Structural and chemical organization
The limbic system (Systema limbicum) has a complex, multi-level structure, uniting areas of the cortex of different phylogenetic ages and deep brain nuclei.
Cortical areas include:
- Paleocortex (old cortex): prepyriform, periamygdaloid, and diagonal cortex, as well as the olfactory bulbs, olfactory tubercle, and septum pellucidum.
- Archicortex (primitive cortex): hippocampus, dentate gyrus, and cingulate gyrus.
- Mesocortex (intermediate cortex): insular cortex (insula) and parahippocampal gyrus.
Subcortical structures include the amygdala, septal nuclei, anterior thalamic nucleus, mammillary bodies, and hypothalamus.
To transmit signals, the limbic system actively uses the monoaminergic system. Its function depends on neurotransmitters produced by brainstem neurons and diffusely projected into the brain: norepinephrine (from the medulla oblongata and pons), serotonin (from the medulla oblongata and midbrain), and dopamine (from the ventral midbrain).
Limbic circuits and conduction functions
A key feature of the limbic system is the presence of closed pathways through which nervous excitation circulates. This is necessary to maintain physiological rhythms (including adrenal hormone secretion and urination).
Three main circuits are distinguished:
- 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 information (afferent input) from all sense organs: 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) are directed to the brainstem, thalamus, and neocortex.
Hippocampus: electrical activity and memory
Morphologically, the hippocampus consists of repeating modules with extremely high background neuronal activity. Its function is closely and reciprocally linked 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. Normally, 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 its connections with the posterior regions 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.
Amygdala (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 are capable of responding to stimuli of completely different natures, and they also feature the highest sensitivity to chemical substances.
The amygdala performs two global functions:
- Autonomic regulation: the activity of amygdalar neurons strictly correlates with respiratory rate and heart rate. It affects digestion and the genitourinary system.
- Emotion control: operates in conjunction with the ventral frontal cortex. If the pathways between them or the frontal cortex itself are damaged, the person develops severe emotional disorders.
Briefly: answers to topical questions
What are the main functions of the amygdala (Corpus amygdaloideum)?
The main functions of the amygdala consist in autonomic regulation and the control of the emotional sphere.
- Autonomic regulation — neuronal activity correlates with respiration and heart rate, and is closely linked to the regulation of digestion, heart function, and the genitourinary and respiratory systems.
- Emotional sphere — the structure plays a key role in the formation of fear and aggression, and is closely connected to the ventral frontal cortex.
Where do efferent pathways from the limbic system structures lead?
Efferent signals from limbic structures project to:
- The thalamus.
- Subcortical nuclei.
- Most brainstem structures.
- Specific areas of the cerebral cortex.
What are the clinical manifestations of Kluver-Bucy syndrome in bilateral amygdala destruction?
In literature, Kluver-Bucy syndrome is described in bilateral lesions or removal of the temporal regions, including the hippocampus and amygdala.
Main manifestations:
- Feeding behavior disorder (hyperorality) — compulsive examination of objects with the mouth, attempts to eat inedible items; the mechanism is indicated as the loss of assessment of the biological significance of the stimulus.
- Complete loss of the sense of fear — lack of protective reactions to threatening stimuli, impaired emotional memory and learning.
- Hypersexuality — loss of the ability to adequately assess sexual activity.
- Memory and cognitive impairment.
- Loss of the ability to visually recognize objects.
- Loss of social status — loss of hierarchical place in the herd.
Class and exam questions
How are the neocortex and hippocampus connected?
They work reciprocally. An increase in slow rhythms in the neocortex causes desynchronization in the hippocampus, while desynchronization in the neocortex triggers a theta rhythm in the hippocampus.
What does the appearance of a pronounced θ-rhythm (theta rhythm) in the hippocampus indicate?
Normally, it reflects processes of learning, attention, and the orienting reflex. Its pathological enhancement indicates strong negative emotions: fear, aggression, severe hunger, or thirst.
What happens to memory when the hippocampus is damaged?
Memory consolidation is impaired. The patient loses old skills and cannot form new ones (for example, does not remember the current date or the way to their hospital room).
What else to study on this topic
- Anatomy and connections of the olfactory tract with the paleocortex
- Polysensory properties of amygdalar neurons
- Mechanisms of limbic system involvement in epileptogenesis
- Influence of the monoaminergic system on limbic function
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