Limbric system; its components and its functions

Limbic system is a complex system of cortical and subcortical structures that form a ring around the hilus of cerebral hemisphere. Limbus means ring. It is also known as limbic lobe. Earlier, it was called rhinencephalon. In terms of evolutionary development (phylogeny), limbic system is one of the oldest parts of the brain and it is related to olfactory lobe. It is found as a prominent structure in fish, amphibians, reptiles and mammals. Limbic system is primarily related to emotional part of our life and is extensively concerned with memory.

COMPONENTS OF LIMBIC SYSTEM

Structures of limbic system are classified into four

groups :

1. Archicortical structures

2. Paleocortical structures

3. Juxtallocortical structures

4. Subcortical structures.

ARCHICORTICAL STRUCTURES

Archicortex forms allocortex along with paleocortex. Archicortex is the phylogenetically oldest

structure. It is concerned with memory.

PALEOCORTICAL STRUCTURES

Paleocortex is in between archicortex and neocortex. It is concerned with olfaction.

JUXTALLOCORTICAL STRUCTURES

Juxtallocortex or mesocortex is situated between paleocortex and neocortex.

SUBCORTICAL STRUCTURES

Structures situated below the level of cortex are called subcortical structures. Limbic system includes

six subcortical structures.

CONNECTIONS OF LIMBIC SYSTEM

Connections of limbic system are complex. Following are the major (afferent and efferent) connections of

limbic system:

1. Fornix: It includes fibers connecting:

i. Hippocampus and septal nuclei with the mamillary body

ii. Hippocampus with hypothalamic nuclei.

2. Lateral hypothalamus receives afferent fibers from:

i. Hippocampus

ii. Septal nuclei

iii. Olfactory tubercle

iv. Head of caudate nucleus

v. Piriform area

vi. Periamygdaloid area.

3. Caudate nucleus receives fibers from:

i. Cingulate gyrus

ii. Intralaminar nuclei of thalamus.

4. Brainstem reticular formation receives fibers from:

i. Hippocampus

ii. Cingulate gyrus.

5. Papez circuit.

Papez Circuit

Papez circuit is the interconnections between various structures of limbic system, which form a complex of closed circuit. This circuit was described by Papez. Hippocampus is connected to mamillary bodies of

hypothalamus via fornix. Mamillary bodies are connected to anterior thalamic nucleus via mamillothalamic

tract. Anterior thalamic nucleus is projected into cingulate gyrus through medial thalamocortical fibers.

Cingulate gyrus is in turn connected to hippocampus. Papez circuit plays a role in memory encoding.

FUNCTIONS OF LIMBIC SYSTEM

1. OLFACTION

Piriform cortex and amygdaloid nucleus form the olfactory centers. In lower animals, the amygdaloid

nucleus is concerned primarily with olfaction.

2. REGULATION OF ENDOCRINE GLANDS

Hypothalamus plays an important role in regulation of endocrine secretion.

3. REGULATION OF AUTONOMIC FUNCTIONS

Hypothalamus plays an important role in regulating the autonomic functions (Chapter 149) such as:

i. Heart rate

ii. Blood pressure

iii. Water balance

iv. Body temperature.

4. REGULATION OF FOOD INTAKE

Along with amygdaloid complex, the feeding center and satiety center present in hypothalamus regulate food intake.

5. CONTROL OF CIRCADIAN RHYTHM

Hypothalamus is taking major role in the circadian fluctuations of various physiological activities.

6. REGULATION OF SEXUAL FUNCTIONS

Hypothalamus is responsible for maintaining sexual functions in both man and animals .

7. ROLE IN EMOTIONAL STATE

Emotional state of human beings is maintained by hippocampus along with hypothalamus.

8. ROLE IN MEMORY

Hippocampus and Papez circuit play an important role in memory.

9. ROLE IN MOTIVATION

Reward and punishment centers present in hypothalamus and other structures of limbic system are

responsible for motivation and the behavior pattern of human beings.

 

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