Limbic System

• The term "limbic" is from the Latin word Limbus, for "border" or "edge".

• The limbic system is a set of primitive brain structures located on top of the brainstem and buried under the cortex

• It separates the medial surface of the cerebral cortex from diencephalon

Limbic System

• The limbic system consists of a number of cortical & subcortical structures with complex and often looped connections that all project to the hypothalamus.

Limbic System

The limbic system includes:1. Limbic lobe.2. Hippocampal formation.3.Septal area.4.Prefrontal area.5. Amygdala6. Anterior thalamic nuclei7. Hypothalamus (mammillary body)8. Fornix9.Olfactory system.10. Habenular nuclei

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Limbic System Components

• C-shaped ring of grey matter on the medial side of each cerebral hemisphere, surrounding the corpus callosum.

• It includes:1. Subcallosal area 2. Cingulate gyrus3. Isthmus4. Parahippocampal

gyrus5. Uncus.

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Limbic Lobe

Hippocampus• It is a seahorse

shaped paired structure, one in each hemisphere.

• Located in the inferomedial part of the temporal lobe.

• Involved in formation, organization, storage and retrieval of memory

• Its principal efferent projection is to the mammillary body via a C-shaped bundle of fibers called the Fornix.

• Fornix consists of: 1. Fimbria2. Crus 3. Body4. Column• The fornix is an

important component of PAPEZ CIRCUIT

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• In 1937: Papez was the first to describe a relationship between limbic system components.

• Papez’s circuit connects the parahippocampal gyrus, hippocampus, fornix, mamillary body, anterior thalamic nucelus and cingulate gyrus.

Since the initial description, connections to additional subcortical structures have been identified.

Papez Circuit

Hippocampal Formation• The hippocampal formation is

a compound structure in the medial temporal lobe of the brain

• It consists of: Hippocampus Dentate gyrus: Which lies

between hippocampus & Parahippocampal gyrus.

Subiculum (at the base of the hippocampus)

Entorhinal area (area 28) Indusium griseum (grey

matter on the upper surface of the corpus callosum).

Dentate gyrus

Indusium griseum

HippocampusEntorhinal area

Fornix

The entorhinal cortex is part of the medial temporal lobe or hippocampal memory system and constitutes the major gateway between the hippocampal formation and the neocortex.The name entorhinal (inside rhinal) cortex derives from the fact that it is partially enclosed by the rhinal (olfactory) sulcus.

Amygdala• Almond shaped mass of nuclei,

lies near the temporal pole, close to the tail of the caudate nucleus

• It is the behavior awareness area which is responsible for the behavioral response appropriate to each occasion.

Connections:• Input: from association areas of

visual, auditory & somatosensory cortices.

• Output: to hypothalamus & brainstem autonomic nuclei, to control the autonomic centers.

Function: It is involved in emotional responses, fear, anger,

hormonal secretions, and memory.• Relate environmental stimuli to coordinated

behavioral autonomic and endocrine responses seen in species-preservation.

• Responses include:Feeding and drinkingAgnostic (fighting) behaviorMating and maternal careResponses to physical or emotional stresses.

Amygdala

Amygdaloid lesions

• Voracious appetite• Increased (perverse) sexual activity• Docility:

Loss of normal fear/anger response• Memory loss:

Alzheimer`s ???• lack of emotional responses

Septal NucleiSite: Located anterior to the

interventricular foramen below the rostrum of corpus callosum

Main connections:1. To hypothalamus

through medial forebrain bundle.

2. To habenular nuclei through stria medullaris thalami.

Function: It provides critical interconnections and it is the pleasure zone.

Septal area

Interventricular foramen

The Fornix-Made up of fibres arising from hippocampus-Body of the fornix is suspended from corpus callosum by septum pellucidum-Posteriorly,divides into two crura.-The crura are interconnected by fibres passing from one another forming hippocampal commissure. Anterior commissure

Fornix Septum pellucidumFornix

Fornix

Habenular nuclei• Located in habenular

triangle (area in the posterior part of the diencephalon, just anterior to pineal gland)

• Have connections with limbic system

Serves autonomic function and emotional drives

Functions of Limbic system

• Energy and water balance• Autonomic functions• Temperature• Endocrine functions• Sexual behavior• Emotional behavior• Learning and memory

• Korsakoff’s psychosis (Retrograde & anterogdrade amnesia)

• Temporal lobe epilepsy: The hippocampus is a common focus site in epilepsy, and can be damaged through chronic seizures.

• Alzheimer’s disease: The hippocampus is one of the first brain areas to show damage in Alzheimer's disease

The hippocampus is sometimes damaged in diseases such as herpes encephalitis & Schizophrenia.

Limbic Lobe Disorders