Diencephalon
By Rashid Alshahoumi
Outline:
Overview
 Development of
Diencephalon
 Basic Organization
 Dorsal Thalamus
(Thalamus)

Hypothalamus
 Ventral Thalamus
( Subthalamus)
 Epithalamus
 Vasculature of the
Diencephalon

Diencephalon

The diencephalon includes
- Dorsal thalamus
- Hypothalamus
- Ventral thalamus
- Epithalamus

Situated between
telencephalon & brainstem.

Main processing center for
information

Rt & Lt halves of the
diencephalon,contain
symmetrically
distributed cell groups
separated by the
space of the 3rd
ventricle
Development of the
Diencephalon


The cell groups that give rise to the
diencephalon form in the caudomedial
portion of the prosencephalon, bordering on
the space that will become the 3rd ventricle.
The developing brain at this level consists
initially of a roof plate and the two alar
plates; it lacks a well-defined floor plate and
basal plates.

-
-



The hypothalamic sulcus
A shallow groove appears in the wall of the 3rd
ventricle & extends rostrally from the developing
cerebral aqueduct to the ventral edge of the
interventricular foramen
Divides the alar plate into :
Superior (dorsal) area : future dorsal thalamus
Inferior (ventral) portion : future hypothalamus
The dorsal thalamus
- On each side of the 3rd ventricle increases rapidly in
size & will partially fuse across the space of the 3rd
ventricle to form :
- massa intermedia, or interthalamic adhesion .
(present in about 80% of the general population)

The epithalamus
- Develops from the caudal portion of the roof plate.
- By 7th week, a small thickening of the roof plate
forms. It gradually increases in size & evaginates to
form the epiphysis, which develops into the pineal
gland of the adult .
- The portion of the roof plate immediately rostral to
the epiphysis gives rise to the habenula, a small
thickening in which the habenular nuclei will develop

Just anterior to the habenular region, the roof plate
epithelium & adjacent pia mater give rise to the
choroid plexus of the third ventricle, This choroid
plexus is continuous through the interventricular
foramina with that of the lateral ventricles.






In locations around the perimeter of the 3rd
ventricle, specialized patches of ependyma
lie on the midline & form unpaired structures
called the circumventricular organs
These structures include :
Subfornical organ
Organum vasculosum of the lamina
terminalis
Subcommissural organ,
Pineal gland.

The development of the pituitary gland
during the 3rd week is linked to that of
the diencephalon .

A downward extension of the floor of the
3rd ventricle, the infundibulum, meets
the Rathke pouch, an upward
outpocketing of the stomodeum, the
primitive oral cavity.

By the end of the 2nd month, the
Rathke pouch loses its connection
with the developing oral cavity but
maintains its attachment to the
infundibulum.

As development continues, the
Rathke pouch gives rise to the
anterior lobe (adenohypophysis)
and pars intermedia of the pituitary
gland
Infundibulum differentiates into the
posterior lobe of the pituitary
gland, or neurohypophysis



A craniopharyngioma
(Rathke pouch tumor)
can arise from a portion
of the Rathke pouch that
fails to undergo proper
migration & apposition
to the infundibulum.
These tumors mimic
lesions of the pituitary &
may cause visual
problems, diabetes
insipidus, & ↑ ICP
Basic Organization

The junction between the diencephalon & midbrain lies along a line
extending from the posterior commissure to the caudal edge of the
mammillary body on the medial aspect of the hemisphere .

On the surface of the hemisphere, this interface is represented by a
line starting at the caudal aspect of the mammillary body, extending
anterolaterally over the edge of the crus cerebri & following the
caudal edge of the optic tract .

The boundary between the diencephalon & surrounding
telencephalon is less distinct & represented :
- laterally by the internal capsule
- rostrally by the interventricular foramen, lamina terminalis & optic
chiasm .

-
-
-
The 3rd third ventricle has
small evaginations or
recesses associated with
Optic chiasm (supraoptic
recess)
Infundibulum (infundibular
recess)
Pineal gland (pineal&
suprapineal recesses)






The dorsal thalamus
Located superior to the hypothalamic sulcus
Extends from the interventricular foramen
caudally to the level of the splenium of the
corpus callosum.
The hypothalamus
Located inferior to the hypothalamic sulcus
Bordered :
- Rostrally by the lamina terminalis
- Caudally by a line that extends from the
posterior aspect of the mammillary body
- Superiorly to intersect with the hypothalamic
sulcus.


-



The ventral thalamus (subthalamus)
Located :
Caudal to the hypothalamus
Rostral to the diencephalon-midbrain junction
Lateral to the midline
Epithalamic structures
Located posteriorly & caudally in close apposition to the
posterior commissure
Include : - pineal gland
- habenular nuclei
- main afferent bundle of these nuclei
- stria medullaris thalami .
Dorsal Thalamus
(Thalamus)

The dorsal thalamus (or thalamus) is a
massive collection of neuronal cell groups
that participate in a widely diverse array of
functions involving motor, sensory & limbic
systems.

Typically, thalamic output neurons project to
the cerebral cortex → !! very little information
reaches the cerebral cortex without first being
processed by thalamic neurons→ the
thalamus is functional "gateway" to the
cerebral cortex

In turn, nearly all
regions of the
cerebral cortex give
rise to reciprocal
projections that
return to the
thalamic region
from which they
originally received
input.

The thalamus is covered on its lateral
aspect by a layer of myelinated axons,
the external medullary lamina ( includes
fibers that enter or leave the subcortical
white matter)

Within the external medullary lamina are
clusters of neurons that form the
thalamic reticular nucleus.
An internal medullary lamina:
- Consisting of myelinated fibers
- Extends into the substance of the thalamus,
where it forms partitions or boundaries that
divide the thalamus into its principal cell
groups :
- anterior, medial, lateral & intraluminar
nuclear groups.
 There are midline thalamic nuclei located just
superior to the hypothalamic sulcus.
 Finally, attached to the caudolateral portion of
the thalamus are the medial and lateral
geniculate bodies (and their nuclei) .

Anterior Thalamic Nuclei



This group of cells consists of a large
principal nucleus & two smaller nuclei →
form the anterior nucleus of the thalamus
The anterior nucleus forms a prominent
wedge on the rostral aspect of the dorsal
thalamus just caudolateral to the
interventricular foramen → this wedge is
the anterior thalamic tubercle.
Rostrally, the internal medullary lamina
divides to partially encapsulate the
anterior nucleus.
Anterior Thalamic Nuclei


The cells of this nucleus receive dense
limbic-related projections from (1) the
mammillary nuclei via the
mammillothalamic tract and (2) the
medial temporal lobe (hippocampus) via
the fornix.
The output of this nucleus is primarily
directed to the cingulate gyrus through
the anterior limb of the internal capsule.
Medial Thalamic Nuclei
Comprises the dorsomedial nucleus
 Composed of :
-Large parvicellular (located caudally)
-Magnocellular (located rostrally)
-Small paralaminar adjacent to the internal
medullary lamina
 2 larger portions are linked to parts of the
frontal & temporal lobes & to the amygdaloid
complex .
 Cells of the paralaminar subdivision receive
input from the frontal lobe & substantia nigra
(may play a role in the control of eye movement)

Lateral Thalamic Nuclei
2 subdivisions
 Dorsal subdivision
 Lateral dorsal
- Functionally part of anterior group (limbic
system)
 Lateral posterior
 Pulvinar
 Border with lateral
posterior is vague
Inputs
􀂄 Pretectal area
􀂄 Superior colliculus


 Reciprocal
connections:
􀂄 Lateral geniculate
nucleus
􀂄 Parietal lobe
􀂄 Temporal lobe
􀂄 Occipital lobe
Roles:
􀂄 Visual relay center
􀂄 Selective attention
􀂄 Speech
 Ventral subdivision
- Ventral anterior
- Ventral lateral
- Ventral posterior
 Receive direct input from long
ascending tracts
 Reprocal connections with cortex
 Retrograde degeneration on cortical
lesions
Ventral Anterior
 Input
 Roles:
􀂄 Globus pallidus
􀂄 Substantia nigra
􀂄 Intralaminar nucleus
(thalamus)
􀂄 Premotor/prefrontal cortex
􀂄 Motor relay station
- Regulate movement
(Control of voluntary
movement)
􀂄 Medial part
- Eye, head, neck
􀂄 Lateral part
- Body, limb

Output (reciprocal
connections) :
􀂄 Premotor cortex
􀂄 Prefrontal cortex
􀂄 Intralaminar nucleus
Ventral Lateral Nucleus
 Input
􀂄Deep cerebellar nuclei
􀂄Globus pallidus
􀂄Primary motor cortex
 Output
􀂄Primary motor cortex
(reciprocal)
􀂄Parietal lobe
-Somatosensory areas
􀂄Premotor/Supplementary
motor areas
 Role:
􀂄 Motor relay station
- Cerebellum/basal
ganglia/cortex
Ventral Posterior Nucleus
 2 Divisions

Ventral posterior medial
(VPM)
 Ventral posterior lateral
(VPL)
 Inputs
􀂄 Medial lemniscus
- VPL
􀂄 Spinothalamic
-VPL
􀂄 Trigeminal lemniscus (taste)
- VPM
􀂄 Primary somatosensory
cortex
- VPM & VPL
 Output
􀂄 Primary somatosensory
cortex (reciprocal)
􀂄 Parietal operculum (taste)

The lateral (LGB) and medial (MGB) geniculate nuclei
are considered parts of the lateral thalamic nuclear
group .

MGB receives ascending auditory input via the brachium
of the inferior colliculus → projects to the primary
auditory cortex in the temporal lobe.

LGB receives visual input from the retina via the optic
tract → projects to the primary visual cortex on the
medial surface of the occipital lobe .

Located in the posterior thalamus at about the level of
the pulvinar and geniculate nuclei is a cluster of cell
groups collectively called the posterior nuclear complex.
This complex consists of :
- Suprageniculate nucleus
- Nucleus limitans
- Posterior nucleus
 These nuclei are positioned superior to the medial
geniculate and medial to the rostral pulvinar.


The posterior nuclear complex receives& sends to the
cortex nociceptive cutaneous input that is transmitted
over somatosensory pathways
Intralaminar Nuclei

Embedded within the internal medullary lamina are the
discontinuous groups of neurons that form the
intralaminar nuclei.

Projections to the neostriatum & to other thalamic nuclei,
along with diffuse projections to the cerebral cortex.

-
2 of the most prominent cell groups are :
Centromedian : projects to the neostriatum & to motor
areas of the cerebral cortex
Parafascicular nuclei : projects to rostral & lateral areas of
the frontal lobe.
-

Other intralaminar nuclei receive input from ascending
pain pathways and project to somatosensory and parietal
cortex.
Midline Nuclei

The midline nuclei are the least understood components
of the thalamus??

The largest is the paratenial nucleus, which is located
just ventral to the rostral portion of the stria medullaris
thalami; other cells are associated with the interthalamic
adhesion (massa intermedia).


Inputs are poorly defined
Efferent fibers reach the amygdaloid complex &the
anterior cingulate cortex, suggesting a role in the limbic
system.
Thalamic Reticular Nucleus

The cells are situated within the external medullary
lamina & between this lamina and the internal capsule .

Axons of these cells project medially into the nuclei of
the dorsal thalamus or to other parts of the reticular
nucleus, but not into the cerebral cortex.

Afferents are received from the cortex and from nuclei of
the dorsal thalamus via collaterals of thalamocortical &
corticothalamic axons.

Thalamic reticular neurons modulate, or gate, the
responses of thalamic neurons to incoming cerebral
cortical input .








Thalamic nucleus : efferent projections
(thalamocortical axons) → corterx
Cortex → reciprocal projection (corticothalamic
axons) → thalamic nucleus
VL/motor/precentral gyrus and anterior paracentral
gyrus
VPL/sensory for the body/postcentral gyrus and
posterior paracentral gyrus
VPM/sensory for the face/postcentral gyrus
MGB/auditory/transverse temporal gyrus
LGB/vision/cortex on the calcarine sulcus
The anterior nucleus projects primarily to the
cingulate gyrus and functions in the broad area of
behavior
 Thalamic
nuclei : relay nuclei or
association nuclei
 Thalamic
nuclei : specific or nonspecific
Hypothalamus
The hypothalamus is mainly involved in
visceromotor, viscerosensory & endocrine
activities.
The hypothalamus & related limbic structures
receive sensory input regarding the internal
environment & in turn, regulate through four
mechanisms the motor systems that modify
the internal environment.
Hypothalamus
Is a principal modulator of
autonomic nervous
system function.
Is a viscerosensory
transducer, containing
neurons with specialized
receptors capable of
responding to changes in
the temperature or
osmolality of blood, as
well as to specific
hormonal levels in the
general circulation.
It regulates the activity of
the anterior pituitary
through the production of
releasing factors
(hormone-releasing
hormones)
It performs an endocrine
function by producing &
releasing oxytocin
&vasopressin into the
general circulation within
the posterior pituitary.
• The hypothalamus can be divided into lateral,
medial & periventricular zones :
Lateral Hypothalamic Zone
Composed of diffuse clusters of neurons
intermingled with longitudinally oriented
axon bundles
Cells are involved in cardiovascular
function & in the regulation of food & water
intake.
Medial Hypothalamic Zone
-
In contrast to the lateral zone, it contains
discrete groups of neurons whose function &
connections are established.
Within the chiasmatic (anterior) region are 5
nuclei:
Preoptic
supraoptic
paraventricular
Anterior
Suprachiasmatic
(Preoptic, supraoptic, periventricular) →
are generally involved in regulating
hormone release
Anterior → cardiovascular function
Suprachiasmatic → circadian rhythms
Preoptic → body temperature & heat loss
mechanisms
In the tuberal region :
- Dorsomedial
- Ventromedial
- Arcuate nuclei
The ventromedial nucleus
- Food intake (satiety) center.
- Bilateral lesions produce hyperphagia, a greatly
increased food intake with resultant obesity.
Cells of the arcuate nucleus deliver peptides to
the portal vessels & through these channels, to
the anterior pituitary.
Peptides
Releasing factors that
↑ secretion of
specific hormones
by the anterior pituitary
Inhibiting factors that
↓ secretion of
specific hormones
by the anterior pituitary.
At caudal levels, the mammillary region
- Posterior nucleus
- Mammillary nuclei
The mammillary nuclei consist of a large
medial & a small lateral nucleus.
Although both of these nuclei receive
input via the fornix, only the medial
nucleus projects to the anterior thalamic
nucleus through the mammillothalamic
tract.
-
The neurons of the posterior nucleus are
involved in :
↑ BP
Pupillary dilation
Shivering or body heat conservation.
The mammillary nuclei are involved in
Control of various reflexes associated with
feeding
Mechanisms relating to memory formation.
Afferent Fiber Systems

The fornix & stria terminalis are 2 major afferent fiber
bundles that reach the hypothalamus

The fornix consists of axons that largely originate in the
hippocampus, and the stria terminalis arises from
neurons in the amygdaloid complex .

Fibers composing the ventral amygdalofugal bundle exit
the amygdala and course through the substantia
innominata to enter the hypothalamus and thalamus .

The medial forebrain bundle passes bidirectionally
through the lateral hypothalamic region.
Ascending axons → forebrain & brainstem.

Efferent Fibers

Several nuclei give rise to descending fibers that
contribute to the dorsal longitudinal fasciculus and the
medial forebrain bundle and to diffuse projections that
pass into the tegmentum.

These fiber systems project directly to numerous
brainstem nuclei, as well as to preganglionic sympathetic
and parasympathetic neurons in the spinal cord.

Other projections reach the thalamus and frontal cortex,
and still others extend to the posterior pituitary or to the
tuberohypophysial portal system for delivery of
substances to the anterior pituitary.
Ventral Thalamus (Subthalamus)

3 main structures
􀂄 Subthalamic
nuclei
Inputs:
􀂄 Globus pallidus
􀂄 Cerebral cortex
􀂄 Thalamus
􀂄 Reticular formation
􀂄 Contralateral
subthalamic nucleus
􀂄 supramamillary
commisure
Outputs:
􀂄 Globus pallidus
􀂄 Substantia nigra
Hemiballismus
􀂄 Damage
􀂄 Involuntary violent
hyperkinesia of
contralateral upper &
lower extremities
Ventral Thalamus (Subthalamus)
3 main structures
􀂄 Fields of Forel



H field of Forel
􀂄 Prerubral
H1 field of Forel
􀂄 Thalamic fasciculus
H2 field of Forel
􀂄 Lenticular fasciculus
Ventral Thalamus (Subthalamus)
3 main structures
􀂄 Zona incerta
􀂄 Implicated in many functions :
 Locomotion,
 Oculomotor, arousal,
 Attention, feeding,
 Sociosexual,
 Somatosensory
Epithalamus




Principal components :
Pineal gland
Habenular nuclei
Stria medullaris thalami
The pineal gland consists of richly vascularized
connective tissue containing glial cells &
pinealocytes but no true neurons.
Pinealocytes are cells that synthesize melatonin
from serotonin via enzymes that are sensitive to
diurnal fluctuations in light.
Production of melatonin by pinealocytes is
rhythmic & calibrated to the 24-hour cycle of
photic input to the retina = circadian rhythm.
Epithalamus




The habenular nuclei consist of a large lateral
nucleus and a small medial nucleus .
Both nuclei contribute axons to the
habenulointerpeduncular tract (fasciculus
retroflexus), which terminates in the midbrain
interpeduncular nucleus.
The stria medullaris thalami, conveys input to
both habenular nuclei.
The habenular commissure, a small bundle of
fibers riding on the upper edge of the posterior
commissure, connects the habenular regions of
the two sides.
Vasculature of the Diencephalon

The diencephalon is supplied by
-
Smaller vessels that branch from the various
arteries making up the cerebral arterial circle
(circle of Willis)
Larger arteries that originate from the
proximal parts of the posterior cerebral
artery
-

The hypothalamus & subthalamus are
supplied by central branches of the circle
(perforating or ganglionic)

Anterior parts of the
hypothalamus are served
by central branches

Caudal hypothalamic
regions and the ventral
thalamus are supplied by
branches of the
(posteromedial group)

arising from the posterior
communicating artery
and the P1 segment of
the posterior cerebral
artery.
(anteromedial group)

arising from the anterior
communicating artery
and the A1 segment of
the anterior cerebral
artery and from branches
of the proximal part of
the posterior
communicating artery.


Some of the branches of the
posteromedial group that arise from the
P1 segment near the basilar bifurcation
are called the thalamoperforating arteries.
Supply → rostral areas of the thalamus
Slightly more distal branches, which usually arise
from the P2 segment, are the posterior choroidal
and thalamogeniculate arteries.
 Supply → portions of the diencephalon
 A narrow portion of the caudal and medial
thalamus bordering on the 3rd ventricle is
supplied by the medial posterior choroidal artery,
whereas the thalamogeniculate branches irrigate
the caudal thalamus, including the pulvinar &
geniculate nuclei
 Branches of the medial posterior choroidal artery
also serve the choroid plexus of the 3rd ventricle.

The anterior choroidal artery originates
from
 Cerebral portion of the internal carotid
artery

penetrating branches into the genu
& posterior limb of the internal capsule
 Serves the optic tract, inferior portions of
the lenticular nucleus, the choroid plexus
of the inferior horn of the lateral ventricle,
and large parts of the hippocampal
formation.
 Sends

Although the thalamus receives a blood
supply largely separate from that of the
internal capsule , vascular lesions in the
thalamus may extend into the internal
capsule or vice versa.

Ischemic or hemorrhagic strokes in the
hemisphere may result in contralateral
hemiparesis in combination with
hemianesthesia.

These losses correlate with damage to
corticospinal and thalamocortical fibers in
the internal capsule.
Strokes involving the larger thalamic
arteries, such as the thalamogeniculate
artery, may result in total or dissociated
sensory losses.
 These patients may subsequently
experience persistent, intense pain

(thalamic pain, Dejerine-Roussy
syndrome).
Thank You