The limbic lobe is one of the six lobes that make up the telencephalic hemispheres, together with the frontal, parietal, occipital, temporal and insula lobes.
The limbic system consists of a set of interconnected regions belonging to the central nervous system. The limbic system includes the mesencephalic, diencephalic, telencephalic structures, the septal region, the preoptic region, the hypothalamus, some nuclei of the thalamus, the ventral tegmental area, the girdle's turn, the parahippocampal turn, the hippocampus, the amygdala, the olfactory cortex plus all the beams that connect the different parts.
The limbic system acts in the integration of smell and short-term memory; it performs important functions in relation to emotions, mood, and sense of self-awareness. The limbic system also performs elementary functions such as the integration between the vegetative nervous system and the neuroendocrine. Moreover, some parts of the limbic system are involved in the mnestic, visceral, defensive and reproductive processes.
The limbic system is often confusing and nicknamed the limbic lobe, which is actually only one component of the limbic system after recent discoveries have established that many activities attributed to the limbic system do not correspond to the anatomical limits of the limbic lobe.
The limbic system consists of a series of projections:
- Receives dopaminergic projections from the midbrain in relation to the phenomena of gratification and the effect of psychoactive substances.
- Noradrenergic projections, on the other hand, are involved in panic attacks, anxiety, fear of dying, sense of suffocation and symptoms inherent in epileptic seizures of the limbic cortex.
- Cholinergic projections are essential for memory maintenance, and when lesions of these nuclei occur, some forms of dementia occur.
- Is closely related to the prefrontal cortex, and for this reason, is involved in decision-making mechanisms in response to emotional aches and pains.
Limbic system: the hippocampus
The hippocampus is contained in the temporal lobe and is formed by the archicortex and a continuation of the parahippocampal gyrus and the entorhinal cortex. It has a "C" shape that allows it to be called "Ammon horn". In the space between the hippocampus and the subcool, there is the dentate gyrus. From the hippocampus depart axons that form the bed, a veil of a white substance tells in a beam, the fimbria, which above forms the columns of the fornix.
The hippocampus consists of 3 layers: a deep one, the lacunar molecular layer, continuation of the molecular layer of the neocortex; one composed of pyramidal cells, continuously with the 5th layer of the neocortex, from which depart the efficiencies of the hippocampus; one of the polymorphous cells continuously with the 6th layer of the neocortex. The dentate gyrus also has the same layered structure as the hippocampus, but instead of the pyramidal layer, it has a granular layer.
Hippocampal circuits and files
The hippocampus is involved in several circuits and receives several afferents. Therefore, there is an internal circuit in which from the entorhinal cortex, through the subicolo, a projection to the dentate gyrus starts. From the dentate gyrus start projections that end substantially in the deficient layer forming Schaffer's collateral. The entorhinal cortex at the hippocampus receives sensitive-sensory information and information about the other activities of the cortex. Other fibers instead emerge in the bed and continue in the fimbria and then in the fornix. The fornix is born from the parahippocampal gyrus and surrounds the thalamus below the corpus callosum. The two fornices, right and left, on the midline, join to form the body of the fornix and through this union, the fibers of one side can pass to the other side. Again, at the anterior complexus, the fornix divides again into a right column for the right hypothalamus and a left column for the left hypothalamus, ending in the mammary nuclei. A small part of the fornix constitutes the pre-commissural fornix placed in front of the measure, from which it receives other differences and emits further files.
The hippocampus, through the fornix, receives information from the nuclei of the septum, from the cholinergic nuclei, from the hypothalamus, from the thalamus and from the nuclei of the serotoninergic raphe, and projects information to the amygdala and to the nucleus accumbens.
The hippocampus is involved in the process of recalling a memory. The memory is something that is built up each time in the hippocampus, and its recovery from memory (mnesic activity) is a momentary construction. Moreover, the hippocampus is central in epilepsies, being in most cases its point of origin.
Limbic system: the fornix
The fornix is composed of white substance and represents the commissural organ of the hippocampus. It is located under the corpus callosum and is the continuation of the alveus and the fimbria which, bending forward and up, form the legs of the fornix. The legs continue in the body, placed dorsally to the thalamus constitute the union of the right and left components of the fornix. In front of the thalamus, the columns of the fornix are formed: two symmetrical bundles that go downwards and head towards the anterior comexus, dividing into a postcommissural portion, which leads to the hypothalamus and pre-commissural portion aimed at the septal nuclei.
The transversal fibers are present in the caesura of the fornix between the two legs before they melt to form the body, making the fornix the commodified formation of the limbic system.
Limbic system: the amygdala
The amygdala is placed in the temporal lobe between the hippocampus and the olfactory area, medially at the anterior edge of the temporal horn of the lateral ventricle and inferiorly at the lenticular nucleus. The amygdala is connected to the frontal, temporal, cingular, olfactory and hippocampal cortex in both directions. The amygdala receives afferents from the intraluminal nuclei of the thalamus and from the reticular formation and from the trunk. In addition, it has cholinergic afferents from the base nuclei, dopaminergic from the ventral tegmental area and from the black substance.
The amygdala projects, through the terminal stripe to the preoptic area, to the region of the septum and to the anterior hypothalamus, to the vegetative nuclei of the brainstem through the medial procephalic fascicle. Other fibers reach the septum region, the nucleus accumbens, the middle dorsal nucleus of the thalamus and the limen insulae.
Being part of the limbic system, the amygdala contributes to the emotional component. In fact, it is the point from which it generates fear. Those who suffer damage in this area are docile, or indifferent to emotional stimuli.