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Mostly hidden from view
Between cerebral hemispheres
2% of CNS by weight
Widespread and important sensory
connections
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Majority of sensory, motor and limbic
pathways involve one or more stops in this
region
4 parts – each part includes the term
‘thalamus’ [ inner chamber]
1.
2.
3.
4.
Epithalamus –including pineal gland and few
nearby neural structures
Dorsal thalamus=thalamus
Subthalamus
Hypothalamus
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Visible part of diencephalon is inferior surface
of hypothalamus
Includes mammillary bodies and
infundibulum
Entire medial surface is wall of 3rd ventricle,
visible in a hemisected brain
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Superiorly, it borders body of lateral ventricle
Laterally- internal capsule
Caudal boundary-plane through posterior
commissure and caudal edge of mammillary
bodies
Rostral boundary-plane through back of
anterior commissure and front of optic
chiasm
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Boundaries are approximate
Neural tissue is continuous across boundaries
Certain thalamic nuclei protrude through
posterior boundary to a position alongside
midbrain
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Includes pineal gland and habenular nuclei
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Midline, unpaired
Resembles a pine cone
Rostral to superior colliculi
Once considered to be the seat of the soul
Pineal tumours compress midbrain leading to
1. Hydrocephalus
2. Deficits in eye movements and pupillary
reactions
3. Altered sexual development
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Receives light – regulated input by a circuitous
pathway
 Retina →hypothalamus→ intermediolateral cell
column→ postganglionic fibres of superior cervical
ganglion→pineal gland
 No known neural output
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Secretes a hormone- melatonin [derived from
serotonin]
Secretion increases during darkness
Related in humans to sleep-wake cycles
Gland undergoes calcification after the age of
17
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Calcified gland is a useful radiologic landmark
Slight shifts in pineal position can be
indicative of expanding masses of different
types
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Small part of diencephalon [ 4g in weight]
Important as a nodal point in pathways
concerned with autonomic, endocrine,
emotional and somatic functions designed to
promote homeostasis
Widespread sets of connections
1. Various components of limbic system
2. Outputs influencing pituitary gland
3. Interconnections with various visceral and
somatic nuclei[ motor and sensory,of
brainstem and spinal cord]
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Optic tracts, optic chiasma, mammillary
bodies
This area exclusive of mammillary bodies is
called tuber cinerium [‘gray swelling’]
Medial eminence protrudes from surface of
tuber cinerium , and is continuous with
infundibular stalk, which in turn is continuous
with posterior lobe of pituitary
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Infundibular stalk +posterior lobe of
pituitary=neurohypophysis
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Anterior extent-lamina terminalis
Superiorly- hypothalamic sulcus
Posteriorly- caudal edge of diencephalon
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Neural tissue anterior to a plane passing
through anterior edge of optic chiasma and
posterior edge of anterior commissure is
functionally continuous with
hypothalamus=preoptic area
Considered a part of anterior hypothalamus
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Anterior
Tuberal
Posterior
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Anterior region- above optic chiasma
Tuberal – above and including tuber cinerium
Posterior – above and including mammillary
bodies
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Periventricular- in the wall of 3rd ventricle
[rostral continuation of PAG]
Lateral –lateral to fornix
Medial zone [in between the two] –populated
by series of hypothalamic nuclei
The 1st 2 zones contain neurons and are
avenues via which ascending and descending
axons enter, leave or traverse hypothalamus
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Traversed by dorsal longitudinal
fasciculus[bundle of hypothalamic afferents
and efferents]
Contains suprachiasmatic and arcuate nuclei
Suprachiasmatic – tiny – less than 1 mm
square and fewer than 10,000 neurons
‘master clock’ for our circadian rhythms
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Receives direct retinal projections which
entrain it to the actual day length
Its neurons also contain melatonin receptors
Night-time rise in pineal melatonin secretion
probably helps ‘set’ the circadian rhythm
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Arcuate nucleus- critically involved in
feeding behavior
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Mainly scattered cells interspersed among
longitudinally running fibers of Medial
forebrain bundle
Anteriorly- continuous with lateral preoptic
nucleus- an important sleep-promoting area
Caudally- continuous with midbrain reticular
formation
Also has
1. Parts of supraoptic nucleus
2. Lateral tuberal nuclei
3. Tuberomammillary nucleus [source of
histaminergic fibers that project widely to
cerebral cortexand thalamus-participate in
sleep-wake cycles]
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Anteriorly has 2 nuclei containing large
neurosecretory cells- paraventricular ,
supraoptic
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Sits astride optic tract
Extends to lateral hypothalamic zone
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Located higher up in the wall of 3rd ventricle
Most cells of supraoptic nucleus and many
cells of paraventricular nucleus secrete
hormones that travel down axons of these
cells and are released in neurohypophysis
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Divided into dorsomedial and ventromedial
nuclei
Also has clusters of orexin-containing
neurons near fornix extending into lateral and
medial hypothalamus
Source of second set of wakefulness
promoting neurons
Contains
 Mammillary body [complex of many nuclei]
 Posterior hypothalamic nuclei continuous
with PAG [periaqueductal gray matter]of
midbrain