Structural and Chemical Organization
The limbic system (Systema limbicum) has a complex multi-level structure, combining cortical areas of different phylogenetic ages with deep brain nuclei.
Cortical structures include:
- Ancient cortex (paleocortex): prepyriform, periamygdaloid, and diagonal areas, as well as the olfactory bulbs, olfactory tubercle, and septum pellucidum.
- Old cortex (archicortex): hippocampus, dentate gyrus, and cingulate gyrus.
- Mesocortex: insular cortex and parahippocampal gyrus.
Subcortical structures include the amygdala, septal nuclei, anterior thalamic nucleus, mammillary bodies, and hypothalamus.
The limbic system actively uses the monoaminergic system for signal transmission. Its function depends on neurotransmitters produced by brainstem neurons that project diffusely into the brain: norepinephrine (from the medulla oblongata and pons), serotonin (from the medulla and midbrain), and dopamine (from the ventral midbrain).
Limbic Circuits and Pathway Functions
A key feature of the limbic system is the presence of closed loops through which neural excitation circulates. This is necessary to maintain physiological rhythms (including adrenal hormone secretion and urination).
There are three main circuits:
- Major hippocampal Papez circuit: mammillary bodies $\rightarrow$ anterior thalamus $\rightarrow$ hippocampus $\rightarrow$ entorhinal cortex $\rightarrow$ septal nuclei $\rightarrow$ mammillary bodies.
- Septo-amygdaloid circuit: septum $\rightarrow$ diagonal band $\rightarrow$ amygdala $\rightarrow$ stria terminalis $\rightarrow$ septum.
- Septo-hippocampal circuit: septum $\rightarrow$ supracallosal striae $\rightarrow$ hippocampus $\rightarrow$ fornix $\rightarrow$ septum.
The system collects input (afferent input) from all senses: olfactory signals go to the septum and amygdala, vision and hearing are relayed via the thalamus to the hippocampus and insula, and tactile and visceral impulses go directly to the insular cortex. Efferent pathways (output) project to the brainstem, thalamus, and neocortex.
Hippocampus: Electrical Activity and Memory
Morphologically, the hippocampus consists of repeating modules with extremely high baseline neuronal activity. Its function is closely and reciprocally related to the neocortex: when slow activity dominates in the neocortex, desynchronization is observed in the hippocampus, and vice versa.
During neocortical desynchronization, rhythmic high-amplitude activity—the $\theta$-rhythm (theta rhythm)—is generated in the hippocampus. Under normal conditions, it accompanies attention, learning, alertness, and the orienting reflex. However, excessive $\theta$-rhythm is a marker of severe negative states (rage, fear, critical hunger).
Through connections with the posterior areas of the frontal cortex, the hippocampus is responsible for memory consolidation—the transfer of information from short-term to long-term memory. When damaged, a person loses the ability to learn and forgets the past: patients do not remember their date of birth, do not recognize doctors, and cannot find their hospital room.
Amygdaloid Body (Corpus amygdaloideum)
The amygdala is located deep within the temporal lobe. Its main functional feature is the presence of polysensory neurons. They exhibit broad convergence, meaning they can respond to stimuli of completely different natures, and are also highly sensitive to chemical substances.
The amygdaloid body performs two global functions:
- Autonomic regulation: amygdalar neuronal activity strictly correlates with respiratory rate and heart rate. It influences digestion and urogenital function.
- Emotional control: works in concert with the ventral frontal cortex. If the pathways between them or the frontal cortex itself are damaged, the individual experiences severe emotional disorders.