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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