The Limbic System and Its Function

The limbic system is composed of numerous structures, including the amygdala, thalamus, hypothalamus, hippocampus, corpus callosum (callus), and several other brain segments. The limbic lobe manages psychological responses to emotional stimuli (1). It is associated with memory, attention, emotions, sexual urges, character, and behavior.

In this article, we will talk about
the position, structure, anatomy, and functions of the limbic lobe. Finally, we
will address some limbic lobe lesion and damages, as well as their
consequences.

Boundaries, Position, and Structure
of the Limbic Lobe

The concept that was previously known as the "Visceral brain" (2) and denoted a complex anatomical and physiological association, started being called the "limbic system" in 1952. Today, we still know it as the limbic system of the limbic lobe of our brain.

The inner and lower surfaces of the brain hemispheres unite in the so-called limbic (marginal) cortex together with the almond-shaped nucleus from the group of subcortical nuclei, the olfactory tract and the bulb, frontal, temporal, and parietal lobes of the cerebral hemisphere, as well as the subarticular region and reticular formation of the trunk. The limbic cortex is combined into one functional system - the limbic-reticular complex (1).

The
girdle and the hippocampal (parahippocampal) gyrus combine into the limbic
region, which has numerous connections with structures of the reticular
formation, forming with it the limbic-reticular complex that provides a wide
range of physiological and psychological processes.

The limbic lobe is commonly attributed to elements of the old bark (archiocortex), which cover the dentate gyrus and hippocampal gyrus; the ancient cortex (paleocortex) of the anterior hippocampus; as well as the middle, or middle, mesocortex cingulate gyrus.

The term "Limbic system" includes components of the limbic lobe and related structures - the entorhinal region (which occupies most of the parahippocampal gyrus) and the septal region, as well as the amygdala and mastoid body (1).

The mastoid body connects the structures of this system to the midbrain and reticular formation. Impulses that arise in the limbic system can be transmitted through the anterior nucleus of the thalamus into the cingulate gyrus and the new cortex along the pathways that form associative fibers.

Impulses arising in the hypothalamus may reach the orbitofrontal cortex and medial dorsal nucleus of the thalamus.

Numerous direct and inverse relationships ensure the interconnection and interdependence of limbic structures and there are many formations responsible for this.

Those include parts of diencephalons and oral carcass parts (non-specific nucleus of the thalamus, hypothalamus, shell, bridle, reticular formation of the brainstem), as well as subcortical nuclei (pale sphere, shell, caudate, caudate, shell) with new crust of large hemispheres, first of all bark of roots and frontal lobes.

The cingulate gyrus is located on the inner surface of the hemisphere above the corpus callosum. This gyrus, with a coating behind the corpus callosum, passes into a gyrus near the cavalry - parahippocampus gyrus. The cingulate gyrus together with the parahippocampal gyrus makes up the superimposed gyrus.

Slightly deviating from the furrow of the corpus gland and the hippocampal grooves are the marginal corpus, subtop, and nasal sulcus, which are a continuation of each other. These grooves limit the outer portion of the arcuate portion of the median surface of the cerebral hemisphere, known as the limbic lobe.

There are two gyruses in the limbic lobe. The upper limb lobe is the upper limbic (upper edge), or shingle, gyrus (gyrus cinguli), the lower part forms the lower limbic gyrus or gyrus of the cavalry (gyrus hippocampi), or the parahippocampal gyrus (gyrus parahyppocampalis), in front of which is the hook (medically called uncus).

Finally, it is important to say that
the formations of the inner surface of the frontal, parietal, occipital and
temporal lobes are located around the limbic lobe of the brain.

The function of the Limbic Lobe

The main function of the limbic system of the limbic lobe is not so much to provide communication with the outside world but to regulate the tone of cortex and the urges. This system regulates complex, multiple functions of the internal organs and behavioral responses. As a result, we can state that the limbic-reticular complex is the most important integrative system of the body (1).

The limbic system is also important
in the formation of motivation. Motivation (or intrinsic motivation) involves
the most complex instinctive and emotional reactions (regarding food, defensive
reactions, sexual reactions).

The limbic system is also involved in the regulation of sleep and wakefulness. Likewise, it has an important olfactory function. Odor sense allows us a proper perception of chemicals in the air. The olfactory brain of a person provides odor recognition as well as the organization of complex forms of emotional and behavioral reactions. The olfactory brain is part of the limbic system.

The olfactory brain consists of two parts - the peripheral and the central part. The peripheral division is represented by the olfactory nerve, olfactory bulbs, and primary olfactory centers. The central compartment includes the seahorse gyrus - hippocampus, dental and vaulted gyrus.

The odor receptor is located in the nasal mucosa. The nerve conduction system transmits information from the receptor to the cortical part of the olfactory analyzer. The cortical part of the olfactory analyzer is located in the cingulate gyrus, seahorse gyrus, and in the hook of the seahorse, which together forms a closed ring region. The periphery of the olfactory analyzer is connected to the cortical portions of both hemispheres (1).

As the olfactory analyzer plays an
important role in the regulation of emotions, its central part refers to the limbic
system, figuratively called the "common denominator" for various
emotional and visceromotor reactions of the body.

Despite phylogenetic, morphological and cytotarchitectonic differences, many of these structures (limbic region, central and medial thalamus structure, hypothalamus, reticular trunk formation) are usually involved in the so-called limbic-reticular complex and act as a zone of integration of many functions, ensuring the organization of polymodal, holistic reactions of the body to various effects, which is especially pronounced in stressful situations. 

The structures of the limbic-reticular complex have a large number of inputs and outputs, through which closed circuits of numerous afferent and efferent connections pass, thus ensuring the combined functioning of the formations involved in this complex. Likewise, they ensure their interaction with all parts of the brain, including the cerebral cortex.

In structures of the
limbic-reticular complex, there is a convergence of sensitive impulses that
occur in interoperative and exterior receptors, including the receptor fields
of the senses. On this basis, the limbic-reticular complex is the primary
synthesis of information on the state of the internal environment of the
organism, as well as environmental factors that affect the organism and
elemental needs, biological motivations. Finally, accompanying emotions are
formed.

The limbic-reticular complex determines the state of the emotional sphere, participates in the regulation of autonomic-visceral ratios in order to maintain the relative constancy of the internal environment (homeostasis), as well as energy supply and correlation of motor acts.

The level of consciousness, the possibility of automated movements, the activity of motor and mental functions, speech, attention, ability to orientate, memory, change of wakefulness and sleep, etc. all depend on its state.

Damage of the Limbic Lobe

Damage to the structures of the limbic lobe and the limbic-reticular complex, in general, can be accompanied by different clinical symptoms. Those include pronounced changes in the emotional sphere of permanent and paroxysmal nature, anorexia or bulimia, sexual disorders, memory impairment, and especially signs of the Korsak syndrome, in which the patient loses the ability to remember current events (3).

Moreover, the symptoms of the limbic
system damage include vegetative-endocrine disorders, sleep disorders,
psychosensory disorders in the form of illusions and hallucinations, changes in
consciousness, clinical manifestations of akinetic mutism, and seizures.

Pathological processes in the limbic
area cause marked disturbances of the vegetative-visceral functions.

Conclusion

The limbic system is a complex system in our brain that contains many different structures working together to build up a center in the brain responsible for managing psychological responses to emotional stimuli, controlling memory, attention, shaping our behavior and character, as well as affecting our emotions, and sexual urges.

The limbic cortex also has an
important olfactory function. Odor - the perception of chemicals in the air,
heavily depends on this system. A person's olfactory brain processes odor stimuli
and is responsible for smell perception, as well as organizing complex forms of
emotional and behavioral responses. The olfactory brain is part of the limbic
system.

The most important parts of this system include the amygdala, hippocampus, thalamus, hypothalamus, and corpus callosum. The limbic lobe damage may cause a series of serious disorders and conditions.

Data from various studies indicate a
dual (activating and inhibitory) effect of the hippocampus and other structures
of the limbic region on behavioral responses, emotions, mental status
characteristics, and bioelectric activity of the cortex.

References

1. Rajmohan V, Mohandas E. The limbic system. Indian J Psychiatry. 2007 Apr;49(2):132-9. doi: 10.4103/0019-5545.33264. PMID: 20711399; PMCID: PMC2917081.  Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2917081/
2. Roxo MR, Franceschini PR, Zubaran C, Kleber FD, Sander JW. The limbic system conception and its historical evolution. ScientificWorldJournal. 2011;11:2428-41. doi: 10.1100/2011/157150. Epub 2011 Dec 8. PMID: 22194673; PMCID: PMC3236374.  Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3236374/
3. Wen J, Yablonskiy DA, Salter A, Cross AH. Limbic system damage in MS: MRI assessment and correlations with clinical testing. PLoS One. 2017 Nov 9;12(11):e0187915. doi: 10.1371/journal.pone.0187915. PMID: 29121642; PMCID: PMC5679614. Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5679614/