Brain stem

The brainstem is a continuation of the spinal cord in the rostral direction. The conventional boundary between them is the place where the first cervical roots emerge and the pyramidal decussation. The brainstem is divided into the hindbrain and midbrain. The former includes the medulla oblongata, the pons, and the cerebellum. Its continuation is the midbrain, consisting of the quadrigeminal bodies and cerebral peduncles and bordering the diencephalon (thalamus, hypothalamus, subthalamus). Ontogenetically, the spinal cord and brainstem develop from the medullary tube, while the remaining parts of the brain (cerebellum, forebrain) are derivatives of these formations. The spinal cord and brainstem are considered to be the central tubular core of the brain, consisting of a relatively undifferentiated neuronal mass, to which specific neuronal groups are attached as appendages from the outer surface. If in the spinal cord the sensory and motor groups form continuous semi-columns in the form of anterior and posterior horns, then in the brain stem the said formations already look like independent nuclei, in the topography of which traces of continuous columns of the spinal cord can be traced. Thus, the dorsomedial row is made up of the motor nuclei of the XII, VI, IV, III pairs of cranial nerves, and the anterolateral column is made up of the branchial motor nuclei (XI, X, VII, V). The system of the V nerve clearly corresponds to the dorsal horn of the spinal cord, while the true branchial sensory nuclei (X, IX) are less clearly separated from the core. A special position is occupied by the VIII nerve: one of the parts of its nuclei - the vestibular - is part of the core of the brain, while the auditory nuclei have a separate highly differentiated structure.

Thus, part of the brainstem formations (namely, the nuclei of the cranial nerves) are homologues of the anterior and posterior horns of the spinal cord and provide segmental innervation. The second component specific part of the brainstem are the ascending classical afferent systems that carry information to the brain from the extero-, proprio- and interoreceptors, as well as the descending pyramidal tract from the cerebral cortex to the spinal cord. The latter position should be accepted with reservation, since the fibers from the Betz cells (motor cortex) make up a small part of the pyramidal tract. The latter includes both descending fibers from the vegetative apparatus of the brain and fibers that carry the efferent function of the cortical-subcortical formations that organize the motor act. In addition, the brainstem contains clearly differentiated formations: olives, red nucleus, black substance, which plays an important role in the cortical-subcortical-stem-cerebellar systems that regulate the maintenance of posture and the organization of movements. The red nucleus is the beginning of the rubrospinal tract, which has been described in animals and is absent, according to the latest data, in humans.

In addition to the three groups of formations (the nuclei of the cranial nerves, the classical afferent and efferent pathways, and clearly differentiated nuclear groups), the brainstem includes a reticular formation, represented by a diffuse accumulation of cells of different types and sizes, separated by many multidirectional fibers. The anatomy of the reticular formation of the brainstem has been described for a long time. In recent decades, the most serious studies have been carried out by J. Olscewski (1957), A. Brodal (1958), A. L. Leontovich (1968), and others.

Along with the ideas about the diffuseness of influences and the absence of patterns of morphological organization, a theory about the presence of a morphofunctional structure of the reticular formation has been developed. The most general cytoarchitectonic patterns consist of the identification of large and even giant neurons in the medial sections of the reticular formation of the medulla oblongata and the pons, small and medium neurons are found in the lateral sections of the same level; in the reticular formation of the midbrain there are predominantly small neurons. In addition, J. 0I-scewski (1957) identified 40 nuclei and subnuclei within the reticular formation, the largest of which are the following:

1. lateral reticular nucleus, located lateral and inferior to the inferior olive;
2. reticular nucleus of the pons tegmentum - dorsal to the proper nuclei of the pons;
3. paramedian reticular nucleus - near the midline, dorsal to the inferior olive;
4. reticular giant cell nucleus - from the olive to the level of the VIII pair of nuclei;
5. caudal reticular nucleus of the pons;
6. oral pontine reticular nucleus;
7. reticular parvocellular nucleus of the medulla oblongata;
8. reticular central nucleus of the medulla oblongata.

Less differentiated is the reticular formation of the midbrain, the functional organization of which is being clarified as the chodological patterns are studied. Efferent projections are clearly divided into two groups: those projecting and those not projecting to the cerebellum. Three of the nuclei described above send their neurons to the cerebellum, while the neurons do not have any other projections and are regularly associated with certain parts of the cerebellum. Thus, the lateral reticular nucleus sends fibers through the rope-shaped bodies to the homolateral parts of the vermis and the cerebellar hemisphere, the paramedian reticular nucleus - mainly homolaterally to the vermis and nuclei of the cerebellum, the reticular nucleus of the pons tegmentum - to the vermis and hemispheres. In addition, the paramedian reticular nucleus transmits impulses mainly from the cerebral cortex, and the lateral nucleus - from the spinal cord.

Among the systems that do not project to the cerebellum, descending and ascending projections are distinguished. The main descending pathway is the reticulospinal, which descends into the spinal cord along the anterior (ventral fascicle) and lateral (medial and lateral fascicles) columns of the spinal cord. The reticulospinal pathway originates from the nuclei of the pons (the fibers go ipsilaterally in the ventral columns) and the medulla oblongata (the fibers go in the lateral column to both halves of the spinal cord). In addition to the fibers listed, the reticulospinal pathway includes tectospinal, vestibulospinal, and rubrospinal (in animals) pathways.

The ascending reticular tracts begin in the medial parts of the pons and medulla oblongata and, as part of the central tegmental bundle, reach the thalamus ( centrum medianum, reticular and intralaminar nuclei), hypothalamus, preoptic region and septum. Fibers from the neurons of the midbrain go mainly to the hypothalamus, and from more caudal parts - to the thalamus and subthalamus.

The afferent connections of the reticular formation are determined by interaction with the cerebellum, spinal cord and parts of the brain lying above. The cerebellar-reticular pathways begin from the nuclei of the cerebellum and end on the neurons of the reticular formation, from where they are directed mainly to the red nucleus and thalamus.

The spinoreticular tracts originate at all levels of the spinal cord, run in its lateral columns, and terminate in the reticular formation of the medulla oblongata and pons. The reticular formation is also where the collaterals from all classical sensory tracts terminate.

Descending pathways to the reticular formation are formed from fibers coming from the fronto-parietal-temporal cortex with the pyramidal tract; from the hypothalamus (periventricular system to the middle - longitudinal posterior fasciculus - and medulla oblongata); from the mammillary-tegmental fasciculus from the mammillary bodies to the reticular formation of the brainstem; from the tectoreticular tract (straight and crossing) - from the upper section to the underlying ones.

The vestibular nuclei complex, isolated from the neurons that comprise it, is in close interaction with the reticular formation of the brainstem. The largest is the vestibular lateral nucleus (Deiters' nucleus). The vestibular superior nucleus (Bechterew's nucleus), medial and inferior vestibular nuclei are also clearly differentiated. These formations have characteristic hodological connections that allow us to understand their functional purpose. Efferent pathways from the vestibular lateral nucleus are directed to the spinal cord (the homolateral vestibulospinal tract, which has a somatotopic organization) and other vestibular nuclei. Pathways from the vestibular lateral nuclei to the cerebellum have not been found. The superior vestibular nucleus projects in the oral direction and follows as part of the medial longitudinal fasciculus to the nuclei of the oculomotor nerves. The medial and inferior vestibular nuclei are less specific, and their neurons send their axons in oral and caudal directions, ensuring the implementation of integrative processes.

The reticular formation of the brain stem can be considered as one of the important integrative apparatuses of the brain. It has an independent significance and at the same time is part of a wider integrative system of the brain. Some authors therefore include in the reticular formation the caudal parts of the hypothalamus, the reticular formation of the hypothalamus, and the reticular nuclei of the hypothalamus.

K. Lissak (1960) subdivides the integrative functions of the reticular formation as follows:

1. sleep and wakefulness control;
2. phasic and tonic muscle control;
3. decoding of environmental information signals by modifying the reception and transmission of pulses arriving through various channels.

The brainstem also contains structures that occupy an intermediate position between the so-called specific and non-specific systems. These include clusters of neurons that are designated as the respiratory and vasomotor centers. There is no doubt that these vital structures have a complex organization. The respiratory center has sections that separately regulate inhalation (inspiratory) and exhalation (expiratory), and within the vascular center, populations of neurons that determine the slowing or acceleration of the heart rate, a decrease or increase in blood pressure have been described. In recent years, homeostasis of blood pressure has been studied in detail. Impulses from baroreceptors located in the heart, carotid sinus, aortic arch, and other large vessels are transmitted to the brainstem structures - the nucleus of the solitary tract and the paramedian nuclei of the reticular formation. From these structures, efferent influences go to the nuclei of the X nerve and the vegetative nuclei of the spinal cord. Destruction of the nucleus of the solitary tract leads to an increase in arterial pressure. We designate these formations as semi-specific. The same nuclei of the solitary tract participate in the regulation of sleep and wakefulness, and their irritation, in addition to circulatory or respiratory effects, is manifested by a change in the EEG and muscle tone, i.e. it forms a certain pattern of integral forms of activity.

Descending influences of the reticular formation are realized through the reticulospinal tract, which has a facilitating or inhibitory effect on the segmental apparatus of the spinal cord. The inhibitory field corresponds to the giant-cell reticular nucleus, with the exception of its rostral part, and the reticular nucleus of the medulla oblongata. Facilitating zones are less clearly localized, they cover a larger area - part of the giant-cell nucleus, the pontine nucleus; facilitating influences from the level of the midbrain are realized through polysynaptic connections. Descending influences of the reticular formation act on the alpha and y-motor neurons, which affect the muscle spindles and interneurons.

It has been shown that most of the fibers of the reticulospinal tract terminate not lower than the thoracic segments and only the vestibulospinal fibers can be traced to the sacral segments. The reticulospinal tract also regulates the activity of the cardiovascular system and respiration.

Undoubtedly, the central integration of somatic and vegetative activity is one of the basic needs of the body. A certain stage of integration is carried out by the reticular formation of the trunk. It is important to note that somatic and vegetative influences go through the reticulospinal pathway and that the fields that increase the activity of motor neurons, arterial pressure and respiratory rate are very close to each other. The opposite somatovegetative reactions are also connected with each other. Thus, irritation of the carotid sinus leads to inhibition of respiration, cardiovascular activity and postural reflexes.

Of great importance are the ascending flows of the reticular formation, which receive abundant collaterals from the classical afferent pathways, the trigeminal and other sensory cranial nerves. At the first stages of studying the physiology of the reticular formation, it was assumed that stimuli of any modality cause a nonspecific activation flow directed to the cerebral cortex. These ideas were shaken by the works of P.K. Anokhin (1968), who revealed the specific nature of this pulsation depending on various biological forms of activity. At present, the participation of the reticular formation in decoding information signals from the environment and regulating diffuse, to a certain extent, specific flows of ascending activity has become obvious. Data have been obtained on the specific connections of the brainstem and forebrain for organizing situationally specific behavior. Connections with the structures of the forebrain are the basis for the processes of sensory integration, elementary processes of learning, and memory function.

It is obvious that for the implementation of integral forms of activity it is necessary to integrate ascending and descending flows, the unity of mental, somatic and vegetative components of integral acts. There are sufficient facts indicating the presence of a correlation of descending and ascending influences. It was found that EEG reactions of awakening correlate with vegetative shifts - pulse rate and pupil size. Stimulation of the reticular formation simultaneously with the EEG reaction of awakening caused an increase in the activity of muscle fibers. This relationship is explained by the anatomical and functional features of the organization of the reticular formation. Among them there are a large number of relationships between different levels of the reticular formation, carried out with the help of neurons with short axons, neurons with dichotomous division, axons with ascending and descending projections directed rostrally and caudally. In addition, a general pattern has been revealed, according to which neurons with a rostral projection are located more caudally than neurons constituting descending pathways, and they exchange many collaterals. It has also been found that the corticoreticular fibers terminate in the caudal parts of the reticular formation, from which the reticulospinal tract originates; the spinoreticular tracts terminate in zones where ascending fibers to the thalamus and subthalamus arise; the oral sections, receiving impulses from the hypothalamus, in turn direct their projections to it. These facts indicate an extensive correlation of descending and ascending influences and an anatomical and physiological basis for the implementation of this integration.

The reticular formation, being an important integrative center, in turn represents only a part of more global integrative systems, including limbic and neocortical structures, in interaction with which the organization of purposeful behavior aimed at adaptation to changing conditions of the external and internal environment is carried out.

The rhinencephalic formations, the septum, the thalamus, the hypothalamus, and the reticular formation are separate links of the functional system of the brain that provides integrative functions. It should be emphasized that these structures are not the only brain apparatuses involved in organizing integral forms of activity. It is also important to note that, being part of one functional system built on a vertical principle, individual links have not lost their specific features.

The medial bundle of the forebrain, which connects the forebrain, intermediate and midbrain, plays a significant role in ensuring the coordinated activity of these formations. The main links, united by the ascending and descending fibers of the bundle, are the septum, amygdala, hypothalamus, and reticular nuclei of the midbrain. The medial bundle of the forebrain ensures the circulation of impulses within the limbic-reticular system.

The role of the neocortex in vegetative regulation is also obvious. There are numerous experimental data concerning stimulation of the cortex: this causes vegetative responses (it should only be emphasized that the effects obtained are not strictly specific). When stimulating the vagus, splanchnic or pelvic nerve, evoked potentials are recorded in various zones of the cerebral cortex. Efferent vegetative influences are carried out through fibers that are part of the pyramidal and extrapyramidal pathways, where their specific weight is large. With the participation of the cortex, vegetative support for such forms of activity as speech and singing is carried out. It has been shown that when a person intends to make a certain movement, an improvement in the blood circulation of the muscles participating in this act develops ahead of this movement.

Thus, the leading link participating in suprasegmental vegetative regulation is the limbic-reticular complex, the features of which, distinguishing it from the segmental vegetative apparatuses, are the following:

1. irritation of these structures does not entail a strictly specific vegetative reaction and usually causes combined mental, somatic and vegetative shifts;
2. their destruction does not entail certain natural disturbances, except in cases where specialized centers are affected;
3. there are no specific anatomical and functional features characteristic of segmental vegetative apparatuses.

All this leads to an important conclusion about the absence of sympathetic and parasympathetic divisions at the studied level. We support the point of view of the largest modern vegetologists who consider it expedient to divide the nvdsegmental systems into ergotropic and trophotropic, using the biological approach and the different roles of these systems in the organization of behavior. The ergotropic system facilitates adaptation to changing environmental conditions (hunger, cold), ensures physical and mental activity, the course of catabolic processes. The trophotropic system causes anabolic processes and endophylactic reactions, ensures nutritional functions, and helps maintain homeostatic balance.

The ergotropic system determines mental activity, motor readiness, and vegetative mobilization. The degree of this complex reaction depends on the importance and significance of the novelty of the situation encountered by the organism. The segmental sympathetic system is widely used. Optimal blood circulation of the working muscles is ensured, arterial pressure increases, the minute volume increases, the coronary and pulmonary arteries dilate, the spleen and other blood depots contract. Powerful vasoconstriction occurs in the kidneys. The bronchi dilate, pulmonary ventilation and gas exchange in the alveoli increase. Peristalsis of the digestive tract and secretion of digestive juices are suppressed. Glycogen resources are mobilized in the liver. Defecation and urination are inhibited. Thermoregulatory systems protect the organism from overheating. The efficiency of the striated muscles increases. The pupil dilates, receptor excitability increases, attention is sharpened. Ergotropic restructuring has a first neural phase, which is enhanced by a secondary humoral phase, depending on the level of circulating adrenaline.

The trophotropic system is associated with the period of rest, with the digestive system, some stages of sleep ("slow" sleep) and mobilizes mainly the vagus-insular apparatus when activated. Slowing of the heart rate, decrease in the strength of systole, lengthening of diastole, decrease in arterial pressure are observed; breathing is calm, somewhat slow, the bronchi are slightly narrowed; intestinal peristalsis and secretion of digestive juices increase; the action of the excretory organs is enhanced: inhibition of the motor somatic system is observed.

Within the limbic-reticular complex, zones are identified whose stimulation can produce predominantly ergotropic or trophotropic effects.

Often the fundamental difference between the sympathetic and parasympathetic effects, on the one hand, and the ergotropic and trophotropic ones, on the other, is not clearly grasped. The first concept is anatomical and functional, the second is functional and biological. The first apparatuses are associated exclusively with the segmental vegetative system, and their damage has certain manifestations; the second do not have a clear structural base, their damage is not strictly determined and manifests itself in a number of areas - mental, motor, vegetative. Suprasegmental systems use certain vegetative systems to organize correct behavior - predominantly, but not exclusively, one of them. The activity of the ergotropic and trophotropic systems is organized synergistically, and only the predominance of one of them can be noted, which in physiological conditions is precisely correlated with a specific situation.