The human nervous system is peculiarcoordinator in our body. It transmits commands from the brain to the musculature, organs, tissues and processes signals coming from them. As a kind of data carrier, a nerve impulse is used. What is it? How fast does it work? These questions, as well as a number of other questions, can be found in this article.
What is the nerve impulse?
This is the name of the wave of excitation thatspreads through the fibers as a response to neuronal irritation. 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). And what is this process is at the physiological level? The mechanism of nerve impulse transmission is that the membranes of neurons can change their electrochemical potential. And the process that interests us is accomplished in the field of synapses. The velocity of the nerve impulse can vary within the range of 3 to 12 meters per second. More details about it, as well as the factors that affect it, we'll talk more.
Study of structure and work
For the first time the passage of a nerve impulse wasdemonstrated by German scientists E. Goering and H. Helmholtz on the example of a frog. At the same time, it was established that the bioelectric signal propagates with the speed indicated above. In general, this is possible due to the special construction of nerve fibers. In some ways they resemble an electrical cable. So, if you draw parallels with it, the conductors are axons, and the insulators are their myelin sheaths (they are a membrane of the Schwann cell, which is wound in several layers). And the speed of the nerve impulse depends primarily on the diameter of the fibers. The second in importance is the quality of electrical insulation. By the way, the lipoprotein myelin is used as a material by the body, which has the properties of a dielectric. Other things being equal, the larger the layer, the faster the nerve impulses will pass. Even at the moment it can not be said that this system has been fully investigated. Much that concerns nerves and impulses, still remains a riddle and a subject of research.
Features of the structure and functioning
If we talk about the path of a nerve impulse, thenit should be noted that the fiber does not cover its entire length with the myelin sheath. The design features are such that the situation is best compared with the creation of insulating ceramic couplings, which are densely threaded on the rod of the electrical cable (although in this case the axon). As a result, there are small non-isolated electrical sections from which the ion current can safely flow from the axon to the environment (or vice versa). This irritates the membrane. As a result, the action potential is generated in areas that are not isolated. This process is called Ranvier interception. The presence of such a mechanism makes it possible to make the nerve impulse spread much faster. Let's talk about this with examples. Thus, the speed of the nerve impulse in a thick myelinated fiber, the diameter of which varies within the range of 10-20 microns, is 70-120 meters per second. While those who have a non-optimal structure, this figure is less than 60 times!
Where are they created?
Nerve impulses arise in neurons. The possibility of creating such "messages" is one of their main properties. The nerve impulse ensures rapid propagation of the same type of signals along axons over a long distance. Therefore, this 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 occurrence. Here there is a complex system of periodicals that can count hundreds of nerve impulses in one second. By a somewhat similar principle, although considerably more complicated, computer electronics works. So, when nerve impulses arise in neurons, they are coded in a certain way, and only then they are transmitted. Thus the information is grouped in special "packs", which have different number and character of following. All this, combined together, is the basis for the rhythmic electrical activity of our brain, which can be registered thanks to an electroencephalogram.
Types of cells
Speaking about the sequence of passagenervous impulse, you can not ignore the nerve cells (neurons), through which the transmission of electrical signals. So, thanks to them, different parts of our body exchange information. Depending on their structure and functional, three types are distinguished:
- Receptor (sensitive). They are encoded and transformed into nerve impulses of all temperature, chemical, sound, mechanical and light stimuli.
- Insert (also called conductor or closing). They serve to process and switch impulses. The largest number of them is in the human brain and spinal cord.
- Effectoral (motor). They receive commands from the central nervous system to ensure that certain actions have been performed (in the bright sun, close your eyes and so on).
Each neuron has a cell body and an outgrowth. The path of the nerve impulse along 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 organ.
Interesting aspect of activity
Speaking about carrying out a nerve impulse by cells,it's hard not to tell about one interesting moment. So, when they are at rest, then, let's say, the sodium-potassium pump moves the ions in such a way as to achieve the effect of fresh water inside and salty outwardly. Due to the resulting imbalance of the potential difference, up to 70 millivolts can be observed on the membrane. For comparison, this is 5% of conventional AA batteries. But as soon as the state of the cell changes, the resulting equilibrium is broken, 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 index, then the reverse processes begin, and the cell reaches a state of rest.
On the action potential
Speaking about the transformation of a nerve impulse and itsdistribution, it should be noted that it could amount to a miserable millimeter per second. Then the signals would pass from the hand to the brain in minutes, which is clearly not good. Here and plays a role in enhancing the potential of the action previously considered the shell of myelin. And all of its "gaps" are placed in such a way that they only have a positive effect on the speed of signal transmission. So, when the end of the main part of one axon body is reached by the impulse, it is transmitted either to the next cell, or (if to speak of the brain) to the numerous branches of the neurons. In the latter cases, a slightly different principle works.
How does it work in the brain?
Let's talk, what transferthe nerve impulse sequence works in the most important parts of our CNS. Here neurons from their neighbors are separated by small slits, which are called synapses. The potency of action can not pass through them, so it seeks a different way to get to the next nerve cell. At the end of each process there are small sacs, which are called presynaptic vesicles. In each of them there are special compounds - neurotransmitters. When the action potential comes to them, then the molecules are released from the sacs. They cross the synapse and join the special molecular receptors that are located on the membrane. At the same time, the equilibrium is disturbed and, probably, a new action potential appears. It is not certain yet, neurophysiologists are engaged in studying the issue to this day.
When they transmit nerve impulses, then there are several options that will happen to them:
- They will diffuse.
- Undergo chemical cleavage.
- Go back to their bubbles (this is called a reverse 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 ejection and re-acquisition) can change a person's mental condition in a fundamental way. So, for example, a number of antidepressants like "Prozac" block the re-seizure of serotonin. There are some reasons to believe that Parkinson's disease is responsible for the deficiency in the brain of the neurotransmitter dopamine.
Now researchers who study borderthe state of the human psyche, trying to figure out how this all affects the human mind. In the meantime, we do not have an answer to such a fundamental question: what makes a neuron create the potential for action? While the mechanism of "launching" this cell for us is a secret. Particularly interesting from the point of view of this puzzle is the work of the neurons of the brain.
In short, they can work with thousandsneurotransmitters, which are sent by their neighbors. Details regarding the processing and integration of this type of impulse are almost unknown to us. Although there are many research groups working on this. At the moment it turned out that all the impulses received are integrated, and the neuron decides whether it is necessary to maintain the action potential and transfer them further. On this fundamental process, the functioning of the human brain is based. Well, then it's no wonder that we do not know the answer to this riddle.
Some theoretical features
In the article "nervous 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 we go into details, then the action potential is only a part of the nerve impulse. With a detailed review of the scientific books, one can learn that this is only called a change in the membrane charge from positive to negative, and vice versa. Whereas a nervous impulse is understood as a complex structural-electrochemical process. It propagates along the neuron's membrane as a traveling wave of changes. 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 the nerve impulses created?
Where do they start their journey? The answer to this question can be given by any student who diligently studied the physiology of excitement. There are four options:
- Receptor end of 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 in a similar way.
- Membrane of excitatory synapse. As a rule, this is possible only if there is strong irritation or their summation.
- Trigger zone of dentrida. In this case, local excitatory postsynaptic potentials are formed as a response to the stimulus. If the first interception of Ranvier is myelinated, then they are summed up on it. Due to the presence of a membrane site, which has a high sensitivity, a nerve impulse appears here.
- Axon mound. This is the place where the axon begins. A hillock is the most frequent 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 mound reacts first of all to them.
Example of propagating excitation
The narrative with medical terms can cause misunderstanding of certain moments. To eliminate this, it is worthwhile to briefly go over the above knowledge. As an example, let's take a fire.
Recollect summaries from last summer's news (alsoit will soon be possible to hear again). The fire is spreading! In this case, the trees and shrubs that burn, remain in their places. But the front of the fire goes farther from the place where the fire was. The nervous system works in a similar way.
Often it is necessary to calm theexcitation of the nervous system. But it is not so easy to do, as in the case of fire. For this purpose, they perform artificial interference in the work of a neuron (for therapeutic purposes) or use various physiological means. This can be compared to flooding a fire with water.