Discoveries and scientific works of I.M. Sechenov. Modern ideas about the mechanisms of central inhibition Sechenov’s experiment with a frog
Psychology and esoterics
Sechenov's experiment: a section of the frog's brain was made at the level of the visual thalamus and the cerebral hemispheres were removed, after which the time of the withdrawal reflex was measured hind legs When they are immersed in a solution of sulfuric acid. Stimulation of this area of the brain causes the reflex time to sharply lengthen. Based on this, he came to the conclusion that in the thalamic region of the frog’s brain there are nerve centers that have an inhibitory effect on spinal reflexes. In the brain, along with excitatory neurons, there are also inhibitory axons.
0015 Inhibition in the central nervous system
The phenomenon of central inhibition was discovered by I.M. Sechenov's experiment: a section of the frog's brain was made at the level of the visual thalamus and the cerebral hemispheres were removed, after which the time of the withdrawal reflex of the hind legs was measured when they were immersed in a solution of sulfuric acid.
This reflex is carried out by spinal centers and its time is an indicator of the excitability of the centers. I.M. Sechenov discovered that if a crystal of table salt is placed on the cut of the visual tuberosities or a weak electrical stimulation is applied to this area of the brain, the reflex time increases sharply. Based on this, he came to the conclusion that in the thalamic region of the frog’s brain there are nerve centers that have an inhibitory effect on spinal reflexes. The intensity of reflex inhibition depends on the force ratio
irritations - stimulating and inhibiting the nerve center. If the irritation causing the reflex is strong and the inhibitory irritation is weak, then the intensity of inhibition is low. And vice versa. If several weak inhibitory stimuli are applied to the nerve, then the inhibition turns out to be enhanced, i.e. summation of inhibitory influences. Currently, several types of inhibition in the central nervous system have been established, having different nature and different localization.
Postsynaptic in various parts of the goal. brain, along with excitatory neurons, there are also inhibitory ones; axons form on the bodies and dendrites of excitatory neurons. cells, nerve endings, in cat. an inhibitory transmitter is produced. It does not depolarize, but hyperpolarizes the postsynaptic membrane, an inhibitory postsynaptic potential arises. The stronger the reflex, i.e., the larger number nerve cells are involved in its occurrence, the greater the strength of the inhibitory stimulation must be to suppress such a reflex. Eliminated under the influence of strychnine, cat. blocks inhibitory synapses.
Presynaptic is localized in presynaptic elements, namely in the thinnest branches of axons before their transition to the nerve ending. At these presynaptic terminals there are located the endings of other nerve cells, which here form special inhibitory synapses, cat mediators. depolarize the terminal membrane causing partial or complete blockade conduction of nerve impulses. Widely distributed in the central nervous system.
Pessimal inhibition in nerve centers. This inhibition, without the participation of inhibitory structures, develops in excitatory synapses as a result of strong depolarization of the postsynaptic membrane under the influence of frequent receipt of nerve impulses. Intermediate neurons of the spinal cord and neurons are prone to it reticular formation and etc.
Inhibition following excitation. Occurs if, after the end of the burst of excitation, strong hyperpolarization develops in the cell. Excitatory postsynapse. The potential turns out to be insufficient for critical depolarization of the membrane and propagating excitation does not occur.
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The beginning of the study of inhibition in the central unequal system is associated with the publication of the work of I.M. Sechenov "Reflexes of the Brain", in which he showed the possibility of inhibiting the motor reflexes of a frog with chemical stimulation of the visual thalamus of the brain.
Inhibition in the central nervous system - an active nervous process manifested in the suppression or weakening of the excitation process.
Central inhibition (experience of I.M. Sechenov) - a process characterized by an increase in reflex time or its complete absence, which occurs when a cross-section of the brain stem is irritated by a crystal of table salt in the area of the visual chambers.
Sechenov's classic experiment is as follows: in a frog with a cut brain at the level of the visual thalamus, the time of the flexion reflex was determined when the paw was irritated with sulfuric acid. After this, a crystal of table salt was placed on the visual tuberosities and the reflex time was again determined. It gradually increased until the reaction completely disappeared. After removing the salt crystal and washing the brain with saline solution, the reflex time was gradually restored. This allowed us to say that inhibition is an active process that occurs when certain parts of the central nervous system are stimulated.
Later I.M. Sechenov and his students showed that inhibition in the central nervous system can occur when strong stimulation is applied to any afferent pathway.
Peripheral inhibition was discovered by the Weber brothers in 1845. They found that irritation of the vagus nerve inhibits the heart until it stops completely.
Types and mechanisms of braking
Thanks to microelectrode research technology, it has become possible to study the process of inhibition at the cellular level.
There are two types of inhibition depending on the mechanisms of its occurrence: depolarization and hyperpolarization. Depolarizing inhibition occurs due to prolonged depolarization of the membrane, and hyperpolarizing - due to hyperpolarization of the membrane.
The onset of depolarization inhibition is preceded by a state of excitation. Due to prolonged irritation, this excitation turns into inhibition. The occurrence of depolarization inhibition is based on the inactivation of the membrane for sodium, as a result of which the action potential and its irritating effect on neighboring areas are reduced, and ultimately the conduction of excitation ceases.
One of the types of this inhibition is pessimal, described by N.E. Vvedensky (1886), who showed that excitation can be replaced by inhibition in any area that has low lability.
Hyperpolarization inhibition occurs with the participation of special inhibitory structures and is associated with a change in membrane permeability with respect to potassium and chlorine, which causes an increase in membrane and threshold potentials, as a result of which a response becomes impossible.
Central inhibition (I.M. Sechenov’s experiment): a - motor reflex to a painful stimulus; 6 - distribution of nerve impulses from inhibitory neurons of the brain stem to the spinal cord when a NaCI crystal is applied to the area of the visual chambers and the absence of a motor reflex to a painful stimulus
Classification of types of central nervous system inhibition
Primary inhibition- the process of activation of inhibitory neurons that form synaptic connections with the cell to which inhibition is directed, while this process is primary for the cell, not related to its preliminary excitation.
Secondary braking- a process that develops in a cell without the participation of specific inhibitory structures and is a consequence of its own excitation.
Extreme braking - depletion of nerve cells under the influence of high-intensity stimuli.
Pessimal braking- blocking of high-frequency impulses in unmyelinated nerve terminals due to their lower lability.
Presynaptic inhibition - a process that occurs when the axo-axonal inhibitory synapse is activated and blocks excitatory impulses directed at a given cell.
Postsynantic inhibition - a process that develops upon activation of axo-somatic and axo-dendritic inhibitory synapses and is localized on the cell’s own membrane to which inhibition is directed.
Reciprocal inhibition- mutual suppression of the activity of antagonistic nerve structures.
Afferent collateral inhibition - a special case of reciprocal inhibition, localized in the afferent part of the reflex arc.
Efferent collateral (return) inhibition- a process in which inhibitory interneurons act on the same nerve cells that activated them, and the more intense the previous excitation, the stronger the inhibition.
Lateral inhibition- a process in which intercalary inhibitory neurons suppress the activity of not only the cell that initiated them, but also others nearby.
Lateral inhibition (T - inhibitory neuron)
Recurrent inhibition (T-inhibitory interneuron (Renshaw cell); M - motor neuron)
Reciprocal inhibition (T - inhibitory interneuron (Renshaw cell); M - motor neuron)
Translational inhibition (T - inhibitory neuron)
Inhibition processes in the central nervous system
The processes of excitation and inhibition in the nervous system are closely interrelated.
Inhibition is a biological process aimed at weakening or preventing the occurrence of the excitation process. For the first time, the idea that in the central nervous system, in addition to excitation processes, there is a process of inhibition, was put forward by I.M. Sechenov in 1862. In experiments on frogs with intact visual buffs, he analyzed the time of the flexion reflex. When salt crystals were placed on the visual thalamus, the reflex time increased (inhibition). Subsequently, this type of braking was called “Sechenov, or central, braking.”
Inhibition in the central nervous system contributes to a certain coordination of the function performed. At the same time, the activity of neurons and centers that are this moment are not required to perform an adaptive reaction. In addition, inhibition also performs a protective function, protecting nerve cells from overexcitation and exhaustion when exposed to strong stimuli.
There are several types of inhibition in the nervous system.
Postsypaptic inhibition develops in cases when the inhibitory transmitter released by the nerve ending changes the properties of the postsynaptic membrane in such a way that the nerve cell cannot generate an action potential. Postsynaptic inhibition can be caused by prolonged depolarization or hyperpolarization that occurs in the postsynaptic membrane due to the interaction of the transmitter with receptors that open potassium and chloride channels. The most common inhibitory neurotransmitters are gamma-aminobutyric acid and glycine. Glycine is secreted by special inhibitory cells (Renshaw cells) at the synapses formed by these cells on the membrane of another neuron. Acting on the receptor of the postsynaptic membrane, glycine increases its permeability to CI- ions, while chloride ions enter the cell according to the concentration gradient, resulting in hyperpolarization. When gamma-aminobutyric acid acts on the postsynaptic membrane, postsynaptic inhibition develops as a result of the entry of chloride ions into the cell or the exit of potassium ions from the cell. Concentration gradients During the development of neuronal inhibition, K + ions are supported by the Na + /K + pump, and CI - ions are supported by the CI - pump.
Recurrent postsynaptic inhibition - This is an inhibition in which inhibitory interneurons (Renshaw cells) act on the same nerve cells that innervate them. An example of recurrent postsynaptic inhibition is inhibition in spinal cord motor neurons. This type of inhibition provides, for example, alternate contraction and relaxation of skeletal muscles - flexors and extensors, which is necessary for coordinating the movements of the limbs when walking.
Lateral postsynaptic inhibition is due to the fact that inhibitory interneurons are connected in such a way that they are activated by impulses from the excited center and influence neighboring cells with the same functions. As a result, a very deep inhibition develops in these neighboring cells, called lateral, since the resulting zone of inhibition is located laterally in relation to the excited neuron and is initiated by it.
Reciprocal inhibition, an example of which is inhibition of the nerve centers of antagonist muscles, lies in the fact that excitation of the proprioceptors of the flexor muscles simultaneously activates the motor neurons of these muscles and intercalary inhibitory neurons. Excitation of interneurons leads to postsynaptic inhibition of motor neurons of extensor muscles. If the centers of the flexor and extensor muscles were simultaneously excited, flexion of the limb at the joint would be impossible.
Presynaptic inhibition This is due to the fact that prolonged depolarization of the membrane can develop in the presynaptic terminal, which leads to the development of inhibition. In the focus of depolarization, the process of propagation of excitation is disrupted and impulses cannot pass through the depolarization zone. Consequently, the transmitter is not released into the synaptic cleft in sufficient quantities and the postsynaptic neuron is not excited. The central nervous system contains a huge number of inhibitory neurons, in particular Renshaw cells. These inhibitory neurons synthesize specific inhibitory transmitters and carry out the inhibition reaction. Activation of an inhibitory neuron causes depolarization of the terminal membrane in afferent neurons, which makes it difficult to conduct an action potential. The mediator at such axonal synapses is gamma-aminobutyric acid or another inhibitory neurotransmitter. Depolarization is a consequence of increased membrane permeability to chlorine ions, as a result of which these ions leave the cell.
Inhibition mechanisms manifest themselves in the cessation or reduction of nerve cell activity. In contrast to excitation, inhibition is a local, non-propagating process that occurs on the cell membrane.
Sechenov braking. The presence of an inhibition process in the central nervous system was first demonstrated by Sechenov in 1862 in experiments on a frog. An incision was made in the frog's brain at the level of the visual thalamus and the time of the reflex to withdraw the hind paw when immersed in a sulfuric acid solution was measured (Türk's method). When a crystal of table salt was placed on the incision of the visual tuberosities, the reflex time increased. Cessation of the effect of salt on the visual thalamus led to the restoration of the original reflex reaction time. The paw withdrawal reflex is caused by excitation of the spinal centers. A crystal of salt, irritating the visual tuberosities, causes excitement, which spreads to the spinal centers and inhibits their activity. THEM. Sechenov came to the conclusion that inhibition is a consequence of the interaction of two or more excitations on the neurons of the central nervous system. In this case, one excitation inevitably becomes inhibited, and the other - inhibitory. Suppression by one excitation of another occurs both at the level of postsynaptic membranes (postsynaptic inhibition) and by reducing the effectiveness of excitatory synapses at the presynaptic level (presynaptic inhibition).
Presynaptic inhibition. Presynaptic inhibition develops in the presynaptic part of the synapse due to the effect of axo-axonal synapses on its membrane. As a result of both depolarizing and hyperpolarizing effects, the transmission of excitation impulses along presynaptic pathways to the postsynaptic nerve cell is blocked.
Postsynaptic inhibition. The most widespread mechanism in the central nervous system is postsynaptic inhibition, which is carried out by special inhibitory intercalary nerve cells (for example, Renshaw cells in the spinal cord or Purkinje cells (piriform neurons) in the cerebellar cortex). The peculiarity of inhibitory nerve cells is that their synapses contain mediators that cause IPSPs (inhibitory postsynaptic potentials) on the postsynaptic membrane of the neuron, i.e. short-term hyperpolarization. For example, for motor neurons of the spinal cord, the hyperpolarizing transmitter is the amino acid glycine, and for many neurons of the cerebral cortex, gamma-aminobutyric acid - GABA - is such a transmitter. A special case of postsynaptic inhibition is recurrent inhibition.
Reciprocal inhibition. The mechanism of postsynaptic inhibition underlies such types of inhibition as reciprocal and lateral. Reciprocal inhibition is one of the physiological mechanisms for coordinating the activity of nerve centers. Thus, the inhalation and exhalation centers, the pressor and depressor vasomotor centers in the medulla oblongata are alternately inhibited reciprocally. Reciprocal inhibition manifests itself at the level of the spinal cord during strictly coordinated motor acts (walking, running, scratching). At the level of spinal cord segments, excitation of a group of motor neurons that causes contraction of the flexor muscles is accompanied by reciprocal inhibition of another group of motor neurons, leading to relaxation of the extensor muscles.
Lateral inhibition. The activity of neurons or receptors located next to the excited neurons or receptors ceases. The mechanism of lateral inhibition provides the discriminatory ability of analyzers. Thus, in the auditory analyzer, lateral inhibition ensures differentiation of the frequency of sounds; in the visual analyzer, lateral inhibition sharply increases the contrast of the contours of the perceived image, and in the tactile analyzer it contributes to the differentiation of two points of contact.
When excitations arrive at the synapses of a nerve cell, hyperpolarization processes can occur on the postsynaptic membranes. Hyperpolarization leads to an increase in the critical level of membrane depolarization and, therefore, makes it difficult for excitation to occur. Such postsynaptic potentials are called “inhibitory postsynaptic potentials” (IPSPs); they arise at synapses where the transmitter causes hyperpolarization of the postsynaptic membrane.
Each neuron synthesizes in its body and then releases the same transmitter in all its synapses, therefore neurons with acetylcholine transmission of excitation are called cholinergic, and with adrenaline - adrenergic. Hyperpolarizing mediators include GABA glycine. These mediators, interacting with chemoreceptors of the postsynaptic membrane, lead to the development of IPSP.
2. mechanism of capillary flicker
The term “Capillary Flicker” was first used by Krogh, this concept was explained by the fact that not all capillaries are functioning at any given time. In fact, only part of them functions, because the total capacity of the capillaries is greater than the volume of circulating blood. Therefore, some of the capillaries are closed and excluded from the blood circulation, and blood flows only through the “standby” capillaries. And these duty capillaries operate in an “opening-closing” mode, which is regulated by local metabolic products. During the period of intense activity of organs, when metabolism in them increases, the number of functioning capillaries increases significantly.
The tone of the vascular wall is influenced by: endothelial cells, which synthesize and secrete factors that influence the relaxation of smooth muscle cells of the vascular wall; also muscle relaxation is influenced by: carbon monoxide, ADP, AMP, phosphoric and lactic acids.
Vasopressin and angiotensin provide contraction of precapillary sphincters and a decrease in capillary circulation.
3. Regulation of gastrointestinal motility is carried out by three mechanisms:
1) reflex;
2) humoral;
3) local.
The reflex component causes inhibition or activation of motor activity when receptors are excited. The parasympathetic department increases motor function: for the upper part - the vagus nerves, for the lower part - the pelvic nerves. The inhibitory effect is carried out due to the celiac plexus of the sympathetic nervous system. When the underlying part of the gastrointestinal tract is activated, the higher part is inhibited.
In reflex regulation there are three reflexes:
1) gastroenteral (when the stomach receptors are excited, other parts are activated);
2) entero-enteric (have both inhibitory and stimulating effects on the underlying sections);
3) recto-enteral (when the rectum is filled, inhibition occurs).
Humoral mechanisms predominate mainly in the duodenum and the upper third of the small intestine.
The stimulating effect is exerted by:
1) motilin (produced by cells of the stomach and duodenum, has an activating effect on the entire gastrointestinal tract);
2) gastrin (stimulates gastric motility);
3) bambezin (causes the separation of gastrin);
4) cholecystokinin-pancreosinin (provides general stimulation);
5) secretin (activates the motor muscle, but inhibits contractions in the stomach).
The braking effect is exerted by:
1) vasoactive intestinal polypeptide;
2) gastroinhibitory polypeptide;
3) somatostatin;
4) enteroglucagon.
Hormones from the endocrine glands also affect motor function. For example, insulin stimulates it, and adrenaline inhibits it.
Local mechanisms are carried out due to the presence of the methsympathetic nervous system and predominate in the small and large intestines. (myenteric plexus - Auerbach, submucosal - Meissner)
The following have a stimulating effect:
1) coarse undigested foods (fiber);
2) hydrochloric acid;
4) end products of the breakdown of proteins and carbohydrates.
THEM. Sechenov wrote: “The inhibition of reflexes during irritation of the visual palaces corresponds to the excited state of the mechanisms contained in them... These mechanisms, in other words, delay the reflexes. The pathways for the spread of this type of reflex inhibition throughout the spinal cord lie in the anterior parts of the latter.”
It should be noted an important circumstance of the experiments of I.M. Sechenov, namely: reflexes used by I.M. Sechenov's experiments were nociceptive.
According to Sechenov, inhibition of reflex activity necessarily occurs after preliminary excitation of some mechanisms in the visual palaces to which salt is applied, and, therefore, only this primary excitation leads to the final inhibitory effect in the form of cessation of activity, expressed in the cessation of movements in response to nociceptive irritation of the lower extremities. According to modern ideas, I.M. Sechenov studied inhibition in frogs caused by irritation of the reticular formation of the brain stem.
THEM. Sechenov objected to the understanding of inhibition in the central nervous system as fatigue due to overexcitation of nervous structures. He wrote: “Suppression of reflexes is a product of excitation, and not overexcitation of any nervous mechanisms. This is proven by the fact that the effect develops in the first moments after the application of stimulation, before movements appear. In addition, from the sections of the visual chambers, irritation always produces, along with inhibition of reflexes, a diastolic arrest of the blood heart, that is, it clearly excites the medulla oblongata.”
Results of experiments by I.M. Sechenov and our studies of some effector manifestations of the action of ketamine indicate a commonality of physiological mechanisms leading to inhibition of reflex activity when irritating the “visual palaces” of a frog and during anesthesia with ketamine.
Indeed, analysis of electroencephalograms revealed an active, active state of the brain characteristic of ketamine, which, according to modern concepts, is associated with the excited state of the reticular formation of the brain stem, which corresponds to the excitation of the same structures by salt in the experiments of I.M. Sechenov.
Irritation (excitation) of the frog's visual tuberosities according to the method of I.M. Sechenov leads to excitation of motor neurons of the extensor muscles of the lower extremities - and, consequently, an increase in the amplitude of the monosynaptic reflex - which causes a tonic contraction of the extensor muscles with simultaneous inhibition of flexion reflexes to nociceptive stimulation. An increase in the excitability of spinal motor neurons of the extensor muscles, revealed by an increase in the amplitude of the Hoffmann reflex with simultaneous inhibition of reflexes to nociceptive stimulation, was also found during ketamine anesthesia.
On the localization of the process of inhibition itself in the system of the integral reflex apparatus I.M. Sechenov wrote: “In relation to the whole problem of delaying reflected movements... in no way can one look for the basis of the inhibition of reflexes that occurs as a result of irritation of the brain in changes in the motor apparatus... delaying reflected movements is carried out in the central formations of the reflex apparatus.”
The results of our study of the excitability of spinal motor neurons during ketamine anesthesia (H-reflex technique) showed that, in contrast to a significant increase in the amplitude of the reflex H-response, the magnitude of the direct (peripheral) M-response did not change. This gives grounds to believe that the noted changes in H- and M-responses during ketamine anesthesia are not associated with the action of ketamine directly at the neuromuscular synapse, but are caused by changes in the excitability of muscle innervation centers.
The found changes in central hemodynamics and vascular tone during ketamine anesthesia indicate the excitation of cardiovasomotor formations of the reticular formation of the medulla oblongata, as in common source excitation systems.
Comparison made effector manifestations of ketamine anesthesia with effector manifestations of the process central braking according to I.M. Sechenov quite definitely point to them identity, which, in our opinion, is determined by the identity of neurophysiological mechanisms.
Braking- an active process that occurs when stimuli act on tissue, manifests itself in the suppression of other excitation, there is no functional function of the tissue.
Inhibition can develop only in the form of a local response.
There are two types of braking:
1) primary. For its occurrence it is necessary to have special inhibitory neurons. Inhibition occurs primarily without previous excitation under the influence of inhibitory mediator .
There are two types of primary inhibition:
- presynaptic at the axo-axonal synapse;
- postsynaptic at the axodendritic synapse.
2) secondary. It does not require special inhibitory structures, occurs as a result of changes in the functional activity of ordinary excitable structures, and is always associated with the process of excitation.
Types of secondary braking:
- transcendental, which occurs when there is a large flow of information entering the cell. The flow of information lies beyond the functionality of the neuron;
- pessimal, which occurs with a high frequency of irritation; parabiotic, which occurs with strong and long-lasting irritation;
Inhibition following excitation, resulting from a decrease in the functional state of neurons after excitation;
Braking based on the principle of negative induction;
Inhibition of conditioned reflexes.
The processes of excitation and inhibition are closely related to each other, occur simultaneously and are different manifestations of a single process. Foci of excitation and inhibition are mobile, cover larger or smaller areas of neuronal populations and can be more or less pronounced. Excitation is certainly replaced by inhibition, and vice versa, that is, there is an inductive relationship between inhibition and excitation.
Braking lies in basis coordination of movements, protects central neurons from overexcitation. Inhibition in the central nervous system can occur when nerve impulses of varying strength from several stimuli simultaneously enter the spinal cord. Stronger stimulation inhibits reflexes that should have occurred in response to weaker ones.
In 1862 I.M. Sechenov discovered phenomenon central braking. He proved in his experiment that irritation with a sodium chloride crystal of the visual thalamus of a frog (the cerebral hemispheres have been removed) causes inhibition of spinal cord reflexes. After the stimulus was removed, the reflex activity of the spinal cord was restored. The result of this experiment allowed I.M. Secheny to conclude that in the central nervous system, along with the process of excitation, a process of inhibition develops, which is capable of inhibiting the reflex acts of the body. N. E. Vvedensky suggested that the phenomenon of inhibition is based on the principle of negative induction: a more excitable area in the central nervous system inhibits the activity of less excitable areas.
Modern interpretation of the experience of I. M. Sechenov(I.M. Sechenov irritated the reticular formation of the brain stem): excitation of the reticular formation increases the activity of inhibitory neurons of the spinal cord - Renshaw cells, which leads to inhibition of α-motoneurons of the spinal cord and inhibits the reflex activity of the spinal cord.
Inhibitory synapses formed by special inhibitory neurons (more precisely, their axons). The mediator may be glycine, GABA and a number of other substances. Typically, glycine is produced at synapses through which postsynaptic inhibition occurs. When glycine as a mediator interacts with glycine receptors of a neuron, hyperpolarization of the neuron occurs ( TPSP ) and, as a consequence, a decrease in the excitability of the neuron up to its complete refractoriness. As a result, the excitatory influences exerted through other axons become ineffective or ineffective. The neuron shuts down completely.
Inhibitory synapses open mainly chlorine channels, allowing chloride ions to easily pass through the membrane. To understand how inhibitory synapses inhibit a postsynaptic neuron, we need to remember what we know about the Nernst potential for Cl- ions. We calculated it to be approximately -70 mV. This potential is more negative than the resting membrane potential of the neuron, equal to -65 mV. Consequently, the opening of chloride channels will promote the movement of negatively charged Cl- ions from the extracellular fluid inward. This shifts the membrane potential towards more negative values compared to rest to approximately -70 mV.
The opening of potassium channels allows positively charged K+ ions to move outward, resulting in greater negativity inside the cell than at rest. Thus, both events (the entry of Cl- ions into the cell and the exit of K+ ions from it) increase the degree of intracellular negativity. This process is called hyperpolarization. Increased negativity membrane potential compared to its intracellular level at rest, the neuron is inhibited, therefore the departure of negative values beyond the limits of the initial resting membrane potential is called TPSP.
Functional Features somatic and autonomic nervous system. Comparative characteristics sympathetic, parasympathetic and metasympathetic divisions of the autonomic nervous system.
The first and main difference The structure of the ANS from the structure of the somatic consists in the location of the efferent (motor) neuron. In the SNS, intercalary and motor neurons are located in the gray matter of the SC; in the ANS, the effector neuron is moved to the periphery, beyond the SC, and lies in one of the ganglia - para-, prevertebral or intraorgan. Moreover, in the metasympathetic part of the ANS, the entire reflex apparatus is located entirely in the intramural ganglia and nerve plexuses of the internal organs.
The second difference concerns exit of nerve fibers from the central nervous system. Somatic NVs leave the SC segmentally and cover at least three adjacent segments with innervation. The fibers of the ANS emerge from three sections of the central nervous system (GM, thoracolumbar and sacral sections of the SM). They innervate all organs and tissues without exception. Most visceral systems have a triple (sympathetic, para- and metasympathetic) innervation.
The third difference concerns innervation of somatic and ANS organs. Transection of the ventral roots of the SC in animals is accompanied by complete degeneration of all somatic efferent fibers. It does not affect the arc of the autonomic reflex due to the fact that its effector neuron is located in the para- or prevertebral ganglion. Under these conditions, the effector organ is controlled by the impulses of a given neuron. It is this circumstance that emphasizes the relative autonomy of this department of the National Assembly.
The fourth difference concerns to the properties of nerve fibers. In the ANS, they are mostly pulpless or thin pulpy, such as preganglionic fibers, the diameter of which does not exceed 5 μm. Such fibers belong to type B. Postganglionic fibers are even thinner, most of them are devoid of a myelin sheath, they belong to type C. In contrast, somatic efferent fibers are thick, pulpy, their diameter is 12-14 microns. In addition, pre- and postganglionic fibers are characterized by low excitability. To evoke a response in them, a much greater force of irritation is required than for somatic motor fibers.
ANS fibers are characterized by a long refractory period and long chronaxy. The speed of NI propagation along them is low and is up to 18 m/s in preganglionic fibers, and up to 3 m/s in postganglionic fibers. Action potentials of ANS fibers are characterized by a longer duration than in somatic efferents. Their occurrence in preganglionic fibers is accompanied by a long trace positive potential, in postganglionic fibers - a trace negative potential followed by a long trace hyperpolarization (300-400 ms).
VNS provides extraorgan and intraorgan regulation of body functions and includes three components:
1) sympathetic;
2) parasympathetic;
3) methsympathetic.
The autonomic nervous system has a number of anatomical and physiological features that determine the mechanisms of its operation.
Anatomical properties:
1. Three-component focal arrangement of nerve centers. The lowest level of the sympathetic department is represented by the lateral horns from the VII cervical to the III-IV lumbar vertebrae, and the parasympathetic - by the sacral segments and the brain stem. The higher subcortical centers are located on the border of the hypothalamic nuclei (the sympathetic department is the posterior group, and the parasympathetic division is the anterior group). The cortical level lies in the area of the sixth to eighth fields Brodman(motosensory zone), in which point localization of incoming nerve impulses is achieved. Due to the presence of such a structure of the autonomic nervous system, the work of the internal organs does not reach the threshold of our consciousness.
2. Availability autonomic ganglia. In the sympathetic department, they are located either on both sides along the spine, or are part of the plexuses. Thus, the arch has a short preganglionic and a long postganglionic path. The neurons of the parasympathetic division are located near the working organ or in its wall, so the arc has a long preganglionic and short postganglionic path.
3. Effetor fibers belong to groups B and C.
Physiological properties:
1. Features of the functioning of the autonomic ganglia. Presence of a phenomenon animations(simultaneous occurrence of two opposite processes - divergence and convergence). Divergence- divergence of nerve impulses from the body of one neuron to several postganglionic fibers of another. Convergence- convergence on the body of each postganglionic neuron of impulses from several preganglionic ones.
This ensures the reliability of the transfer of information from the central nervous system to the working organ. An increase in the duration of the postsynaptic potential, the presence of trace hyperpolarization and synoptic delay contribute to the transmission of excitation at a speed of 1.5-3.0 m/s. However, the impulses are partially extinguished or completely blocked in the autonomic ganglia. In this way they regulate the flow of information from the central nervous system. Due to this property, they are called nerve centers located on the periphery, and the autonomic nervous system is called autonomous.
2. Features of nerve fibers. Preganglionic nerve fibers belong to group B and conduct excitation at a speed of 3-18 m/s, postganglionic nerve fibers belong to group C. They conduct excitation at a speed of 0.5-3.0 m/s. Since the efferent pathway of the sympathetic department is represented by preganglionic fibers, and the parasympathetic one is represented by postganglionic fibers, the speed of impulse transmission is higher in the parasympathetic nervous system.
Thus, the autonomic nervous system functions differently, its work depends on the characteristics of the ganglia and the structure of the fibers.
Sympathetic nervous system innervates all organs and tissues (stimulates the heart, increases the lumen of the respiratory tract, inhibits the secretory, motor and absorption activity of the gastrointestinal tract, etc.). It performs homeostatic and adaptive-trophic functions.
Her homeostatic role is to maintain consistency internal environment the body is in an active state, i.e. the sympathetic nervous system is activated only during physical activity, emotional reactions, stress, pain, and blood loss.
Adaptation-trophic function aimed at regulating the intensity of metabolic processes. This ensures the body's adaptation to changing environmental conditions.
Thus, the sympathetic department begins to act in an active state and ensures the functioning of organs and tissues.
Parasympathetic nervous system is an antagonist of the sympathetic and performs homeostatic and protective functions, regulates the emptying of hollow organs.
The homeostatic role is restorative in nature and acts in a state of rest. This manifests itself in the form of a decrease in the frequency and strength of heart contractions, stimulation of the gastrointestinal tract with a decrease in blood glucose levels, etc.
All protective reflexes rid the body of foreign particles. For example, coughing clears the throat, sneezing clears the nasal passages, vomiting removes food, etc.
Emptying of hollow organs occurs when the tone of the smooth muscles that make up the wall increases. This leads to the entry of nerve impulses into the central nervous system, where they are processed and sent along the effector pathway to the sphincters, causing them to relax.
Metsympathetic nervous system is a collection of microganglia located in organ tissue. They consist of three types of nerve cells - afferent, efferent and intercalary, therefore they perform the following functions:
Provides intraorgan innervation;
They are an intermediate link between the tissue and the extraorgan nervous system. When exposed to a weak stimulus, the metosympathetic department is activated, and everything is decided at the local level. When strong impulses arrive, they are transmitted through the parasympathetic and sympathetic departments to the central ganglia, where they are processed.
The methsympathetic nervous system regulates the functioning of smooth muscles that make up most organs of the gastrointestinal tract, myocardium, secretory activity, local immunological reactions, etc.
The role of the SM in the processes of regulation of the activity of the musculoskeletal system and the vegetative functions of the body. Characteristics of spinal animals. Principles of the spinal cord. Clinically important spinal reflexes.
SM - the most ancient education CNS. Feature buildings - segmentarity.
SM neurons form it Gray matter in the form of anterior and posterior horns. They perform the reflex function of the SC.
Hind horns contain neurons ( interneurons), which transmit impulses to the overlying centers, to the symmetrical structures of the opposite side, to the anterior horns of the spinal cord. The dorsal horns contain afferent neurons that respond to pain, temperature, tactile, vibration, and proprioceptive stimuli.
Front horns contain neurons ( motor neurons), giving axons to the muscles, they are efferent. All descending pathways of the central nervous system of motor reactions end in the anterior horns.
IN lateral horns The neurons of the sympathetic division of the autonomic nervous system are located in the cervical and two lumbar segments, and the parasympathetic ones are located in the second to fourth segments.
The SC contains many interneurons that provide communication with the segments and with the overlying parts of the central nervous system; they account for 97% of total number spinal cord neurons. They include associative neurons - neurons of the SC's own apparatus; they establish connections within and between segments.
White matter The SM is formed by myelin fibers (short and long) and plays a conductive role.
Short fibers connect neurons of the same or different segments of the spinal cord.
Long fibers (projection) form the pathways of the spinal cord. They form ascending pathways to the brain and descending pathways from the brain.
The spinal cord performs reflex and conductive functions.
Reflex function allows you to realize all motor reflexes of the body, reflexes of internal organs, thermoregulation, etc. Reflex reactions depend on the location, strength of the stimulus, the area of the reflexogenic zone, the speed of impulse transmission along the fibers, and the influence of the brain.
Reflexes are divided into:
1) exteroceptive(occur when irritated by agents external environment sensory stimuli);
2) interoceptive(occur when irritation of presso-, mechano-, chemo-, thermoreceptors): viscero-visceral - reflexes from one internal organ to another, viscero-muscular - reflexes from internal organs to skeletal muscles;
3) proprioceptive(own) reflexes from the muscle itself and the formations associated with it. They have monosynaptic reflex arc. Proprioceptive reflexes regulate motor activity due to tendon and postural reflexes. Tendon reflexes (knee, Achilles, triceps brachii, etc.) occur when muscles are stretched and cause relaxation or contraction of the muscle, occurring with every muscle movement;
4) posotonic reflexes (occur when vestibular receptors are excited when the speed of movement and position of the head relative to the body changes, which leads to a redistribution of muscle tone (increased extensor tone and decreased flexor tone) and ensures body balance).
The study of proprioceptive reflexes is carried out to determine the excitability and degree of damage to the central nervous system.
Conductor function ensures communication of SC neurons with each other or with overlying parts of the central nervous system.
Spinal animal- an animal in which the spinal column is crossed, often at the level of the neck, but the function of most of the spinal column is preserved;
Immediately after transection of the SC, most of its functions below the point of intersection in the spinal animal are sharply inhibited. After a few hours (in rats and cats) or several days, weeks (in monkeys), most of the functions inherent in the spinal cord are restored almost to normal, making it possible experimental research drug.