Limbic system - connections and function (cortical areas, hippocampal formation, amygdalar complex)

The limbic system can be understood as a very complex, interconnected complex of various structures located on the medial surface of the cerebral hemisphere on either side of the diencephalon , surrounded from above by the corpus callosum .

Structure of the limbic system

The structure of the limbic system is widely debated today, but according to the latest divisions, the limbic system can be divided into:

Localization of the limbic system

Limbic system

Limbic cortical area (lobus limbicus)

• Neocortical field – uncus gyri parahippocampalis, gyrus subcallosus, gyrus cinguli, gyrus parahippocampalis;
• mesocortical or transitional field – entorhinal and perirhinal cortical area, praesubiculum;
• archicortical field – hippocampal formation (subiculum, hippocampus, dentate gyrus);
• paleocortical field – olfactory cortical area.

Limbic subcortical structures

• Amygdala;
• septum verum;
• a large part of the hypothalamus ;
• thalamic nuclei ;
• habenula nuclei (epithalamus);
• some nuclei of the reticular formation ;
• striatum and ventral pallidum.

Functions of the limbic system

The most important functions of the limbic system include the control of anxiety, fear, social and emotional behavior (mainly thanks to the amygdala), participation in short-term memory processes (long-term memory is more related to thalamo-cortical and intracortical connections) and even the control of heart activity, breathing (thanks to the connection to the hypothalamus ) or the secretion of endocrine glands .

Other functions include the connection with sexual expressions or care for offspring. The entire complex also functions thanks to the supply of acetylcholine as a mediator from the septum verum. Mainly due to extensive connections with the association areas of the frontal, parietal and temporal lobes, the limbic system participates in sensory perception and its evaluation.

Limbic system involvement

Cingular cortical area - gyrus cinguli

- associated with thalamus, hypothalamus and ncc. raphes

- the central part sends descending fibers to the spinal cord and is the seat of M3 and M4

- regulator of mental, emotional and motor manifestations, which are stimulated by sensitive painful stimuli and also accompany changes in the autonomic nervous system - changes in blood pressure, breathing, bowel motility, pulse

A highly heterogeneous area composed of areas 23, 24, 25, 29, 30, 31. Afferent and efferent inputs come mainly from the association areas of the temporal, parietal and frontal lobes. A very important bundle of fibers called the cingulum , which forms part of the Pape's circuit and goes to the parahippocampal gyrus. It also projects to subcortical structures - mainly to the striatum, cerebellum (via the nuclei pontis), thalamus (the anterior cingulate area to the medial and intralaminar nuclei, the posterior to the lateral, anterior and pulvinar nuclei). The anterior cingulate area is oriented towards emotional reactions, while the posterior to verbal memory or spatial orientation.

Gyrus parahippocampalis and uncus gyri parahippocampalis

The cingulate gyrus passes at the level of the splenium of the corpus callosum into the parahippocampal gyrus (about 5 cm long), which ends rostrally as the uncus gyri parahippocampalis, on which we distinguish the gyrus semilunaris (here the cortical nuclei of the amygdala are located), the gyrus ambiens and the end of the gyrus dentatus. At the bottom of the uncus gyri parahippocampalis is an area of ​​fundamental importance for the connection of the neocortex with the hippocampal formation called the entorhinal cortical area (area 28). The perirhinal cortical area (area 35, 36) along the sulcus rhinalis and sulcus collateralis is equally important .

Afferentation comes from the association areas of the neocortex, from the prefrontal and olfactory cortex, the hippocampal formation, the amygdala, the thalamus and other structures. Efferentation is practically reciprocal (amygdala, anterior thalamic nuclei, ventral striatum...), but the bundle leading to the hippocampal formation is very important. The main task of the parahippocampal gyrus is spatial memory (place cells ) and orientation and the ability to distinguish and recognize objects

Area subcallosa and gyrus paraterminalis

- in front of the rostrum corporis callosi, olfactory fibers from the pars olfactoria of the nasal mucosa go here

Hippocampus

- at the bottom of the temporal process

- sulcus hippocampalis separates the hippocampus from the parahippocampal gyrus, the transitional area is called the subiculum

Fornix

- efferent fibers emerging from the hippocampus

Corpus mammillare

- part of the hypothalamus just behind the infundibulum

Rhinencephalon structures

- bulbus olfactorius, tractus olfactorius, trigonum olfactorium, stria olfactoria medialis et lateralis

-importance of smell

Amygdala

-in the depth of the temporal lobe

- the area of ​​the cortex above the nuclei is called the cortex periamygdaloideus

Corpus amygdaloideum

Amygdala This is a set of nuclei (gray matter of the CNS) on the underside of the uncus gyri parahippocampalis, which belongs developmentally to the basal ganglia, but functionally belongs to the limbic system.

• Corticomedial nuclei – on the surface of the uncus, developmentally oldest, connected to the olfactory bulb.
• Central nuclei – are small, located dorsally, under the influence of brainstem structures .
• Basolateral nuclei – make up over 70% of the volume, in contact with the neocortex, rich in monoamines.

The amygdala gives off two important bundles of predominantly efferent fibers:

• The ventral amygdalofugal system projects from the basolateral part of the amygdala to the thalamus , hypothalamus , brainstem , and cerebral cortex;
• The stria terminalis arc from the corticomedial part of the amygdala through the caudate nucleus and thalamus to the hypothalamus .

Afferentation is provided mainly by the association areas of the neocortex, then by the olfactory bulb, hippocampal formation, but also by numerous subcortical structures ( thalamus , hypothalamus , brainstem , cholinergic projections from the nucleus basalis Meynerti). Efferentation is represented by reciprocal connections (insular cortex, entorhinal area, paleocortex, septum verum). Within the intraamygdalar connections, information from the lateral nucleus (afferentation from the neocortex) reaches the basal nucleus, where it is connected with information from the hippocampal formation or prefrontal cortex. Finally, it travels to the central nucleus, which acts as the main efferent structure projecting to the hypothalamus and brainstem .

The amygdala has very rich connections and when it is stimulated, we can observe a reaction of "increased attention" and conversely, with bilateral damage, fear and anxiety disappear. It can therefore be said that the amygdala is part of a defense mechanism that is able to evaluate the "danger" of an approaching object or other organism. The amygdala also decides on the positive or negative "aura" of a given sensation and is considered the "center of epilepsy". When stimulated, aggressive behavior occurs, when damaged or removed, a calm reaction.

Hippocampal formation

Andersen circuit

We use the term archaecortex or archicortex for it, it is an evolutionarily old, three-layered cortical region composed of 3 parts.

1. Subiculum – located on the upper side of the parahippocampal gyrus, medially from it are the entorhinal cortical area and the presubiculum (mesocortex), laterally it passes into the hippocampus.
2. Hippocampus or cornu Ammonis – the largest structure of the hippocampal formation (about 5 cm long ridge arching into the lateral ventricle of the brain) consisting of 4 units (CA1 – CA4). At the upper edge begins the bundle of fibers of the fornix (fimbria fornicis), which connects the hippocampal formation with the rest of the limbic system.
3. Gyrus dentatus – medial to the hippocampus, narrows anteriorly to form Giacomini's strip. Posteriorly, it follows the fimbria fornicis, smoothes out and is called the gyrus fasciolaris.

On a frontal section, it can be seen that the dentate gyrus (shaped like the letter C) is "bitten" from above into the hippocampus (shaped like a lying S). Rich afferentation comes from the entorhinal cortex, association areas of the neocortex (via the cingulate gyrus and the parahippocampal gyrus), the thalamus , the septum verum (cholinergic stimulation), the locus coeruleus (noradrenergic stimulation), the raphe nuclei of the reticular formation (serotoninergic stimulation). The intensity of the stimuli increases the closer they are to the hippocampal formation, which thus has to cope with a huge onslaught of information. Efferentation is partly reciprocal, but most of the signals go through the fornix to the thalamus and hypothalamus or through the entorhinal cortical area to the neocortex. The efferent fibers are mainly excitatory with the glutamate mediator.

Papez and Andersen circuit

Papez circuit

The Papez and Andersen circuits form the two most important circuits associated with the limbic system. The Papez circuit begins in the hippocampal formation, from where it continues along the fornix to the hypothalamus (corpora mammillaria) and further through the mammillothalamic tract to the anterior nuclei of the thalamus . The signals then reach the cingulate gyrus via the internal capsule fibers and through the cingulum to the parahippocampal gyrus, the entorhinal cortex (area 28) and finally back to the hippocampal formation.

Previously, the Papez circuit and the structures it connected were considered a control mechanism of emotional behavior. However, recently, the idea of ​​the participation of these structures in memory mechanisms (consolidation of memory traces) has been promoted. The Andersen circuit is essentially a part of the Papez circuit (it can be separated from it) and shows the internal connection of the hippocampal formation.

Links

Related articles

• Functions of the Neuronal Circuits
• Memory and its disorders
• Emotion
• Neurobiology

Citations

• DRUGA, Rastislav – GRIM, Miloš – DUBOVÝ, Petr. Anatomie centrálního nervového systému. 1. edition. Praha : Galén; Karolinum, 2011. 219 pp. ISBN 978-80-7262-706-6.

• DRUGA, Rastislav. Mozková kůra, limbický systém [lecture for subject Anatomy, specialization General medicine, 2. LF Univerzita Karlova]. Praha. 17. 05. 2012. 

• ČIHÁK, Radomír. Anatomie 3. 2. edition. Praha : Grada Publishing, 2004. 692 pp. ISBN 978-80-247-1132-4.

• PETROVICKÝ, Pavel. Anatomie s topografií a klinickými aplikacemi. 1. edition. Martin : Osveta, 2002. 542 pp. ISBN 80-8063-048-8.