The human nervous system acts as a kind of coordinator in our body. It transmits commands from the brain to muscles, organs, tissues and processes the signals coming from them. As a kind of data carrier, a nerve impulse is used. What is he like? How fast does it work? These, as well as a number of other questions can be answered in this article.
What is a nerve impulse?
This is the name of the excitation wave that propagates 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 that interests us takes place in the field of synapses. The speed of the nerve impulse can vary from 3 to 12 meters per second. In more detail about it, as well as about the factors that affect it, we will talk more.
Study of the structure and work
The passage of a nerve impulse was first demonstrated by German scientists E. Goering and G. Helmholtz on the example of a frog. Then 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 a way, they resemble an electric cable. So, if we draw parallels with it, then the axons are conductors, and their myelin sheaths are insulators (they are a Schwann cell membrane that 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 the myelin lipoprotein as a material, which has the properties of a dielectric. Other things being equal, the larger its layer, the faster 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 study.
Features of the structure and functioning
If we talk about the path of the nerve impulse, it should be noted that the
myelin sheath does not
cover the fiber along its entire length. The construction features are such that the current situation will be best compared with the creation of insulating ceramic couplings, which are tightly strung on the rod of the electric cable (although in this case, on the axon). As a result, there are small uninsulated electrical areas from which the ion current can safely flow from the axon into the environment (or vice versa). This irritates the membrane. As a result of this, the generation of
action potential in areas that are not isolated is caused. This process is called Ranvier interception. The presence of such a mechanism makes it possible for the nerve impulse to propagate much faster. Let's talk about this with examples. So, the speed of a nerve impulse in a thick myelinated fiber, the diameter of which varies within 10-20 microns, is 70-120 meters per second. Whereas those with a suboptimal structure, this figure is 60 times less!
Where are they created?
Nerve impulses arise in neurons. The ability to create such "messages" is one of their main properties. A nerve impulse provides the rapid propagation of the same type of signal along axons over a long distance. Therefore, this is the most important means of the body to exchange information in it. Data on irritation is transmitted by changing the frequency of their repetition. A complex periodical system works here, which can count hundreds of nerve impulses in one second. By 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 transmitted. Moreover, the information is grouped into special “bundles”, which have a different number and nature of the sequence. All this, put together, forms the basis for the rhythmic electrical activity of our brain, which can be registered thanks to the electroencephalogram.
Cell types
Speaking about the sequence of passage of a nerve impulse, one cannot ignore
nerve cells (neurons), through which electrical signals are transmitted. So, thanks to them, different parts of our body exchange information. Three types are distinguished depending on their structure and functionality:
- Receptor (sensitive). They coded and converted into nerve impulses all temperature, chemical, sound, mechanical and light stimuli.
- Insertion (also called conductive or closing). They serve to process and switch pulses. Their greatest number is in the human brain and spinal cord.
- Effector (motor). They receive commands from the central nervous system to ensure that certain actions are performed (in bright sunshine, 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 have the function of perceiving the irritation of the receptors located on them.
- Axons. Thanks to them, nerve impulses are transmitted from cells to the working body.
An interesting aspect of the activity
Speaking about the conduction of a nerve impulse by cells, it is difficult not to talk about one interesting point. So, when they are at rest, then, let's say, the sodium-potassium pump is involved in the movement of ions in such a way as to achieve the effect of fresh water inside and outwardly salty. Thanks to the resulting imbalance of the potential difference, up to 70 millivolts can be observed on the membrane. For comparison, this is 5% of ordinary
AA batteries. But as soon as the state of the cell changes, the resulting equilibrium is violated, and the ions begin to change places. This happens when a path of 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 indicator, then reverse processes begin, and the cell reaches a state of rest.
About action potential
Speaking about the transformation of the nerve impulse and its distribution, it should be noted that it could be miserable millimeters per second. Then the signals from hand to brain would reach in minutes, which is clearly not good. This is where the previously considered myelin sheath plays its role in enhancing the action potential. And all its “omissions” are placed in such a way that they only have a positive effect on the signal transmission speed. So, when an impulse reaches the end of the main part of one body of an axon, 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 the transmission sequence of a nerve impulse that works in the most important parts of our central nervous system. Here, neurons from their neighbors are separated by small gaps, which are called synapses. The action potential cannot pass through them, so he is looking for another way to get to the next nerve cell. At the end of each process there are small sacs called presynaptic vesicles. In each of them there are special compounds - neurotransmitters. When an action potential arrives at them, molecules are released from the sacs. They cross the synapse and attach to specific molecular receptors that are located on the membrane. In this case, the equilibrium is disturbed and, probably, a new action potential appears. Reliably this is not yet known, 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 be diffused.
- Subjected to chemical cleavage.
- Go back to their bubbles (this is called re-capture).
At the end of the 20th century, a striking discovery was made. Scientists have learned that drugs that affect neurotransmitters (as well as their release and reuptake) can fundamentally change a person’s mental state. So, for example, a number of antidepressants like Prozac block the reuptake of serotonin. There are certain reasons to believe that dopamine neurotransmitter deficiency in the brain is to blame for Parkinson's disease.
Now researchers who study the borderline states of the human psyche are trying to figure out how all this affects the human mind. In the meantime, we have no answer to such a fundamental question: what makes a neuron create an action potential? While the mechanism of the "launch" of this cell is a secret for us. Of particular interest from the point of view of this riddle is the work of neurons in the brain.
In short, they can work with thousands of neurotransmitters sent by their neighbors. Details about the processing and integration of this type of pulse 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 pulses 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 is not surprising that we do not 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 called 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-electrochemical process. It spreads across the membrane of a neuron like a traveling wave of change. The action potential is just an electrical component in the composition of a nerve impulse. It characterizes the changes that occur with the charge of the local portion of the membrane.
Where are nerve impulses created?
Where do they start their journey from? The answer to this question can be given by any student who diligently studied the physiology of arousal. There are four options:
- The receptor ending of the dendrite. If it is (which is not a fact), then it is possible to have an adequate stimulus, which will first create a generator potential, and then a nerve impulse. Pain receptors work similarly.
- Membrane of the exciting synapse. As a rule, this is possible only in the presence of severe irritation or their summation.
- Trigger zone of the dentride. In this case, local exciting postsynaptic potentials are formed as a response to the stimulus. If the first interception of Ranvier is myelinated, then they are summed on it. Due to the presence of a portion of the membrane there, which has an increased sensitivity, a nerve impulse arises here.
- Axon hill. This is the place where the axon begins. The knoll is the most frequent 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 reduced critical level of depolarization. Therefore, when the summation of numerous exciting postsynaptic potentials begins, the mound first reacts to them.
An example of pervasive arousal
The narrative in medical terms can cause misunderstanding of certain points. To eliminate this, it is worth a brief walk through the knowledge presented. Take fire as an example.
Remember the summaries of the news of last summer (also this will soon be heard again). Fire is spreading! At the same time, the trees and shrubs that burn remain in their places. But the front of the fire goes farther from the place where there was a source of ignition. The nervous system works similarly.
It is often necessary to calm the onset of nervous system excitement. But this is not as 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. This can be compared to flooding a fire with water.