The limbic system of the brain

The limbic section of the cerebral hemispheres currently includes the cortical zones of the olfactory analyzer (hippocampus - gyrus hippocampi, transparent septum - septum pellucidum, cingulate gyrus - gyrus cinguli, etc.), and partly the gustatory analyzer (circular sulcus of the insula). These sections of the cortex are connected with other mediobasal areas of the temporal and frontal lobes, with the formations of the hypothalamus and the reticular formation of the brainstem. The listed formations are united by numerous bilateral connections into a single limbic-hypothalamic-reticular complex, which plays a major role in the regulation of all vegetative-visceral functions of the body. The oldest sections of the cerebral cortex, which are part of this complex, differ in their cytoarchitectonics (three-layer type of cellular structure) from the rest of the cortex, which has a six-layer type of structure.

R. Brosa (1878) considered the phylogenetically old telencephalic areas located around the brainstem as a “large limbic lobe”.

These same structures were designated as the "olfactory brain", which does not reflect their leading function in organizing complex behavioral acts. The identification of the role of these formations in the regulation of vegetative-visceral functions led to the emergence of the term "visceral brain" [McLean P., 1949]. Further clarification of the anatomical and functional features and physiological role of these structures led to the use of a less (specific) definition - "limbic system". The limbic system includes anatomical formations united by close functional connections. The structures that make up the limbic system differ in phylogenetic terms:

• ancient cortex (paleocortex) - hippocampus, piriform gyrus, pyriform, periamygdaloid cortex, entorhinal region, olfactory bulb, olfactory tract, olfactory tubercle;
• paraallocortex - an area occupying an intermediate position between the old and new cortex (cingulate gyrus, or limbic lobe, presubiculum, frontoparietal cortex);
• subcortical formations - amygdala, septum, anterior nuclei of the thalamus, hypothalamus;
• reticular formation of the midbrain.

The central links of the limbic system are the amygdala and the hippocampus.

The amygdala receives afferent input from the olfactory tubercle, septum, pyriform cortex, temporal pole, temporal gyri, orbital cortex, anterior insula, intralaminar nuclei of the thalamus, anterior hypothalamus, and reticular formation.

There are two efferent pathways: the dorsal one - through the stria terminalis to the anterior hypothalamus and the ventral one - to the subcortical formations, the temporal cortex, the insula and along the polysynaptic pathway to the hippocampus.

Afferent impulses come to the hippocampus from the anterior basal formations, the frontotemporal cortex, the insula, the cingulate groove, and from the septum through Broca's diagonal ligament, which connects the reticular formation of the midbrain with the hippocampus.

The efferent pathway from the hippocampus goes through the fornix to the mammillary bodies, through the mammillothalamic bundle (Vicq d'Azyr bundle) to the anterior and intralaminar nuclei of the thalamus, then to the midbrain and pons.

The hippocampus is closely connected with other anatomical structures that are part of the limbic system and together with them forms the Papez circle [Papez J., 1937]: hippocampus - fornix - septum - mammillary bodies - anterior nuclei of the thalamus - cingulate gyrus - hippocampus.

Thus, two main functional neuronal circles of the limbic system are distinguished: the large circle of Papez and the small circle, including the amygdala complex - stria terminalis - hypothalamus.

There are several classifications of limbic structures. According to the anatomical classification of H. Gastaut, H. Lammers (1961), there are two parts - basal and limbic; according to the anatomical and functional classification - the oromedial-basal region, which regulates vegetative-visceral functions, behavioral acts associated with the food function, sexual, emotional sphere, and the posterior region (the posterior part of the cingulate groove, hippocampal formation), which takes part in the organization of more complex behavioral acts, mnemonic processes. P. McLean distinguishes two groups of structures: the rostral (orbital and insular cortex, temporal pole cortex, piriform lobe), which ensures the preservation of life of a given individual, and the caudal (septum, hippocampus, lumbar gyrus), which ensures the preservation of the species as a whole, regulating generative functions.

K. Pribram, L. Kruger (1954) identified three subsystems. The first subsystem is considered to be the primary olfactory (olfactory bulb and tubercle, diagonal bundle, cortico-medial nuclei of the amygdala), the second provides olfactory-gustatory perception, metabolic processes and emotional reactions (septum, basal-lateral nuclei of the amygdala, frontotemporal basal cortex) and the third is involved in emotional reactions (hippocampus, entorhinal cortex, cingulate gyrus). Phylogenetic classification [Falconner M., 1965] also identifies two parts: the old one, consisting of mammillary structures closely associated with the formations of the midline and neocortex, and the later one - the temporal neocortex. The first one carries out vegetative-endocrine-somatoemotional correlations, the second one - interpretive functions. According to the concept of K. Lissak, E. Grastian (1957), the hippocampus is considered as a structure that has inhibitory effects on the thalamocortical system. At the same time, the limbic system plays an activating and modeling role in relation to a number of other brain systems.

The limbic system is involved in the regulation of vegetative-visceral-hormonal functions aimed at ensuring various forms of activity (eating and sexual behavior, processes of species preservation), in the regulation of systems that ensure sleep and wakefulness, attention, emotional sphere, memory processes, thus implementing somatovegetative integration.

The functions in the limbic system are presented globally, are poorly differentiated topographically, but at the same time certain sections have relatively specific tasks in organizing holistic behavioral acts. Including neural closed circles, this system has a large number of "inputs" and "outputs" through which its afferent and efferent connections are realized.

Damage to the limbic region of the hemispheres causes primarily various disorders of the vegetative-visceral functions. Many of these disorders of the central regulation of vegetative functions, which were previously attributed only to pathology of the hypothalamic region, are associated with damage to the limbic region, especially the temporal lobes.

Pathology of the limbic region may manifest itself as symptoms of loss with vegetative asymmetry or symptoms of irritation in the form of vegetative-visceral attacks, more often of temporal, less often of frontal origin. Such attacks are usually shorter than hypothalamic ones; they may be limited to short auras (epigastric, cardiac, etc.) before a general convulsive attack.

When the limbic zone is damaged, there is fixational amnesia (memory disorder similar to Korsakov's syndrome) and pseudo-reminiscences (false memories). Emotional disorders (phobias, etc.) are very common. Disorders of the central regulation of vegetative-visceral functions entail a violation of adaptation, adjustment to changing environmental conditions.

Corpus callosum

In the corpus callosum - a massive formation of white matter - commissural fibers pass, connecting paired sections of the hemispheres. In the anterior section of this large commissure of the brain - in the genu (genu corporis callosi) - connections pass between the frontal lobes, in the middle section - in the trunk (truncus corporis callosi) - between the parietal and temporal lobes, in the posterior section - in the thickening (splenium corporis callosi) - between the occipital lobes.

Corpus callosum lesions manifest themselves in mental disorders. With lesions in the anterior sections of the corpus callosum, these disorders have features of the "frontal psyche" with confusion (behavioral, action, and critical disorders). Frontal-callous syndrome is distinguished (akinesia, amimia, aspontaneity, astasia-abasia, oral automatism reflexes, decreased criticism, memory impairment, grasping reflexes, apraxia, dementia). Disconnection of connections between the parietal lobes leads to distorted perceptions of the "body scheme" and the appearance of motor apraxia in the left upper limb; temporal mental changes are associated with impaired perceptions of the external environment, with the loss of correct orientation in it (the syndrome of "already seen", amnestic disorders, confabulations); lesions in the posterior sections of the corpus callosum lead to complex types of visual agnosia.

Pseudobulbar symptoms (violent emotions, oral automatism reflexes) are also common in lesions of the corpus callosum. However, pyramidal and cerebellar disorders, as well as disorders of cutaneous and deep sensitivity, are absent, since their projection innervation systems are not damaged. Of the central motor disorders, dysfunctions of the pelvic sphincters are most often observed.

One of the features of the human brain is the so-called functional specialization of the cerebral hemispheres. The left hemisphere is responsible for logical, abstract thinking, the right - for concrete, figurative thinking. The individuality and features of perception (artistic or thinking type of character) depend on which of the hemispheres is most morphologically developed and dominant in a person.

When the right hemisphere is switched off, patients become verbose (even chatty), talkative, but their speech loses intonational expressiveness, it is monotonous, colorless, dull, acquires a nasal (nasal) tint. Such a violation of the intonational-vocal component of speech is called dysprosody (prosody - melody). In addition, such a patient loses the ability to understand the meaning of the speech intonations of the interlocutor. Therefore, along with the preservation of the formal vocabulary (vocabulary and grammar) and an increase in speech activity, a "right-hemisphere" person loses the figurativeness and concreteness of speech that intonational-vocal expressiveness gives it. The perception of complex sounds is impaired (auditory agnosia), a person stops recognizing familiar melodies, cannot hum them, has difficulty recognizing male and female voices (figurative auditory perception is impaired). The deficiency of figurative perception is also revealed in the visual sphere (does not notice the missing detail in unfinished drawings, etc.). The patient has difficulty performing tasks that require orientation in a visual, figurative situation, where it is necessary to take into account specific features of the object. Thus, when the right hemisphere is switched off, those types of mental activity that underlie figurative thinking suffer. At the same time, those types of mental activity that underlie abstract thinking are preserved or even strengthened (facilitated). Such a state of the psyche is accompanied by a positive emotional tone (optimism, a tendency to joke, faith in recovery, etc.).

When the left hemisphere is damaged, a person's speech abilities are sharply limited, the vocabulary is depleted, words denoting abstract concepts are dropped from it, the patient does not remember the names of objects, although he recognizes them. Speech activity is sharply reduced, but the intonation pattern of speech is preserved. Such a patient recognizes song melodies well and can reproduce them. Thus, when the function of the left hemisphere is impaired, the patient, along with the deterioration of verbal perception, retains all types of figurative perception. The ability to remember words is impaired, he is disoriented in place and time, but notices details of the situation; visual specific orientation is preserved. At the same time, a negative emotional background arises (the patient's mood worsens, he is pessimistic, it is difficult to distract himself from sad thoughts and complaints, etc.).

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