest number of them is in the human brain and spinal cord.
- Effective (motor). They receive commands from the central nervous system to perform certain actions (in the bright sun, close your eyes with your hand, and so on).
Each neuron has a cell body and a process. The path of a nerve impulse through the body begins precisely with the latter. The processes are of two types:
- Dendrites. They are entrusted with the function of perceiving irritation of the receptors located on them.
- Axons. Thanks to them, nerve impulses are transmitted from cells to the working organ.
Interesting aspect of activity
Speaking about the conduction of a nerve impulse by cells, it is difficult not to tell about one interesting point. So, when they are at rest, then, let's saythus, the sodium-potassium pump is engaged in the movement of ions in such a way as to achieve the effect of fresh water inside and s alty outside. Due to the resulting imbalance of the potential difference across the membrane, up to 70 millivolts can be observed. For comparison, this is 5% of conventional AA batteries. But as soon as the state of the cell changes, the resulting equilibrium is disturbed, and the ions begin to change places. This happens when the path of a nerve impulse passes through it. Due to the active action of ions, this action is also called the action potential. When it reaches a certain value, then reverse processes begin, and the cell reaches a state of rest.
About action potential
Speaking of nerve impulse conversion and propagation, it should be noted that it could be a miserable millimeters per second. Then the signals from the hand to the brain would reach in minutes, which is clearly not good. This is where the previously discussed myelin sheath plays its role in strengthening the action potential. And all its "passes" are placed in such a way that they only have a positive effect on the speed of signal transmission. So, when an impulse reaches the end of the main part of one axon body, it is transmitted either to the next cell, or (if we talk about the brain) to numerous branches of neurons. In the latter cases, a slightly different principle works.
How does everything work in the brain?
Let's talk about what nerve impulse transmission sequence works in the most important parts of our central nervous system. Here, neurons are separated from their neighbors by small gaps, which are called synapses. The action potential cannot cross them, so it looks for another way to get to the next nerve cell. At the end of each process are small sacs called presynaptic vesicles. Each of them contains special compounds - neurotransmitters. When an action potential arrives at them, molecules are released from the sacs. They cross the synapse and attach to special molecular receptors that are located on the membrane. In this case, the balance is disturbed and, probably, a new action potential appears. This is not yet known for certain, neurophysiologists are studying the issue to this day.
The work of neurotransmitters
When they transmit nerve impulses, there are several options for what will happen to them:
- They will diffuse.
- Will undergo chemical breakdown.
- Get back into their bubbles (this is called a recapture).
A startling discovery was made at the end of the 20th century. Scientists have learned that drugs that affect neurotransmitters (as well as their release and reuptake) can change a person's mental state in a fundamental way. So, for example, a number of antidepressants like Prozac block the reuptake of serotonin. There are some reasons to believe that a deficiency in the brain neurotransmitter dopamine is to blame for Parkinson's disease.
Now researchers who study the borderline states of the human psyche are trying to figure out how itEverything affects the mind of a person. In the meantime, we do not have an answer to such a fundamental question: what causes a neuron to create an action potential? So far, the mechanism of "launching" this cell is a secret for us. Particularly interesting from the point of view of this riddle is the work of neurons in the main brain.
In short, they can work with thousands of neurotransmitters that are sent by their neighbors. Details regarding the processing and integration of this type of impulses are almost unknown to us. Although many research groups are working on this. At the moment, it turned out to find out that all received impulses are integrated, and the neuron makes a decision - whether it is necessary to maintain the action potential and transmit them further. The functioning of the human brain is based on this fundamental process. Well then, it's no wonder we don't know the answer to this riddle.
Some theoretical features
In the article, "nerve impulse" and "action potential" were used as synonyms. Theoretically, this is true, although in some cases it is necessary to take into account some features. So, if you go into details, then the action potential is only part of the nerve impulse. With a detailed examination of scientific books, you can find out that this is only the change in the charge of the membrane from positive to negative, and vice versa. Whereas a nerve impulse is understood as a complex structural and electrochemical process. It spreads across the neuron membrane like a traveling wave of changes. Potentialactions are just an electrical component in the composition of a nerve impulse. It characterizes the changes that occur with the charge of a local section of the membrane.
Where are nerve impulses generated?
Where do they start their journey? The answer to this question can be given by any student who diligently studied the physiology of arousal. There are four options:
- Receptor ending of the dendrite. If it exists (which is not a fact), then the presence of an adequate stimulus is possible, which will first create a generator potential, and then a nerve impulse. Pain receptors work in a similar way.
- The membrane of the excitatory synapse. As a rule, this is possible only if there is a strong irritation or their summation.
- Dentrid trigger zone. In this case, local excitatory postsynaptic potentials are formed as a response to a stimulus. If the first node of Ranvier is myelinated, then they are summed up on it. Due to the presence of a section of the membrane there, which has increased sensitivity, a nerve impulse occurs here.
- Axon hillock. This is the name of the place where the axon begins. The mound is the most common to create impulses on a neuron. In all other places that were considered earlier, their occurrence is much less likely. This is due to the fact that here the membrane has an increased sensitivity, as well as a lower critical level of depolarization. Therefore, when the summation of numerous excitatory postsynaptic potentials begins, the hillock reacts to them first of all.
Example of spreading excitation
Telling in medical terms can cause misunderstanding of certain points. To eliminate this, it is worth briefly going through the stated knowledge. Let's take a fire as an example.
Remember last summer's news bulletins (also to be heard again soon). The fire is spreading! At the same time, trees and shrubs that burn remain in their places. But the front of the fire goes further and further from the place where the fire was. The nervous system works in a similar way.
It is often necessary to calm the nervous system that has begun to excite. But this is not so easy to do, as in the case of fire. To do this, they make an artificial intervention in the work of a neuron (for medicinal purposes) or use various physiological means. It can be compared to pouring water on a fire.
