Regulation of nervous activity is the processes of excitation and inhibition in the central nervous system. Initially, it arises as an elementary reaction to irritation. In the process of evolution, a complication of neurohumoral functions occurred, leading to the formation of the main departments of the nervous and endocrine systems. In this article we will study one of the main processes - inhibition in the central nervous system, types and mechanisms of its implementation.
Nerve tissue, its structure and functions
One of the varieties of animal tissue, called nervous, has a special structure that provides both the process of excitation and activates the inhibition functions in the central nervous system. Nerve cells consist of a body and processes: short (dendrites) and long (axon), which ensures the transmission of nerve impulses from one neurocyte to another. The end of the axon of the nerve cell is in contact with the dendrites of the next neurocyte in places called synapses. They provide the transmission of bioelectric impulses along the nerve tissue. Moreover, the excitement always moves in one direction - from the axon to the body or the dendrites of another neurocyte.
Another property, in addition to excitement, occurring in the nervous tissue is inhibition in the central nervous system. It is a response of the body to the action of the stimulus, leading to a decrease or complete cessation of motor or secretory activity, in which centrifugal neurons participate. Inhibition in the nervous tissue can occur without prior excitation, but only under the influence of a inhibitory mediator, for example, GABA. It is one of the main braking transmitters. Here you can name such a substance as glycine. This amino acid is involved in enhancing inhibitory processes and stimulates the production of gamma-aminobutyric acid molecules in synapses.
I. M. Sechenov and his work in neurophysiology
An outstanding Russian scientist, the creator of the theory of reflex activity of the brain, proved the presence in the central parts of the nervous system of special cell complexes capable of inactivating bioelectric processes. The opening of centers of inhibition in the central nervous system became possible due to the use of three types of experiments by I. Sechenov. These include: cutting sections of the cortex in various areas of the brain, stimulating individual loci of gray matter with physical or chemical factors (electric current, sodium chloride solution), as well as the method of physiological excitation of brain centers. I.M.Sechenov was an excellent experimenter, making ultra-precise cuts in the area between the visual tubercles and directly in the frog’s thalamus itself. He observed a decrease and complete cessation of motor activity of the limbs of the animal.
So, a neurophysiologist discovered a special type of nervous process - inhibition in the central nervous system. We will consider the types and mechanisms of its formation in more detail in the following sections, and now we will once again focus on such a fact: in such departments as the medulla oblongata and visual tubercles, there is a site called the inhibitory, or “Sechenovskiy” center. The scientist also proved its presence not only in mammals, but also in humans. Moreover, I.M.Sechenov discovered the phenomenon of tonic excitation of inhibitory centers. He understood by this process small excitation in centrifugal neurons and muscles associated with them, as well as in the nerve centers of inhibition themselves.
Do nervous processes interact?
Studies of prominent Russian physiologists I.P. Pavlova and I.M.Sechenov have proved that the central nervous system is characterized by coordination of reflex reactions of the body. The interaction of the processes of excitation and inhibition in the central nervous system leads to a coordinated regulation of body functions: motor activity, respiration, digestion, excretion. Bioelectric processes simultaneously occur in the nerve centers and can consistently change in time. This provides a correlation and timely passage of response reflexes to signals of the internal and external environment. Numerous experiments conducted by neurophysiologists have confirmed the fact that excitation and inhibition in the central nervous system are key nervous phenomena, which are based on some patterns. Let us dwell on them in more detail.
The nerve centers of the cerebral cortex are able to spread both types of processes throughout the nervous system. This property is called irradiation of excitation or inhibition. The opposite phenomenon is a decrease or restriction of the area of the brain that propagates bio-impulses. It is called concentration. Scientists observe both types of interactions during the formation of conditioned motor reflexes. During the initial stage of the formation of motor skills, due to the irradiation of excitation, several muscle groups that are not necessarily involved in the performance of the formed motor act are simultaneously reduced at once. Only after repeated repetitions of the formed complex of physical movements (skating, skiing, cycling), as a result of the concentration of excitation processes in specific nerve foci of the cortex, all human movements become highly coordinated.
Switching in the work of nerve centers can also occur due to induction. It manifests itself when the following condition is met: first, the concentration of inhibition or excitation occurs, and these processes must be of sufficient strength. Two types of induction are known in science: the S-phase (central inhibition in the central nervous system enhances excitation) and the negative form (excitation causes the inhibition process). Sequential induction also occurs. In this case, the nervous process changes to the opposite in the nerve center itself. Studies of neurophysiologists have proved the fact that the behavior of higher mammals and humans is determined by the phenomena of induction, irradiation and concentration of nervous processes of excitation and inhibition.
Unconditional braking
Let us consider in more detail the types of inhibition in the central nervous system and dwell on its form, which is inherent in both animals and humans. The term itself was proposed by I. Pavlov. The scientist considered this process to be one of the innate properties of the nervous system and identified two of its types: dying and permanent. Let us dwell on them in more detail.
Suppose there is a focus of excitation in the cortex, generating impulses to the working organ (to the muscles, secretory cells of the glands). Due to changes in the conditions of the external or internal environment, another excited portion of the cerebral cortex arises. It produces higher intensity bioelectric signals, which inhibits excitation in the previously active nerve center and its reflex arc. Dimming inhibition in the central nervous system leads to the fact that the intensity of the orienting reflex gradually decreases. The explanation for this is as follows: the primary stimulus no longer causes the process of excitation in the receptors of the afferent neuron.
Another type of inhibition observed both in humans and in animals is demonstrated by the experiment conducted by Nobel Prize laureate in 1904 I.P. Pavlov. During the feeding of the dog (with the fistula removed from the cheek), the experimenters turned on a sharp sound signal - the excretion of saliva from the fistula ceased. The scientist called this type of inhibition transcendental.
Being an innate property, inhibition in the central nervous system proceeds according to the unconditionally reflex mechanism. It is quite passive and does not cause the consumption of a large amount of energy, leading to the cessation of conditioned reflexes. Constant unconditional inhibition accompanies many psychosomatic diseases: dyskinesias, spastic and flaccid paralysis.
What is a dimming brake
Continuing to study the mechanisms of inhibition in the central nervous system, let us consider what constitutes one of its types, called the damping brake. It is well known that the orientation reflex is the body's response to the effect of a new extraneous signal. In this case, a nerve center is formed in the cerebral cortex, which is in a state of excitement. It forms the reflex arc, which is responsible for the reaction of the body and is called the orientational reflex. This reflex act inhibits the conditioned reflex that is occurring at the moment. After repeated repetition of an extraneous stimulus, the reflex, called orientational, gradually decreases and finally disappears. And that means that it no longer inhibits the conditioned reflex. Such a signal was called the dying brake.
Thus, the external inhibition of conditioned reflexes is associated with the influence of an extraneous signal on the body and is an innate property of the central and peripheral nervous system. A sudden or new stimulus, for example, pain, extraneous sound, a change in light, not only causes an indicative reflex, but also helps to weaken or even completely stop the conditioned reflex arc that is currently active. If an extraneous signal (except for a pain signal) acts repeatedly, the inhibition of the conditioned reflex is less pronounced. The biological role of the unconditional form of the nervous process is to respond to the stimulus, the most important at the moment.
Internal braking
Its other name used in the physiology of higher nervous activity is conditional inhibition. The main prerequisite for the emergence of such a process is the lack of reinforcement of signals from the outside world with innate reflexes: digestive, salivary. The braking processes that occur under these conditions in the central nervous system require a certain time interval. Consider their types in more detail.
For example, differentiating inhibition occurs as a response to environmental signals that coincide in amplitude, intensity and strength to a conditioned stimulus. This form of interaction between the nervous system and the surrounding world allows the body to more finely distinguish between stimuli and isolate from their totality one that receives reinforcement with a congenital reflex. For example, a dog developed a conditional salivary response to the sound of a bell with a force of 15 Hz, backed up by a feeder with food. If we apply another sound signal to the animal, with a force of 25 Hz, without reinforcing it with food, in the first series of experiments in a dog, saliva from the fistula will be allocated to both conditioned stimuli. After some time, the animal will differentiate these signals, and for sound, with a force of 25 Hz, the saliva from the fistula will cease to stand out, that is, differential inhibition will develop.
To free the brain from information that has lost a vital role for the body - this function is exactly what the inhibition in the central nervous system performs. Physiology empirically proved that conditioned motor reactions, well fixed by the acquired skills, can be preserved throughout a person’s life, for example, ice skating, cycling.
Summing up, we can say that the processes of inhibition in the central nervous system are the weakening or termination of certain reactions of the body. They are of great importance, since all the reflexes of the body are corrected in accordance with the changed conditions, and if the conditioned signal has lost its meaning, then it can even completely disappear. Various types of inhibition in the central nervous system are basic for such abilities of the human psyche as maintaining self-control, distinguishing stimuli, and waiting.
A delayed view of the nervous process
Empirically, you can create a situation in which the body's response to a conditioned signal from the external environment is manifested even before exposure to an unconditioned stimulus, such as food. With an increase in the time interval between the beginning of the action of a conditioned signal (light, sound, for example, a metronome beats) and the moment of reinforcement up to three minutes, salivation to the aforementioned conditioned stimuli is more and more delayed and appears only at the moment when a feeder with food appears in front of the animal. A lag in the response to a conditioned signal characterizes the processes of inhibition in the central nervous system, called a delayed view, in which its flow time corresponds to the delay interval of an unconditioned stimulus, such as food.
The value of inhibition in the central nervous system
The human body, figuratively speaking, is "under the gun" of a huge number of factors of the external and internal environment, to which it is forced to react and form many reflexes. Their nerve centers and arcs form in the brain and spinal cord. Overload of the nervous system with a huge number of excited centers in the cerebral cortex negatively affects the mental health of a person, and also reduces his performance.
The biological basis of human behavior
Both types of activity of the nervous tissue, both excitation and inhibition in the central nervous system, are the basis of higher nervous activity. It determines the physiological mechanisms of human mental activity. The doctrine of higher nervous activity was formulated by I.P. Pavlov. Its modern interpretation is as follows:
- Excitation and inhibition in the central nervous system, occurring in interaction, provide complex mental processes: memory, thinking, speech, consciousness, and also form complex behavioral reactions of a person.
To compose a scientifically based mode of study, work, rest, scientists apply knowledge of the laws of higher nervous activity.
The biological significance of such an active nervous process as inhibition can be determined as follows. Changes in the conditions of the external and internal environment (lack of reinforcement of the conditioned signal by a congenital reflex) entails adequate changes in adaptive mechanisms in the human body. Therefore, the acquired reflex act is suppressed (goes out) or disappears altogether, as it becomes inappropriate for the body.
What is a dream?
I.P. Pavlov experimentally proved the fact that the processes of inhibition in the central nervous system and sleep are of the same nature. During the wakefulness of the body, against the background of the general activity of the cerebral cortex, its individual parts are nevertheless diagnosed, covered by internal inhibition. During sleep, it radiates over the entire surface of the cerebral hemispheres, reaching the subcortical formations: visual tubercles (thalamus), hypothalamus, reticular formation and limbic system. As the outstanding neurophysiologist P.K. Anokhin pointed out, all of the above parts of the central nervous system, responsible for the behavioral sphere, emotions and instincts, decrease their activity during sleep. This entails a decrease in the generation of nerve impulses coming from under the crust. Thus, the activation of the cortex is reduced. This provides the possibility of rest and restoration of metabolism both in the neurocytes of the large brain, and throughout the body as a whole.
The experiments of other scientists (Hess, Economo) established special complexes of nerve cells that are part of the non-specific nuclei of the visual tubercles. The excitation processes diagnosed in them cause a decrease in the frequency of biorhythms of the cortex, which can be regarded as a transition from an active state (wakefulness) to sleep. Studies of such areas of the brain as Silviev’s aqueduct and the third ventricle prompted scientists to the idea of a center for regulating sleep. It is anatomically connected with the area of the brain responsible for wakefulness. The defeat of this locus of the cortex due to trauma or as a result of hereditary disorders in humans leads to pathological conditions of insomnia. We also note the fact that the regulation of the inhibition process vital for the body, such as sleep, is carried out by the nerve centers of the diencephalon and subcortical nuclei: caudate, amygdala, fence and lenticular.