Reticular formation The brainstem is considered as one of the important integrative apparatuses of the brain.
The actual integrative functions of the reticular formation include:
- control over sleep and wakefulness
- muscle (phasic and tonic) control
- processing of information signals of the environment and the internal environment of the body, which come through different channels
A large number of afferent pathways from other brain structures converge in the reticular formation: from the cerebral cortex - the collaterals of the cortico-spinal (pyramidal) pathways, from the cerebellum and other structures, as well as collateral fibers that fit through the brainstem, fibers of sensory systems (visual, auditory, etc.). All of them end in synapses on neurons of the reticular formation. Thus, thanks to this organization, the reticular formation is adapted to combine influences from various brain structures and is able to influence them, that is, to perform integrative functions in the activity of the central nervous system, determining to a large extent the overall level of its activity.
Properties of reticular neurons. Neurons of the reticular formation are capable of sustained background impulse activity. Most of them constantly generate discharges with a frequency of 5-10 Hz. The reason for such a constant background activity of reticular neurons is: firstly, the massive convergence of various afferent influences (from receptors of the skin, muscle, visceral, eyes, ears, etc.), as well as influences from the cerebellum, cerebral cortex, vestibular nuclei and others brain structures on the same reticular neuron. In this case, often in response to this, excitement arises. Secondly, the activity of the reticular neuron can be changed by humoral factors (adrenaline, acetylcholine, CO2 tension in the blood, hypoxia, etc.). These continuous impulses and chemicals contained in the blood support the depolarization of the membranes of the reticular neurons, their ability to sustain impulse activity. In this regard, the reticular formation also has a constant tonic effect on other brain structures.
A characteristic feature of the reticular formation is also the high sensitivity of its neurons to various physiologically active substances. Due to this, the activity of reticular neurons can be relatively easily blocked. pharmacological preparations that bind to the cytoreceptors of the membranes of these neurons. Barbituric acid compounds (barbiturates), chlorpromazine and others are especially active in this regard. medications which are widely used in medical practice.
The nature of nonspecific influences of the reticular formation. The reticular formation of the brain stem is involved in the regulation of the autonomic functions of the body. However, back in 1946, the American neurophysiologist H. W. Megoun and his collaborators discovered that the reticular formation is directly related to the regulation of somatic reflex activity. It has been proven that the reticular formation has a diffuse non-specific, descending and upward influence to other brain structures.
Downward influence. When the reticular formation of the hindbrain is stimulated (especially the giant cell nucleus of the medulla oblongata and the reticular nucleus of the pons, where the reticulospinal pathway originates), inhibition of all spinal motor centers (flexion and extensor) occurs. This inhibition is very deep and prolonged. This position in natural conditions can be observed during deep sleep.
Along with diffuse inhibitory influences, when certain areas of the reticular formation are irritated, a diffuse influence is revealed that facilitates the activity of the spinal motor system.
The reticular formation plays an important role in regulating the activity of muscle spindles by changing the frequency of discharges delivered by gamma efferent fibers to the muscles. Thus, the reverse impulse in them is modulated.
Upward influence. Studies by N. W. Megoun, G. Moruzzi (1949) showed that irritation of the reticular formation (hind, midbrain and diencephalon) affects the activity of the higher parts of the brain, in particular the cerebral cortex, ensuring its transition to an active state. This position is confirmed by these numerous experimental studies and clinical observations. So, if the animal is in a state of sleep, then direct stimulation of the reticular formation (especially the pons) through the electrodes inserted into these structures causes a behavioral reaction of awakening the animal. In this case, a characteristic image appears on the EEG - a change in the alpha rhythm by the beta rhythm, i.e. the reaction of desynchronization or activation is fixed. This reaction is not limited to a certain area of the cerebral cortex, but covers large areas of it, i.e. is generalized. When the reticular formation is destroyed or its ascending connections with the cerebral cortex are turned off, the animal falls into a dream-like state, does not respond to light and olfactory stimuli, and does not actually come into contact with the outside world. That is, the end brain ceases to function actively.
Thus, the reticular formation of the brainstem performs the functions of the ascending activating system of the brain, which maintains the excitability of neurons in the cerebral cortex at a high level.
In addition to the reticular formation of the brain stem, the ascending activating system of the brain also includes nonspecific nuclei of the thalamus, posterior hypothalamus , limbic structures. Being an important integrative center, the reticular formation, in turn, is part of more global integration systems of the brain, which include hypothalamic-limbic and neocortical structures. It is in interaction with them that expedient behavior is formed, aimed at adapting the body to changing conditions of the external and internal environment.
One of the main manifestations of damage to the reticular structures in humans is loss of consciousness. It happens with cerebrovascular accident, tumors and infectious processes in the brain stem. The duration of the state of syncope depends on the nature and severity of dysfunction of the reticular activating system and ranges from a few seconds to many months. Dysfunction of the ascending reticular influences is also manifested by a loss of vigor, constant pathological drowsiness or frequent attacks of falling asleep (paroxysmal hypersomia), restless night sleep. There are also violations (often an increase) in muscle tone, various autonomic changes, emotional and mental disorders, etc.
Reticular formation known since 1845, described by Deiters (O.F.C. Deiters) in 1885. Currently, its study continues. The reticular formation is located between the posterior and lateral horns of the cervical segments of the spinal cord, in the tegmentum of the brain stem, in the central nucleus of the thalamus. It is a complex of anatomically and functionally interconnected neurons surrounded by many fibers running in different directions to the nuclear structures and pathways (Fig. 30).
Rice. 30.: 1 - ascending paths; 2 - descending paths; 3 - specific (lemniscus) sensitive pathway; 4 - pyramidal path.
The reticular formation perceives all impulses (pain, temperature, light, sound, etc.), but there are no specialized neurons in it. Therefore, the same neurons perceive different impulses and transmit them to different parts of the brain, to all parts of the cortex. The reticular formation is the second afferent system of the brain, its non-specific structure. It has two-way connections with all structures of the brain and spinal cord (Fig. 31, 32).
Rice. 31. : 1; 2; 3 - specific (lemniscus) sensitive pathway; 4 - collaterals connecting a specific sensory pathway with the reticular formation of the brain stem; 5 - ascending activating system of the reticular formation; 6 - generalized influence of the reticular formation on the cerebral cortex.
Rice. 32.: 1 - sensory nerve on which the stimulus is applied (pain irritation); 2 - spinal cord; 3 - sympathetic nerves; 4 - adrenal gland; 5 - carotid sinus; 6 - pituitary gland; 7 - reticular formation. Solid arrows indicate nervous influences, dotted arrows indicate hormonal influences, which, through the reticular formation, have an activating effect on the cerebral cortex.
Structural elements of the reticular formation of the brain stem are divided into lateral and medial sections. In the lateral section, fibers from various afferent systems end. Collaterals from the medial and lateral loops, from the sensory nuclei of the cranial nerves approach the scattered cells and nuclei of the reticular formation. From the neurons of the medial section, efferent fibers begin to the motor nuclei of the cranial nerves, to the cerebellum, to the motor nuclei of the anterior horns of the spinal cord.
The main afferent pathways of the reticular formation: tr. spinoreticularis - from the spinal cord, tr. tegmentothalamicus - from the midbrain, reticulothalamicus - from the medulla oblongata and bridge, tr. thalamocorticalis - to all areas and layers of the cerebral cortex. The reticular formation activates the cerebral cortex and cerebellum.
The cerebral cortex, in turn, sends via tr. corticoreticularis impulses into the reticular formation as part of the pyramidal pathways. The main efferent tract is tr. reticulospinalis. This pathway conducts tonic impulses to the gamma motor neurons of the spinal cord. The reticular formation regulates the motor link, providing coordination of movements, synchrony of muscle contractions, provides non-standard movements, a balance reflex, establishes an anti-gravitational muscle tone that keeps the body above the ground. The reticular formation redistributes muscle tone, which in crisis situations leads to the mobilization of the hidden reserves of the body.
The role of the bluish spot and raphe nuclei in the regulation of sleep and wakefulness has been established. bluish spot (locus caeruleus) is located in the upper lateral part of the rhomboid fossa. The neurons of this nucleus produce norepinephrine, which activates the overlying parts of the brain. The activity of the bluish spot neurons is especially high during wakefulness; during deep sleep, it fades almost completely.
Seam cores (nuclei raphes) are located in the midline of the medulla oblongata. The neurocytes of these nuclei produce serotonin, which causes the processes of diffuse inhibition and the state of sleep.
The nuclei of the reticular formation of the medulla oblongata have connections with the autonomic nuclei of Ⅸ, Ⅹ nerves and the sympathetic nuclei of the spinal cord. Therefore, they are involved in the regulation of cardiac activity, respiration, vascular tone, gland secretion, and so on.
The nuclei of Cahal and Darkshevich, related to the reticular formation of the midbrain, to the medial longitudinal bundle ( fasciculus longitudinalis medialis), have connections with the nuclei of the third, fourth, sixth, eighth, ninth, tenth and eleventh pair of cranial nerves. They coordinate the work of this beam, providing combined turns of the head and eyes when changing the posture or when searching for a sound source, fixing the gaze. (These movements are absolutely necessary for labor and play acts).
These relationships explain autonomic disturbances in vestibular overload. Scattered neurons of the reticular formation act as intercalary neurons protective reflexes of swallowing, corneal (Fig. 33), cough vomiting, yawning, sneezing, etc.
Rice. 33.: 1 - receptors located in the cornea; 2 - eye branch trigeminal nerve; 3 - false unipolar cell of the sensory ganglion of the trigeminal nerve; 4 - associative neuron - scattered cell of the reticular formation; 5 - cell of the motor nucleus of the facial nerve; 6 - circular muscle of the eye.
The term reticular formation was proposed in 1865 by the German scientist O. Deiters. By this term, Deiters meant cells scattered in the brainstem, surrounded by many fibers running in different directions. It was the network-like arrangement of fibers that connect nerve cells to each other that served as the basis for the proposed name.
At present, morphologists and physiologists have accumulated rich material on the structure and functions of the reticular formation. It has been established that the structural elements of the reticular formation are localized in a number of brain formations, starting from the intermediate zone of the cervical segments of the spinal cord (lamina VII) and ending with some structures of the diencephalon (intralaminar nuclei, thalamic reticular nucleus). The reticular formation consists of a significant number of nerve cells (it contains almost 9/10 of the cells of the entire brain stem). Common features of the structure of reticular structures are the presence of special reticular neurons and the distinctive nature of the connections.
Rice. 1. Neuron of the reticular formation. Sagittal section of the brainstem of a rat pup.
Figure A shows only one neuron of the reticular formation. It can be seen that the axon is divided into caudal and rostral segments, of great length, with many collaterals. B. Collaterals. Sagittal section of the lower brainstem of a rat pup, showing the connections of the collaterals of the great descending tract (pyramidal tract) to reticular neurons. The collaterals of the ascending pathways (sensory pathways), which are not shown in the figure, are connected to the reticular neurons in a similar way (according to Sheibel M. E. and Sheibel A. B.)
Along with numerous separately lying neurons, different in shape and size, there are nuclei in the reticular formation of the brain. Scattered neurons of the reticular formation primarily play an important role in providing segmental reflexes that close at the level of the brain stem. They act as intercalary neurons in the implementation of such reflex acts as blinking, corneal reflex, etc.
The significance of many nuclei of the reticular formation has been elucidated. So, the nuclei located in the medulla oblongata have connections with the autonomic nuclei of the vagus and glossopharyngeal nerves, the sympathetic nuclei of the spinal cord, they are involved in the regulation of cardiac activity, respiration, vascular tone, gland secretion, etc.
The role of the locus coeruleus and raphe nuclei in the regulation of sleep and wakefulness has been established. blue spot, is located in the upper lateral part of the rhomboid fossa. The neurons of this nucleus produce a biologically active substance - norepinephrine, which has an activating effect on the neurons of the overlying parts of the brain. The activity of locus coeruleus neurons is especially high during wakefulness; during deep sleep, it fades almost completely. Seam cores located in the midline of the medulla oblongata. The neurons of these nuclei produce serotonin, which causes the processes of diffuse inhibition and the state of sleep.
Kernels of Cajal and Darkshevich related to the reticular formation of the midbrain, have connections with the nuclei of III, IV, VI, VIII and XI pairs of cranial nerves. They coordinate the work of these nerve centers, which is very important for ensuring the combined rotation of the head and eyes. The reticular formation of the brain stem is important in maintaining the tone of the skeletal muscles, sending tonic impulses to the motor neurons of the motor nuclei of the cranial nerves and the motor nuclei of the anterior horns of the spinal cord. In the process of evolution, such independent formations as the red core, the black substance emerged from the reticular formation.
According to structural and functional criteria, the reticular formation is divided into 3 zones:
1. Median, located along the midline;
2. Medial, occupying the medial sections of the trunk;
3. Lateral, the neurons of which lie near the sensory formations.
median zone represented by raphe elements, consisting of nuclei, the neurons of which synthesize the mediator - serotonin. The system of raphe nuclei takes part in the organization of aggressive and sexual behavior, in the regulation of sleep.
Medial (axial) zone consists of small neurons that do not branch. The zone contains a large number of nuclei. There are also large multipolar neurons with a large number of densely branching dendrites. They form ascending nerve fibers to the cerebral cortex and descending nerve fibers to the spinal cord. The ascending pathways of the medial zone have an activating effect (directly or indirectly through the thalamus) on the neocortex. Descending paths have an inhibitory effect.
Lateral zone- it includes reticular formations located in the brain stem near sensory systems, as well as reticular neurons lying inside sensory formations. The main component of this zone is a group of nuclei that are adjacent to the nucleus of the trigeminal nerve. All nuclei of the lateral zone (with the exception of the reticular lateral nucleus of the medulla oblongata) consist of neurons of the small and medium size and devoid of large elements. In this zone, ascending and descending paths are located, providing a connection between sensory formations with the medial zone of the reticular formation and the motor nuclei of the trunk. This part of the reticular formation is younger and possibly more progressive; the fact of a decrease in the volume of the axial reticular formation in the course of evolutionary development is associated with its development. Thus, the lateral zone is a set of elementary integrative units formed near and within specific sensory systems.
Rice. 2. Kernels of the reticular formation (RF)(after: Niuwenhuys et. al, 1978).
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1-6 - median zone of the RF: 1-4 - raphe nuclei (1 - pale, 2 - dark, 3 - large, 4 - bridge), 5 - upper central, 6 - dorsal raphe nucleus, 7-13 - medial zone of the RF : 7 - reticular paramedian, 8 - giant cell, 9 - reticular nucleus of the pontine tegmentum, 10, 11 - caudal (10) and oral (11) nuclei of the pons, 12 - dorsal tegmental nucleus (Gudden), 13 - sphenoid nucleus, 14 - I5 - lateral zone of the RF: 14 - central reticular nucleus of the medulla oblongata, 15 - lateral reticular nucleus, 16, 17 - medial (16) and lateral (17) parabrachial nuclei, 18, 19 - compact (18) and scattered (19) parts of the pedunculo -pontine nucleus.
Due to downward influences, the reticular formation also has a tonic effect on the motor neurons of the spinal cord, which in turn increases the tone of the skeletal muscles and improves the afferent feedback system. As a result, any motor act is performed much more efficiently, performs more precise control behind the movement, but excessive excitation of the cells of the reticular formation can lead to muscle trembling.
In the nuclei of the reticular formation there are centers of sleep and wakefulness, and stimulation of certain centers leads either to the onset of sleep or to awakening. This is the basis for the use of sleeping pills. The reticular formation contains neurons that respond to pain stimuli coming from muscles or internal organs. It also contains special neurons that provide a quick response to sudden, vague signals.
The reticular formation is closely connected with the cerebral cortex, due to this, a functional connection is formed between the external parts of the central nervous system and the brain stem. The reticular formation plays an important role both in the integration of sensory information and in the control over the activity of all effector neurons (motor and autonomic). It is also of paramount importance for the activation of the cerebral cortex, for the maintenance of consciousness.
It should be noted that the cerebral cortex, in turn, sends cortical-reticular pathways of impulses into the reticular formation. These impulses originate mainly in the cortex of the frontal lobe and pass through the pyramidal pathways. Cortical-reticular connections have either inhibitory or excitatory effects on the reticular formation of the brain stem, they correct the passage of impulses along efferent pathways (selection of efferent information).
Thus, there is a two-way connection between the reticular formation and the cerebral cortex, which ensures self-regulation in activity. nervous system. The functional state of the reticular formation determines muscle tone, the functioning of internal organs, mood, concentration of attention, memory, etc. In general, the reticular formation creates and maintains conditions for the implementation of complex reflex activity involving the cerebral cortex.
Ticket 15
1. Forms (fragments) of afferent synthesis: Dominant motivation; situational afferentation; Starting afferentation. The role of the reticular formation.
2. Fast and slow muscle fibers.
Question 1
AFFERENT SYNTHESIS- (connection, compilation) - the process of comparison, selection and synthesis of numerous and different in functional significance afferentations caused by various effects on the body, occurring in c. n. s., on the basis of which the purpose of the action is formed.
A. s. according to Anokhin's theory of the functional system, it is the first, universal, stage of any purposeful behavioral act (see Functional systems).
A. s. includes processing of 4 main types of afferent excitations.
1. Motivational excitation reflects the dominant need of the body, which arises under the influence of metabolic, hormonal, and in humans - social factors. Motivation plays a decisive role in shaping the goal of action. Specifically increasing the reactivity of cortical neurons with the help of exploratory reaction, motivational arousal contributes to the processing and active selection of sensory information necessary for the construction of goal-directed behavior.
2. Situational afferentation is the impact on the body of the totality of external factors that make up a specific environment, against the background of which adaptive activity develops. Situational afferentation is formed not only by constant components of the situation, but also by a number of successive afferent influences on the organism. A characteristic feature of situational afferentation is that it gives specificity to a future behavioral response, ensuring its adaptive value only in a given situation.
The role of situational afferentation is most clearly manifested in experiments with conditioned reflexes. In these cases, the animal responds to the same conditioned stimulus with a conditioned defensive reaction in one experimental chamber and a conditioned food reaction in the other (or in the same experimental chamber in the morning the animal responds with a food reaction, and in the evening with a defensive one).
At the stage of afferent synthesis, the questions “what to do?”, “how to do?”, “when to do?” are solved.
Starting afferentation
It is a special stimulus that actually triggers a behavioral response. The significance of the starting stimulus lies in the fact that it is intended to indicate the moment of the onset of a behavioral response.
Goal-directed behavior can begin without an explicit trigger stimulus. Examples of such reactions are regularly occurring physiological functions (eating, sleeping, defecation, urination, etc.), timed to coincide with certain periods of the day.
Afferent synthesis is carried out on the basis of the following neurophysiological mechanisms:
1) mechanisms of ascending activating influences of subcortical formations on the cerebral cortex. First of all, these are the activating influences of the hypothalamus to the frontal cortex, through the anterior nuclei of the thalamus, which reflects motivational excitations. Other limbic systems act similarly. Second in activating value are the reticular structures of the midbrain and pons, which provide an appropriate level of wakefulness.
2) mechanisms of convergence of excitations of different quality on the neurons of the cortex and subcortical structures of the brain. In particular, multisensory convergence from surfaces (visual, tactile, auditory, temperature, etc.); multibiological convergence associated with certain conditions (hunger, pain, etc.), etc.;
3) integration of motivational, situational and triggering afferentations on the neurons of the cerebral cortex;
4) the mechanisms of formation of the dominant, due to which the current activity is suppressed and the newly formed behavioral reaction is retained.
The role of the reticular formation
The reticular formation is characterized by relatively low excitability. The effects of her irritation appear after a long latent period, she reacts slowly and remains active for a long time after the cessation of irritation (long aftereffect). The reticular formation facilitates or suppresses phasic movements and tension of skeletal muscles caused by motor neurons of the spinal cord, as well as movements caused from the cerebral cortex. The reticular formation of the midbrain and diencephalon facilitates the reflex movements of animals; irritation of the diencephalon inhibits the motor reflexes of the spinal cord.
The lateral sections of the reticular formation of the pons and midbrain facilitate, and its middle sections in the medulla oblongata inhibit motor reflexes. Relief and inhibition also depend on the intensity and duration of stimulation of the reticular formation. By gamma neurons, it regulates the functions of muscle spindles, and therefore feedback from skeletal muscles. It also changes the excitability of the ascending afferent pathways of the spinal cord, which can reduce or stop postsynaptic inhibition. Tonic influences of the reticular formation cause EPSP or IPSP in the motor neurons of the spinal cord. It also changes the transmission of impulses in the brain stem and, simultaneously with the effect on the skeletal muscles, causes vasomotor, respiratory, pupillary and other reactions.
The reticular formation has an adaptive-trophic effect on the cerebral cortex, subcortical formations of the diencephalon, cerebellum and spinal cord. There are mutual influences of these parts of the nervous system, both excitatory and inhibitory. It is involved in the physiological processes of sleep and awakening, as well as in emotions, in the stress response (“stress”), etc. Irritation of the reticular formation causes the awakening of sleeping animals, and its destruction and shutdown causes deep sleep in awake animals. The mutual influences of the reticular formation and the cerebral cortex were studied. The participation of the reticular formation in the formation and course of conditioned reflexes was established.
Through sympathetic fibers, the reticular formation regulates the excitability and performance of skeletal muscles, the functional state of the nervous system and sensory organs, exerting an adaptive-trophic effect on them. The regulation of posture reflexes and motor reflexes that move the body is carried out along efferent gamma fibers innervating proprioceptors.
The reticular formation regulates autonomic functions, the activity of internal organs. It affects the formation of hormones in the pituitary gland and other endocrine glands and hormones and mediators are concentrated in it.
Afferent fibers enter it through the sympathetic and vagus nerves. Part of the cells of the reticular formation of the midbrain and varolneval bridge is excited by adrenaline and norepinephrine (adrenergic systems), and the other part, located in diencephalon, somewhat above the midbrain, is excited by acetylcholine and its derivatives (cholinergic systems). The adrenoreactive systems of the midbrain and pons facilitate the onset of motor reflexes, and the adrenoreactive systems of the medulla oblongata inhibit spinal reflexes. Adrenaline also stimulates cholinergic systems. It is assumed that the action of acetylcholine and its derivatives is less limited than the action of adrenaline, and covers many areas of the brain. The action of acetylcholine on the reticular formation is opposite to its peripheral effect on internal organs. The reticular formation of the middle and medulla oblongata excites carbon dioxide.
Hormones and mediators act on the function of the cerebral hemispheres both directly and through the reticular formation. Thus, the reticular formation of the brain stem is the subcortical center of the autonomic nervous system.
Question 2.
The reticular formation (or substance) (Deiters, 1865) of the brain stem, as well as its other sections (spinal cord, etc.) is a collection of nerve cells of different sizes and a system of numerous fibers located in various directions and forming a kind of grid (reticulum ). Nerve cells of the reticular formation are located in the form of clusters - nuclei (more than 90 of them are known) and diffusely in the form of individual cells. The most important accumulations of cells of the reticular formation are:
- 1. The central reticular nucleus of the medulla oblongata, located in the suture area.
- 2. Ventral small cell reticular nucleus of the medulla oblongata.
- 3. Giant cell nucleus, lying posterior to the olive and continuing throughout the entire brain stem.
- 4. Lateral and paramedial reticular nuclei associated with the cerebellum.
In the spinal cord, the reticular formation is represented by fibers of various directions located between the projection "conducting" pathways of the spinal cord. Cells of the reticular formation are located in the region of the reticular process of the lateral horn of the spinal cord.
In the midbrain, the reticular formation is located in the internal parts of the quadrigemina. Its fibers are closely connected with the red nuclei, the substantia nigra, the nuclei of the optic tubercle, with the amygdala, the nuclei of the hypothalamus and the basal ganglia.
In the diencephalon, cells of the reticular formation are located in the thalamus, nipple bodies, subthalamic nucleus, Lewis bodies, and other formations.
The most important ascending (afferent) fiber systems of the reticular formation are:
- 1) spino-reticular path - rises up, passes the medulla oblongata, pons varolii and ends in the cerebral cortex;
- 2) the nucleoreticular path - from the vestibular and auditory nuclei, from the nuclei of a single bundle of vagus nerves, as well as from the cells of the reticular formation itself, goes to the nuclei of the bridge, cerebellum, to the visual tubercle, to the subcortical nodes and ends in the cerebral cortex;
- 3) reticulo-cerebellar path - from the nuclei of the medulla oblongata and the bridge to the nuclei of the cerebellum;
- 4) reticulo-opercular path - from the nuclei of the medulla oblongata and the pons and cerebellum to the nuclei of the quadrigemina. Numerous fibers and collaterals connect the cells and fibers of the reticular formation with the visual tubercle, the right substance and the red nuclei of the quadrigemina, as well as with the hypothalamus - (the reticular formation has great importance maintaining muscle tone).
The whole system, including the reticular formation and pathways that conduct impulses to the cortex, was called the ascending activating system (Fig. 134).
The high level of activity of the formation itself is supported by the flow of afferent impulses. Added to this are the humoral effects. Powerful activators of the reticular formation are adrenaline and carbon dioxide. In maintaining a high level of activity of the reticular formation, an important role is played by the effect that the cerebral cortex has on it. "Encouraging" impulses go not only from the reticular formation to the cortex, but in the opposite direction. This was proved by special experiments, when certain areas of the cortex were irritated and the same diffuse reaction of awakening was obtained, as with direct stimulation of the reticular formation. After damage to the reticular formation, stimulation of these areas of the cortex no longer “activated” diffusely the entire cortex.
All these data perfectly confirm the idea of IP Pavlov about the interdependence and mutual influence of the cortex and subcortex, about the tonic effect of the subcortex on the cortex and the regulating effect of the cortex on the subcortex. I. P. Pavlov figuratively called this role of the subcortex “blind force” or “source of force” for cortical activity.
Thus, with any irritation of the sensory nerves, afferent impulses reach the cerebral cortex in two ways:
- 1) according to known classical conductors (specific system), which excite only limited areas of the cortex;
- 2) through the reticular formation, which activates the entire cortex.
The most important descending pathways of the reticular formation are:
- 1) cortico-reticular path from the cerebral cortex to the reticular formation of the middle and medulla oblongata;
- 2) thalamo-reticular;
- 3) pallido-reticular,
- 4) tectoreticular;
- 5) the reticulo-spinal bundle starts from the cells of the red nucleus and descends to the cells of the reticular formation of the medulla oblongata;
- 6) fastigio-reticular bundle connects the nuclei of the cerebellum with the reticular formation of the midbrain, pons and medulla oblongata.
For the first time, the downward influence of the reticular formation on the spinal cord was shown by I. M. Sechenov in 1863 by Kristallik table salt he irritated the interstitial brain of the frog (the hemispheres of the brain were removed) and obtained inhibition of spinal activity in the form of a lengthening of the reflex time. This inhibition is called Sechenov inhibition.
But only 80 years after Sechenov, thanks to the work of Magun, it became obvious that Sechenov was dealing with an inhibitory fraction of the reticular formation. Now neurophysiologists around the world consider Sechenov's experiment the first experiment in the physiology of the reticular formation.
It has now been proven that when the medial part of the bulbar reticular formation is stimulated, movements caused by cortical irritation and a number of reflexes (regardless of their nature and the level of reflex arc closure) experience significant inhibition up to their complete cessation. If, however, the lateral part of the bulbar reticular formation or the reticular formation of the pons and midbrain is irritated, then the motor reflexes, on the contrary, are facilitated, as they are amplified.
Thus, the downward influence of the reticular formation on the spinal cord can be twofold: facilitating and inhibitory. It is believed that the normal activity of the spinal cord is achieved by a certain balance between the facilitating and inhibitory downward influence of the reticular formation on the spinal cord.
Damage to the reticular formation
Various damage to the reticular formation can occur due to trauma (hemorrhage), tumors, infections (flu, encephalitis, rheumatism, etc.), intoxication and other pathogenic effects. Pathogenic effects cause destruction of the pericellular apparatus of ganglion cells of the reticular formation, damage their protoplasm (Nissl substance, etc.) and the nucleus. Depending on the site of damage, various patterns of dysfunction of the nervous system arise, often involving many forms of nervous activity. The variety of manifestations of damage to various parts of the reticular formation depends on a large number of connections of the reticular formation both with the overlying (cerebral cortex, thalamus, hypothalamus, cerebellum) and with the underlying parts of the central nervous system. Damage to both ascending and descending fibers of the reticular formation causes a variety of disorders, ranging from higher nervous activity to numerous disorders of muscle tone or autonomic functions.
Damage to the reticular formation of the spinal cord manifests itself in the development of trophic disorders of the skin, muscles, bones and other tissues innervated by the nerves of the affected segments. Trophic disorders are expressed in the development of spontaneous gangrene of the affected area of the body, such as fingers. Spontaneous gangrene is preceded by a violation of blood circulation in the tissues affected by dystrophy in the form of alternating blanching with redness. Dystrophic processes develop as a result of damage to the reticular formation of the spinal cord (lateral horn, reticular process of the gray matter) and associated parts of the autonomic sympathetic nervous system. There are cases when the defeat of the reticular formation of the upper thoracic segments of the spinal cord led to myocardial infarction.
Damage to the reticular formation of the medulla oblongata disrupts the activity, coordination and integration of the most important centers of regulation of body functions (respiratory movements, blood pressure, etc.). It is known that the respiratory center (N. A. Mislavsky) is located in the reticular formation of the medulla oblongata. Damage to it, depending on the localization, causes a violation of inhalation, exhalation and coordination of respiratory movements. The processes of coordination of the work of the respiratory and vasomotor centers are also disturbed. Fluctuations in blood pressure and blood composition occur (the content of erythrocytes, leukocytes, ROE and other indicators change). There may be asymmetries in the fluctuations of these indicators, especially blood pressure. Strengthening tendon reflexes.
Damage to the medulla oblongata by mechanical trauma, hemorrhage into the cavity of the IV ventricle of the brain, or a tumor that compresses the substance of the medulla oblongata ( bulbus), causes a severe syndrome called bulbar paralysis .
The most important signs of bulbar paralysis are loss of functions of the motor nucleus of the vagus nerve: paralysis of the muscles of the soft palate, violation of the act of swallowing, loss of voice due to paralysis of the vocal cords (aphonia). Then damage to the cells of the hypoglossal nerve can join these phenomena, which causes paralysis of the muscles of the tongue. The spread of damage to the respiratory center of the medulla oblongata leads to respiratory arrest and death of the animal and man. Bulbar paralysis is a formidable sign indicating the possibility of a fatal outcome of the disease.
Damage to the reticular formation of the diencephalon characterized by a change in the tonic effect of this section on the cells of the cerebral cortex, the influence of this section of the reticular formation on the hypothalamus and pituitary gland is also disturbed. Since the reticular formation combines numerous afferent impulses in the diencephalon and "filters" these impulses into the thalamus and other nuclei of the brain stem, damage to this part of the brain is accompanied by various attacks of autonomic dysfunction (palpitations, cold sweat, weakness, decreased muscle tone or its promotion, etc.). These seizures are known as "diencephalic syndrome". Often it is accompanied by a violation of the activity of analyzers (smell, hearing), a disorder of various types of sensitivity, and sometimes loss of consciousness.
Damage to the reticular formation of the diencephalon it is also accompanied by a disturbance in the processes of higher nervous activity, internal, differential inhibition, and a weakening of the closure of conditioned reflexes. Patients complain of fatigue, fatigue when talking, a feeling of memory lapses, etc.
The most important violations of the function of the reticular formation are disorders of its coordinating and integrating role in the activity of various parts of the nervous system, according to the level of damage (spinal, medulla oblongata or midbrain, etc.).
The clinical expressions of these disorders are somewhat different. However, each of them is based on dysfunctions of the reticular formation of the corresponding level.
