The nervous system plays a leading role in ensuring the integrity of the body, as well as in its regulation. These processes are carried out by the anatomical and physiological complex, which includes the departments of the central nervous system (central nervous system). It has its own name - the nerve center. The properties by which it is characterized: occlusion, central relief, rhythm transformation. They and some others will be studied in this article.
The concept of the nerve center and its properties
Earlier, we identified the main function of the nervous system - integrating. It is possible due to the structures of the brain and spinal cord. For example, the respiratory nerve center, whose properties are the innervation of respiratory movements (inhalation and exhalation). It is located in the fourth ventricle, in the area of the reticular formation (medulla oblongata). According to the research of N. A. Mislavsky, it consists of symmetrically placed parts responsible for inhaling and exhaling.
In the upper zone of the warolium bridge is the pneumotactic section, which regulates the above parts and structures of the brain responsible for respiratory movements. Thus, the general properties of the nerve centers provide for the regulation of the physiological functions of the body: cardiovascular activity, excretion, respiration and digestion.
The theory of dynamic localization of functions I. P. Pavlova
According to the scientist, fairly simple reflex actions have stationary zones in the cerebral cortex, as well as in the spinal cord. Complex processes, such as memory, speech, thinking, are associated with certain parts of the brain and are an integrative result of the functions of many of its parts. The physiological properties of the nerve centers also determine the formation of the basic processes of higher nervous activity. In neurology, from an anatomical point of view, parts of the central nervous system, consisting of the afferent and efferent parts of neurons, began to be called nerve centers. They, as the Russian scientist P.K. Anokhin believed, form functional systems (an association of neurons that perform similar functions and can be found in various parts of the central nervous system).
Excitation irradiation
Continuing to study the basic properties of the nerve centers, we dwell on the form of the distribution of two main processes occurring in the nervous tissue - excitation and inhibition. It is called irradiation. If the strength of the stimulus and the duration of its action are large, nerve impulses diverge along the processes of neurocytes, as well as on intercalary neurons. They combine afferent and efferent neurocytes, causing the continuity of reflex arcs.
Consider inhibition (as a property of nerve centers) in more detail. The reticular formation of the brain provides both irradiation and other properties of nerve centers. Physiology explains the reasons that limit or hinder the spread of arousal. For example, the presence of inhibitory synapses and neurocytes. These structures perform important protective functions, as a result of which the risk of overexcitation of skeletal muscles, which can go into a convulsive state, is reduced.
Having considered the irradiation of excitation, we need to recall the following feature of a nerve impulse. It moves only from a centripetal neuron to a centrifugal neuron (for a two-neural, reflex arc). If the reflex is more complex, then interneurons are formed in the brain or spinal cord - inserted nerve cells. They take excitement from an afferent neurocyte and then transmit it to motor nerve cells. In synapses, bioelectrical impulses are also unidirectional: they move from the presynaptic membrane of the first nerve cell, then to the synaptic cleft, and from it to the postsynaptic membrane of another neurocyte.
Summation of nerve impulses
We continue to study the properties of nerve centers. The physiology of the main departments of the brain and spinal cord, being the most important and complex branch of medicine, is studying the conduct of excitation through a set of neurons that perform common functions. Their properties are summation, may be temporary or spatial. In both cases, weak nerve impulses caused by subthreshold stimuli add up (add up). This leads to copious release of acetylcholine molecules or another neurotransmitter, which generates an action potential in neurocytes.
Rhythm transformation
This term refers to the change in the frequency of excitation, which passes through the complexes of central nervous system neurons. Among the processes characterizing the properties of nerve centers is the transformation of the rhythm of impulses, which can occur as a result of the distribution of excitation to several neurons, the long processes of which form contact points on the same nerve cell (increasing transformation). If a single action potential appears in the neurocyte, as a result of the summation of the excitation of the postsynaptic potential, they speak of a lowering rhythm transformation.
Divergence and convergence of arousal
They are interconnected processes that characterize the properties of nerve centers. Coordination of reflex activity occurs due to the fact that impulses from the receptors of various analyzers simultaneously enter the neurocyte: visual, olfactory, and musculocutaneous sensitivity. In a nerve cell, they are analyzed and summed into bioelectric potentials. Those, in turn, are transmitted to other parts of the reticular formation of the brain. This important process is called convergence.
However, each neuron not only receives impulses from other cells, but also forms synapses with neighboring neurocytes. This is a divergence phenomenon. Both properties provide the spread of excitation in the central nervous system. Thus, the totality of nerve cells in the brain and spinal cord that perform common functions is the nerve center whose properties we are considering. It provides regulation of the work of all organs and systems of the human body.
Background activity
The physiological properties of nerve centers, one of which is spontaneous, that is, the background formation of electrical impulses by neurons, for example, the respiratory or digestive center, are explained by the structural features of the nervous tissue itself. It is capable of self-generation of bioelectric excitation processes even in the absence of adequate stimuli. It is due to the divergence and convergence of excitation, which we examined earlier, that neurocytes receive impulses from excited nerve centers through postsynaptic connections of the same reticular formation of the brain.
Spontaneous activity can be caused by microdoses of acetylcholine entering the neurocyte from the synaptic cleft. Convergence, divergence, background activity, as well as other properties of the nerve center and their characteristics directly depend on the level of metabolism in both neurocytes and neuroglia.
Types of excitation summation
They were considered in the works of I.M.Sechenov, who proved that the reflex can be caused by several weak (subthreshold) stimuli, which quite often act on the nerve center. The properties of its cells, namely: central relief and occlusion, will be examined by us further.
With simultaneous stimulation of the centripetal processes, the response is greater than the arithmetic sum of the strength of the stimuli acting on each of these fibers. This property is called central relief. If the action of pessimal stimuli, regardless of their strength and frequency, causes a decrease in the response - this is occlusion. It is the inverse property of the summation of excitation and leads to a decrease in the strength of nerve impulses. Thus, the properties of the nerve centers — central relief, occlusion — depend on the structure of the synaptic apparatus, which consists of a threshold (central) zone and a subthreshold (peripheral) border.
Nerve tissue fatigue its role
The physiology of nerve centers, the definition, types and properties that we have previously studied and are inherent in neuron complexes, will be incomplete if we do not consider such a thing as fatigue. Nerve centers are forced to conduct through themselves a continuous series of impulses, providing the reflex properties of the central parts of the nervous system. As a result of intense metabolic processes carried out both in the body of the neuron and in glia, the accumulation of toxic metabolic toxins occurs. The deterioration of the blood supply to the nerve complexes also causes a decrease in their activity due to a deficiency of oxygen and glucose. The sites of neuron contacts, synapses, which quickly reduce the release of neurotransmitters into the synaptic cleft, also contribute to the development of fatigue of the nerve centers.
The genesis of nerve centers
Complexes of neurocytes located in the central nervous system and performing a coordinating role in the activity of the body undergo anatomical and physiological changes. They are explained by the complication of physiological and psychological functions that occur during a person’s life. The most important changes affecting the age-related features of the properties of the nerve centers are observed in the formation of such important processes as upright posture, speech and thinking that distinguish Homo sapiens from other members of the mammalian class. For example, the formation of speech occurs in the first three years of a child's life. Being a complex conglomerate of conditioned reflexes, it is formed on the basis of irritations perceived by the proprioreceptors of the muscles of the tongue, lips, vocal cords of the larynx and respiratory muscles. By the end of the third year of a child’s life, all of them are combined into a functional system, which includes a section of the cortex that lies at the base of the lower frontal gyrus. It was called the center of Brock.
The zone of the superior temporal gyrus (center of Wernicke) also takes part in the formation of speech activity . Excitation from the nerve endings of the speech apparatus enters the motor, visual and auditory centers of the cerebral cortex, where speech centers are formed.