CNS

1. The Limbic System
THE LIMBIC SYSTEM IS A DIVERSE
COLLECTION of cortical and
subcortical regions that are crucial for
normal human behavior.
Dr M Idris Siddiqui

9. Limbic structures Functions of the limbic
system
1. Sub callosal, cingulated &
parahippocampal gyri
2. Hippocampal formation
3. Amygdaloid nucleus
4. Mammillary bodies
5. Anterior thalamic nucleus
1. Influence the emotional
behavior
1.Reaction to fear & anger
2.Emotions associated with
sexual behavior
2. Hippocampus is involved in
converting short term
memory to long term
memory (If the
hippocampus is damaged,
patient is unable to store
long term memory –
Anterograde amnesia)
The limbic system

10. The limbic lobe
• The limbic lobe fit many of the criteria for an anatomical substrate for drive-related
and emotional behaviour.
• The limbic cortex is connected in one direction with
widespread neocortical areas and in another direction with
the hypothalamus.
• Although there is no universal agreement on the total list of
structures which compromise the limbic system, it can be
considered to be consisting of:
• The hippocampal formation;
• The cingulate gyrus;
• The amygdala;
• The septal area;
• The mamillary bodies;
• The anterior nuclear group of the thalamus;
• The inferior temporal lobe;
• The prefrontal cortex;
• And the tracts that link these areas (e.g., fornix,
mammillothalamic tract and stria terminalis).

12. Major brain division Structure Component part
Cerebral hemisphere (telencephalon) Limbic association cortex Orbitofrontal
Cingulate
Entorhinal
Temporal pole
Perirhinal
Parahippocampal
Hippocampal formation Hippocampus (Ammon's horn)
Subiculum
Dentate gyrus
Amygdala Corticomedial
Basolateral
Central nucleus1
Ventral striatum Nucleus accumbens
Olfactory tubercle
Ventromedial caudate and putamen
Diencephalon Thalamus Anterior nucleus
Medial dorsal nucleus
Midline nuclei
Hypothalamus Mammillary nuclei
Ventromedial nucleus
Lateral hypothalamic area
Epithalamus2 Habenula
Midbrain Portions of the periaqueductal gray matter and reticular
formation
Compnents of limbic system

13. The Limbic Association Cortex
• Located on the Medial Surface of
– the Frontal,
– Parietal, and
– Temporal Lobes

17. Gross Anatomy of the Hippocampal Formation
• This is in the temporal lobe as the floor of the
inferior horn of the lateral ventricle.
• The hippocampal formation is a curved and
recurved sheet of cortex folded into the
medial surface of the temporal lobe.
• Transverse sections reveal that it has 3 distinct
zones: the dentate gyrus, the hippocampus
proper and the subiculum.
• In such sections, the hippocampal formation
has the appearance to 2 interlocking Cs.
• Its shape has also been described to resemble
a rams horn and is thus also called the cornu
ammonis.

18. Gross Anatomy of the Hippocampal Formation
• Embryologically, the hippocampal
formation is an extension of the medial
edge of the temporal lobe.
• During development, it becomes
invaginated by the hippocampal sulcus
and the tip (dentate gyrus) rotates
around the adjacent hippocampus.
• The entire hippocampal formation has a
length of about 5 cm from its anterior end
at the amygdala to its tapering posterior
end near the splenium of the corpus
callosum.

20. • Alveus
• A thin layer of fibres, the alveus, covers the ventricular surface of the
hippocampus.
• This gives the surface a shiny, white appearance.
• These fibres coalesce to form the fimbria and subsequently the crura of
the fornix (main efferent pathway of the hippocampal formation).
• Subiculum
• This is the transitional zone between the 6-layered entorhinal cortex and
the 3-layered hippocampus.
• Hippocampal Sulcus
• This is between the subiculum and dentate gyrus.
• Fimbriodentate Sulcus
• This is between the fimbria and the dentate gyrus.
• Choroid Fissure
• This is between the fimbria and overlying forebrain.
• Through it runs the anterior and posterior choroidal vessels.

21. Histology of the Hippocampal Formation
• The hippocampus and the dentate gyrus are 3 layered.
• They both have a superficial molecular layer and a deep
polymorphic layer.
• The intermediate stratum is the granule cell layer in the
dentate gyrus.
• It is the pyramidal cell layer in the hippocampus.
•
• Molecular Layer
• This is a synaptic layer that is continuous over the molecular
layers of the dentate gyrus, hippocampus and entorhinal
cortex.
•
• Pyramidal Layer
• This is a mix of dendrites, axons and interneurons.
• It is similar to layer 6 of the neocortex.

22. Hippocampal Afferents
• The most prominent source is the entorhinal
cortex. The entorhinal cortex itself receives almost all
types of sensory information.
• In addition, some septal and hypothalamic fibres reach
the hippocampal formation via the fornix.
• A few fibres also arrive from the contralateral
hippocampal formation passing from one crus to
another via the hippocampal commissure.
• Granule cells
• in the dentate gyrus receive input from the
entorhinal cortex via the perforant pathway.
• The granule cells project via mossy fibres to the
hippocampal pyramidal cells.

23. Hippocampal Efferents
• Many fibres are sent directly back to the entorhinal cortex.
• The most anatomically prominent output pathway is the fornix,
however.
• The pyramidal cells send an axon into the fornix as well as a Schaffer
collateral that projects to CA1 part of the hippocampal formation.
• These fibres arch forward under the corpus callosum.
• At the level of the interventricular foramen, some fibres split off
anterior to the anterior commissure as the precommissural fornix.
• Most of these end in the septal and preoptic areas but some continue
on to reach orbital and anterior cingulate cortex.
• The remaining fibres of the postcommissural fornix do one of 2
things:
• Some turn sharply posteriorly to end in the anterior thalamic nuclei;
• The rest travel through the hypothalamus in the column of the fornix
and end mainly in the mammillary bodies.

24. Functional Aspects of the Hippocampal Formation
• The most prominent role ascribed to the hippocampal formation
has to do with learning and memory.
• The synaptic connections within the hippocampus are readily
modifiable by a single experience (long-term potentiation).
• This may be the circuitry that converts short-term to long-term
memory.
• After bilateral removal of the medial parts of the temporal
lobe, humans have a striking memory deficit, anterograde
amnesia for declarative memories.
• Such a patient could, e.g., learn in repeated attempts to
assemble a jigsaw puzzle more and more skilfully, at the same
rate as a normal individual, despite never remembering having
seen the puzzle before.
• Intelligence is relatively unaffected.
• The pyramidal cells in CA1 are particularly sensitive to anoxia
so these symptoms may arise after revival from drowning.

25. Korsakoff's Psychosis
• Damage to the mamillary bodies (often as a
result of chronic alcoholism) results in a similar
memory deficit.
• The afflicted patients have relatively intact
intelligence but an inability to form new
memories.
• They do, however, typically make up answers
(confabulate) as they go along, concealing to
some extent their memory loss.
• Thus, Korsakoff's psychosis is also known as
amnestic confabulatory syndrome.

30. The Amygdala
Its name is derived from the Greek (amygdalon)
or Latin (amygdalum) word for almond, to
which Burdach compared its shape in the
early nineteenth century.

32. The Amygdala
• This is a collection of nuclei lying beneath the uncus of
the temporal lobe.
• It lies at the anterior end of the hippocampal formation
and the anterior tip of the inferior horn of the lateral
ventricle.
• It merges with the periamygdaloid cortex, which forms
part of the surface of the uncus.
• The amygdala also abuts the tail of the caudate nucleus
as it ends in the temporal lobe.
• The amygdala does have some connections with the
striatum but its overall pattern of connections is typical
of the limbic system.

33. The Amygdala
• This is a collection of nuclei lying beneath the
uncus of the temporal lobe.
• It lies at the anterior end of the hippocampal
formation and the anterior tip of the inferior
horn of the lateral ventricle.
• It merges with the periamygdaloid cortex,
which forms part of the surface of the uncus.
• The amygdala also abuts the tail of the caudate
nucleus as it ends in the temporal lobe.
• The amygdala does have some connections
with the striatum but its overall pattern of
connections is typical of the limbic system.

35. Connections
• Afferent:
–Olfactory system
–Hypothalamus
–Thalamus
–Reticular formation
–Neocortex
• Efferent:
–Via striae terminalis to septal nuclei
–Via siriae terminalis to thalamus
–Thalamus

36. Amygdalo-septal Circuit
• The dorsomedial amygdala blends with the cortex of
the uncus and receive afferents from the olfactory
bulb.
• This part of the amygdala relays olfactory input to the
ventrolateral part of the amygdala.
• Other afferents to the amygdala come from the frontal
and temporal cortex.
• The amygdala projects to the septal area via the stria
terminalis, which runs adjacent and medial to the tail
of the caudate nucleus.
• It also sends fibres to the anterior hypothalamus and
centres in the brainstem (e.g., DMnX, solitary nucleus,
and raphe nuclei) via the medial forebrain bundle.

37. Functional Aspects of the Amygdala
• The amygdala has a high order modulating influence on
autonomic function based on learning and past experiences.
• For example, generalised fear increases the heart rate, sweating,
and respiration.
• This instinctive modulation of autonomic function is different
from the reflexive modulation of the hypothalamus.
• Electrical stimulation
• of the amygdala in humans elicits emotions ranging from pleasure
to aggression though fear (along with the normal autonomic
manifestations) is the most common.
• Bilateral destruction
• of the amygdala causes a great decrease in aggression.
• This also often causes an eating disorder, either hyperphagia or
hypophagia.

38. Amygdala
• The amygdala has three major nuclear divisions,
which collectively are involved in emotions and their
behavioral expression:
– the basolateral nuclei, receive a major input from the
cerebral cortex and project to the medial dorsal nucleus of
the thalamus, the basal nucleus, the ventral striatum, and
back to the cortex (temporal, orbitofrontal, and prefrontal
association areas).
– the central nuclei, are reciprocally connected with
viscerosensory and visceral motor nuclei of the brain stem.
They also project to the hypothalamus to regulate
neuroendocrine functions. and
– the corticomedial nuclei,receive direct olfactory input. They
may play a role in appetitive behaviors and neuroendocrine
functions through their projections to the ventromedial
nucleus of the hypothalamus.

39. • The amygdala has two output pathways:
• (1) The stria terminalis, which is C-shaped,
carries the efferent projection primarily from
the corticomedial nuclei, and
• (2) the ventral amygdalofugal pathway
carries the efferents from the central nuclei,
which descend to the brain stem, and those
from the basolateral nuclei, which ascend to
the thalamus, the ventral striatum, and the
basal nucleus. The bed nucleus of the stria
terminalis runs along with the stria.

40. The Septal Area
• This has extensive reciprocal connections with the
hippocampus (via fornix).
• The septal area projects to the habenula nuclei via the
stria medullaris thalami.
• It also projects to the anterior hypothalamus and
modulates hypothalamic function.
•
• Functional Aspects of the Septal Area
• This area (and the nucleus accumbens) is associated
with pleasure.
• Rats will perform 5000 bar-presses an hour to obtain
self-stimulation of this region from implanted electrodes.

41. • Papez's Circuit
• This circuit approximates the hippocampo-mammillo-cortical circuit.
• This and the amygdalo-septal circuit "begins" in the frontal cortex, and overlap in
the temporal cortex and the hippocampus and septal areas.
•
• Hippocampo-mammillo-cortical Circuit
• The entorhinal cortex receives afferents from the olfactory tract and diverse
areas of the temporal lobe.
• It relays highly processed sensory information to granule cells in the dentate
gyrus.
• The hippocampus sends projections back to the entorhinal and inferior temporal
cortex.
• However, most of the efferents travel in the fornix and terminate in the
mammillary bodies, septal area and contralateral hippocampus.
• The mamillary bodies project to the anterior nuclei of the thalamus via the
mammillothalamic fasciculus.
• The anterior thalamic nuclei project to the cingulate gyrus via the internal capsule.
• The cingulum lies within the cingulate gyrus and serves as a cortical association
pathway for adjacent regions of neocortex in the frontal, parietal and occipital
lobes.
•
• Clinical Aspects of this Circuit
• The hippocampus and entorhinal cortex are both affected at an early stage of
Alzheimer's disease (olfactory toxin?).

45. Clinical Aspects of the Limbic System
• Klüver-Bucy Syndrome
• Complete removal of both temporal lobes in monkeys leads to:
• The animals are fearless and placid, showing an absence of
emotional reactions.
• The male animals become hypersexual and are
indiscriminate in their choice of sexual partners.
• They show in inordinate degree of attention to all sensory
stimuli. They respond to every object within sight or reach by
sniffing it or examining it orally (consequently leading to
hyperphagia).
• Although they incessantly examine all objects, they recognise
nothing, i.e., they have visual agnosia (due to the loss of the
visual association cortex).
• Smaller lesions of the temporal poles produce the same
symptoms minus the visual agnosia.
• A similar set of symptoms can be seen in humans.