HUMAN: LIMBIC SYSTEM

COMPONENTS OF LIMBIC SYSTEM

· The limbic system consists of the phylogenetically old limbic lobe and the related subcortical structures.
· The limbic lobe forms a rim (limbus) around the hilum of the cerebral hemispheres.
· It includes the subcallosal gyrus, cingulate gyrus, the parahippocampal gyrus with the underlying hippocampus and the dentate gyrus.

· The limbic lobe is well preserved throughout the animal kingdom and is associated with the sense of olfaction.
· The subcortical structures that are related to the limbic lobe are the amygdala, the septum, hypothalamus, the habenula, anterior thalamic nucleus and parts of the basal ganglia (ventral striatum).
· The limbic lobe and the associated subcortical structures forming the limbic system are concerned with emotions through their role in visceral regulation.

CONNECTIONS
· The important tracts of the limbic system are:

(1) median forebrain bundle,

(2) fornix &

(3) stria terminalis.

· These tracts are in connection with the hypothalamus.
· Papez circuit: The fornix arises from the hippocampus and projects to the mammillary bodies (hypothalamus).

· The Mammillothalamic Tract (MTT) from the mammillary body connects with anterior thalamic nuclei, which in turn, projects to the cingulate cortex.

· The cingulate cortex has connections with the hippocampus.

· This series of connections is the structural basis for emotions and is referred to as the Papez circuit.
· The limbic structures do not have many connections with the neocortical (newly developed) areas of the cortex.

· However, the limbic structures are under the control of the cerebral cortex.

FUNCTIONS:
· The limbic system is primarily concerned with emotions.

· All emotions have a physical and mental aspect to them.
· The mental aspects of emotions are:

(1) cognition - recognizing the cause,

(2) affect - subjective feeling &

(3) conation - the effect.
· The physical aspects involve the various visceral and somatic changes that occur during emotional states.

The Kluver- Bucy syndrome
· This syndrome is seen in the experimental animals illustrate the role of the limbic system in emotions.

· Wild monkeys are aggressive and are difficult to handle.
· Bilateral removal of the temporal lobe (bilateral temporal lobectomy) in wild monkeys brings about dramatic changes in their behaviour. This was originally demons­trated by, Kluver and Bucy.
· The state of the animal, after temporal lobectomy, is referred to as Kluver-Bucy syndrome. The salient manifestations of this syndrome are - placidity, visual agnosia, oral tendencies, hyperphagia and bizarre sexual behaviour.
· The normally aggressive animals become very docile and become placid with no display of any fear, rage or aggression, on provocation.
· The animals fail to recognize any obstacle (visual agnosia) in their path and compulsively examine any object by placing it in their mouth (oral tendencies).

· They eat voraciously (hyperphagia) and exhibit bizarre sexual behaviour.
· Bilateral temporal lobectomy removes most of the limbic structures, especially the amygdala.
· Sense of fear can be evoked in animals by stimulation of the limbic structures, especially the amygdala.

· A Kluver-Bucy monkey exhibits no sense of fear and does not respond to the presence of a snake, to which monkeys are normally afraid.
· Stimulation of the hypothalamus and amygdala are seen to induce emotions of fear or anger in human beings. Aggressive patients can be made placid with bilateral amygdaloid lesions.

Aversion, Pleasure and Reward
· Self-stimulation studies in experimental animals have given evidence of 'pleasure centres’ and ‘aversion centres'.

· Self-­stimulation of the medial forebrain bundle makes the animal repeatedly stimulate to exhaustion, and is considered to be a 'pleasure centre'.
· In man, stimulation of this region gives a sense of wellbeing.

· On the contrary, self-stimulation of the posterior hypothalamus is associated with aversion and the animal avoids stimulating this area. These regions are thought to lie in the ascending pathways leading to regions in the brain where emotion is experienced.
· Te pleasure centres and aversion centres of the limbic system may be associated with the learning process since the information that accompanies reward is learnt, whereas punishment or aversion related information is normally discarded.

Preservation of Self
· The limbic system is concerned with the emotional aspects of the individual related to his survival in a changing environment.
· This is best seen in animals. Feeding is a basic urge in the survival of the animal. Hunger and thirst initiate the animal to go in search of food and water and feeding counteracts the subjective feeling of hunger. An animal feeds till it feels satiated.

· Hunger and satiation are thus emotional states involved in feeding behaviour. In a similar way, the limbic system is thought to be involved in emotional states associated with other activities concerned with the survival of the individual animal.

Preservation of the Species
· Sexual behaviour is related to the preservation of the species. The limbic system is involved in the emotional aspects of sexual behaviour.
· The Kluver - Bucy animals exhibit hypersexuality and abnormal sexual behaviour. The role of the hypothalamus in sexual behaviour and reproduction has been described earlier.
· Bilateral lesions in or around the amygdaloid nucleus cause hypersexuality in men. Stimulation of the amygdala produces sexual behaviour in animals.

· Lesions in the anterior thalamic nucleus abolish oestrus in animals.

Function of the Limbic Lobe (Cortex)
· The precise role of the limbic lobe in the elaboration of emotions is not well understood. Bilateral temporal lobectomy in animals produces a placid animal with heightened oral tendency and sexual behaviour. The animal shows no fear or anger and indulges in overeating.
· Removal of the cingulate gyrus and the subcallosal gyrus makes the animal vicious and aggressive. The limbic cortex is thus seen as an associative region between the cortical structures and the subcortical structures concerned with emotions.

Functions of the Hippocampal formation
· In the lower animals the hippocampus is associated with the sense of smell and it plays an important role in decision-making.
· The hippocampus has evolved in the higher organisms as a neural structure concerned with memory. The hippocampus is sometimes the site of the epileptic focus. Removal of the hippocampus makes the individual unable to remember new information.
· However, these people can recall old memories of events and learned facts. This type of loss of memory is called anterograde amnesia. Hippocampus is now considered to be involved in short-term memory or recent memory, and the consolidation of memory.

Function of Amygdala
· Amygdala or amygdaloid complex is a group of nuclei situated in the rostral end of the temporal lobe and lies in the inferior horn of the lateral ventricles. It consists of three groups of nuclei – cortico-medial, basolateral and central.
·The corticomedial group is the oldest and has extensive olfactory connections, and is reciprocally connected to the hypothalamus. The basolateral group is the most recent and has reciprocal connections with neocortical association areas in the parietal and temporal lobes. It has also connections with the basal ganglia, thalamus and limbic lobe. The central group gives rise to descending pathways to the autonomic neurons of the brainstem.
· Amygdala serves to integrate the autonomic and visceral functions through its reciprocal connections with the hypothalamus and the visceral nuclei of the brainstem.
· The amygdala is concerned with complex cognitive functions that influence emotion and behaviour. Impulses from the sensory systems, the cerebral cortex and other structures cause excitation of visceral and somatic mechanisms.
· Their combined action provides the physiological basis for emotion (e.g., Fear) and behaviour. The reciprocal connections between the cortical areas and the amygdala appear to be important in correlating emotional expression with behavioural patterns.

Also, Read our other Notes Related to HUMAN:

• Human Digestive System & Digestion
• Circulatory System
• Respiratory System
• Excretory System
• Reproductive System
• Nervous System
• Limbic System
• Human Sense Organs: EYE
• Human Sense Organs: EAR
• Body Immunity
• Human Diseases