The limbic system

Anatomy

The limbic system is one of the oldest parts of the brain. It is present in humans, but also in reptiles and fish.

A term introduced by Paul MacLean in 1952, the limbic system was long considered the seat of emotions (aggressiveness, fear, pleasure, anger), representing the dialogue between the brain and the body. 

Functions of the limbic system

The limbic system is not only involved in emotions but also in:

  • memory learning,
  • olfaction,
  • the control of the endocrine system which participates in the release of hormones,
  • eating behaviors and appetite,
  • the autonomic nervous system which controls respiratory, digestive and cardiovascular functions.

Anatomy of the limbic system

The limbic system is made up of several nuclei located under the cortex (they are said to be subcortical structures) and near the thalamus:

  • The hippocampus (from the ancient Greek hippocampos, meaning « bent horse »): role in learning and storing information in long-term memory.
  • The amygdala (from the Latin amygdala which means « almond »): role in aggression, anger, fear, anxiety and emotional memory. The German physiologist Burdach (1776-1847) is the origin of the term  amygdala. When electrically stimulated, animals become aggressive. And if the amygdala is removed, the animals become very tame and no longer respond to things that would have caused rabies before.
  • The fornix .
  • The limbic cortex (cingulate gyrus, cingulum, insula and parahippocampal gyrus): role in the conscious control of behavior.
  • The septum. One of the first functional roles to be associated with the septum was involvement in the reward (or reinforcement) circuit. The nuclei of the septum have been implicated in a number of other roles such as social behavior and fear expression, and abnormalities in septal functioning have been linked to a variety of illnesses ranging from depression to schizophrenia.
  • The hypothalamus. It is a vital part of the limbic system which is responsible for the production of multiple chemical messengers, called hormones. These hormones control the body’s water levels, sleep cycles, body temperature, and food intake. The hypothalamus is located below the thalamus.
  • The mammillary bodies.
  • The cingulate gyrus. It serves as a channel for transmitting messages between the inner and outer parts of the limbic system.
  • Anterior nucleus of the thalamus.
  • The epiphysis

The limbic system is involved in emotions and memory

The limbic system is involved in the feeling of fear which can be reproduced by stimulating through the hypothalamus and amygdala. Conversely, by destroying the tonsils, the fear and the reaction on the body disappears. For example, when a walker is surprised in the forest by a snake, his tonsil is stimulated, which causes an increase in heart rate and stress.

The limbic system (especially the tonsils) is also the source of anger , as is the phenomenon of addiction (drug use) and pleasure (eg consumption of sugars).

Finally, the tonsils and the hippocampus are involved in the formation, maintenance and extinction of phobic memory (eg fear of certain animals).

The main components

The hippocampus  receives neurons from the limbic cortex, one of the components of which is the parahippocampal gyrus. The parahippocampal gyrus includes the entorhinal cortex, one of the first areas of the brain affected in Alzheimer’s disease. The limbic cortex in turn receives afferents * from other associative cortices (parieto-temporo-occipital and prefrontal).

Afferences: Scientific jargon meaning that the limbic cortex receives neurons from other cortices. We can also say that the associative cortices send efferences towards the limbic cortex, or that they project towards the limbic cortex.

Neurons from the hippocampus project via the fornix into the hypothalamus and the septal nucleus. They also project into the entorhinal cortex which in turn projects into the different cortices (prefrontal, orbital, parahippocampal, cingulate, insular). The mammillary nucleus projects onto the mamillothalamic tract which in turn projects onto the cingulate gyrus. A bidirectional circuit is thus formed: it is the circuit of Papez. It plays a central role in memory .

The amygdala is a set of subcortical nuclei (basolateral, central and corticomedial nuclei) located in the medial temporal lobe. The basolateral nuclei receive projections from the sensory and associative cortical areas of the temporal lobe. They in turn project to the limbic cortex, the prefrontal cortex and the hippocampal formation. The basolateral nuclei project to the thalamus, which projects to the prefrontal cortex. Basolateral nuclei also project to Meynert’s basal nucleus, a nucleus made up of cholinergic neurons that send neuronal projections to the cortex (these neurons are damaged in Alzheimer’s disease). The basolateral nuclei of the amygdala are thought to be involved in the emotional component of sensory stimuli, as well as the memorization of emotional stimuli.

Located below the thalamus, the hypothalamus  sends out neurons that control:

  • the secretion of certain hormones secreted by the pituitary gland (or pituitary gland),
  • the autonomic nervous system (regulation of temperature, the circadian cycle, heart rate, sweating), and
  • certain behaviors (sexual, food, defense, stress).

The basal ganglia

The limbic system is associated with the basal ganglia (or basal ganglia ), a region of the brain comprising a collection of subcortical nuclei located approximately in the center of the human brain. These nuclei are the striatum (grouping together the caudate nucleus and the putamen), the internal and external globus pallidus, the subthalamic nucleus and the substantia nigra.

Connected with the cerebral cortex and the thalamus, they play a fundamental role in voluntary motor skills but also in learning, memory and emotions.

The role of the basal ganglia in emotions is explained by the fact that the ventral part of the striatum receives neuronal projections from regions of the limbic system (amygdala, hippocampal formation and limbic associative cortex).

Neurons from the globus pallidus project into the thalamus, which in turn projects onto the prefrontal cortex.

The limbic system includes cortical and subcortical brain areas involved:

  • in declarative memory and learning (hippocampus and parahippocampal gyrus) and,
  • emotions and behavior (amygdala).

Note that the amygdala is also involved in emotional memory.

Let’s remember that :

  • the diencephalon is made up of the thalamus (from the Greek meaning « chamber »), the hypothalamus and the subthalamus.
  • The telencephalon is made up of the hippocampus (composed of the hippocampus, subiculum and dentate gyrus), amygdala, striatum (caudate nucleus and putamen) and limbic cortex.
Frontal section of a human brain. The regions of the limbic system (limbic cortex, hippocampus) coexist with those of the basal ganglia (caudate nucleus, putamen, substantia nigra.

Legend. Limbic cortex (1); frontal lobe (2); parietal lobe (3); island lobe (4); temporal lobe (5); temporal gyrus (6); hippocampus (7); parahippocampal gyrus (8); cranial nerves (9); substantia nigra (10); red core (11); putamen (12); caudate nucleus (13).