DIENCEPHALONDIENCEPHALON

Dr. Zeenat Zaidi

DiencephalonDiencephalon Paired structure Located between the

brain stem and the cerebral hemisphere

Continuous with the rostral part of the midbrain

Forms the lateral wall of the 3rd ventricle

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Almost entirely surrounded by the cerebral hemispheres

A little part seen externally on the base of the brain caudal to optic chiasma, includes:• Infundibulum• Tuber cinerium• Mamillary bodies

Other parts seen on sagittal & coronal sections

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On the medial surface, the diencephalon is subdivided, by hypothalamic sulcus (indicated by black line) into: Dorsal part Ventral part

Cerebral aqueduct

Dorsal

Fornix

Optic chiasma

Ventral

Midbrain

CC

Dorsal partDorsal part Thalamus & Epithalamus

Subthalamus & HypothalamusVentral partVentral part H

RelationsRelations

Lateral: Internal capsule

Medial: 3rd ventricle

Dorsal: Lateral ventricle

Ventral: Exposed on the base of the brain

EPITHALAMUSEPITHALAMUS

EpithalamusEpithalamusRelatively small

part, located in most caudal and dorsal region

Lies immediately rostral to superior colliculus

Consists of: Pineal gland & Habenular nuclei

Pineal GlandPineal GlandAn endocrine organSynthesizes

melatoninControls:

Sleep/awake cycle

Regulation of onset of puberty

Habenular nucleiHabenular nucleiLocated in habenular

triangle (area in the posterior part of the diencephalon, just anterior to pineal gland)

Have connections with limbic system

Serves autonomic function and emotional drives

THALAMUSTHALAMUS

ThalamusThalamus Large mass of grey matter, in shape and size, resembles

small hen’s egg Forms the lateral wall of the 3rd ventricle Separated from hypothalamus by hypothalamic sulcus May be connected to opposite thalamus by interthalamic

adhesion (massa intermedia)

thalamus

Interthalamic adhesion

Hypothalamic sulcus

Thalamus: In horizontal sections of brainThalamus: In horizontal sections of brain

Higher level

Lower level

Thalamus has a narrow anterior end called tubercle of thalamustubercle of thalamus, that lies in the posterior boundary of the interventricular foramen

The expanded posterior end, called pulvinar, lies above the superior colliculi

RelationsRelations

MedialMedial: 3rd ventricle

DorsalDorsal: lateral ventricle

Ventral: Subthalamus & Hypothalamus

LateralLateral: Internal capsule

CaudalCaudal: midbrain

AnteriorAnterior: interventricular foramen

SurfacesSurfaces

4 Surfaces:• Superior• Inferior• Medial• Lateral

S

l

ML

Superior SurfaceSuperior Surface Bounded laterally by

caudate nucleus, thalamostriate vein and a nerve fiber bundle called stria terminalis

Lateral part lies in the floor of the lateral ventricle & is covered by ependymaependyma

Medial part is related to the choroid plexus choroid plexus of the 3rd ventricle

caudate nucleus

LVLV

ependymachoroid plexus

thalamo-thalamo-striate veinstriate vein

stria terminalis

Lateral SurfaceRelated to the

internal capsuleinternal capsule

Inferior SurfaceRests on the

subthalamus & hypothalamus

Medial SurfaceMedial Surface Stria medullaris thalami Stria medullaris thalami

(a fascicle of nerve fibers) courses along its dorsomedial margin

Below is limited by hypothalamic sulcus

Forms the upper part of the lateral wall of the 3rd ventricle

Covered by ependyma

Stria medullaris thalami

Hypothalamic sulcus

Internal OrganizationInternal Organization Thalamus is composed

of grey mattergrey matter, interrupted by two two vertical sheaths of white vertical sheaths of white matter called matter called medullary medullary laminae. laminae.

External medullary External medullary laminalamina: :

Located laterally, separates reticular reticular nucleus nucleus from the rest of the thalamic mass

Contains thalamocorticalthalamocortical & corticothalamiccorticothalamic fibers

Internal medullary Internal medullary laminalamina

Y- shaped bandY- shaped band, , divides divides thalamusthalamus into Anterior, Medial & Lateral nuclear groups

Contains: Fibers connecting

thalamic nuclei with one another

Neuronal collections called intralaminar intralaminar nucleinuclei

Nuclear GroupsNuclear Groups AnteriorAnterior nuclear group Lateral Lateral nuclear group:

Divided into: dorsal & dorsal & ventral tiersventral tiers

MedialMedial nuclear group IntralaminarIntralaminar nuclei Reticular Reticular nucleus MidlineMidline nuclei

Functional OrganizationFunctional Organization All the nuclei of the

thalamus except reticular nucleus, project to ipsilateral ipsilateral cerebral cortexcerebral cortex

The whole of the cerebral cortex receives input from the thalamus

All thalamic nuclei receive corticofugal fibersfibers in a basically reciprocal fashion

Based on their connection with the cerebral cortex, the thalamic nuclei are divided into:Specific nucleiSpecific nucleiNonspecific nucleiNonspecific nuclei

Specific nucleiSpecific nuclei:: Have well-defined

sensory and motor functions

Have highly organized point-to-point connection with sensory & motor regions of cerebral cortex

Lie within the ventral group of the lateral nuclear group

Non-specific Nuclei: Receive less functionally

distinct afferent input Connect with wider area

of cortex, including associative and limbic regions

Include nuclei of the dorsal tier of lateral group, and whole of the anterior and medial group

Lateral Nuclear GroupLateral Nuclear GroupVentral TierVentral Tier

• Ventral anterior• Ventral lateral• Ventral posterior:

• (VPL)• (VPM)

• Lateral geniculate • Medial geniculate

Ventral Anterior NucleusVentral Anterior Nucleus

Influences motor activity

Premotor & supplementary motor cortex

Ipsilateral globus pallidus & substantia nigraMotor cortex

Ventral Lateral NucleusVentral Lateral Nucleus

Influences motor activity

Ipsilateral globus pallidus &substantia nigraContralateral dentate nucleusPrimary motor cortex

Primary motor cortex

Ventral Posterior NucleusVentral Posterior Nucleus Chief sensory relay station

General sensory afferents from the contralateral half of the Head & neck (VPM) and Body (VPL)

Primary somatosensory cortex

Lateral Geniculate BodyLateral Geniculate Body

Part of the Visual Pathway

Ipsilateral temporal hemiretinaContralateral nasal hemiretina

Optic radiation to the primary visual cortex

Medial Geniculate BodyMedial Geniculate Body

Part of the Auditory Pathway

Inferior colliculus

Auditory radiation to the primary auditory cortex

Lateral Nuclear GroupLateral Nuclear Group Dorsal TierDorsal Tier

• Lateral Dorsal• Lateral Posterior• Pulvinar

Pulvinar:Pulvinar: Sensory association cortices of parietal, temporal & occipital lobes

Lateral posterior nucleussensory association cortex of parietal lobe

Lateral dorsal Lateral dorsal nucleusnucleus Part of Limbic Part of Limbic SystemSystem Hippocamus

Cingulate gyrus

Medial Nuclear GroupMedial Nuclear Group

Integrates emotion, thought, and judgmentIntegrates emotion, thought, and judgment

Hypothalamus, amygdala, other thalamic nuclei, prefrontal cortex

Prefrontal cortex & limbic structures

Mediodorsal nucleus Mediodorsal nucleus & Nucleus reuniens& Nucleus reuniens

Anterior Nuclear GroupsAnterior Nuclear Groups

3 parts: Anteroventral Anteromedial Anterodorsal•

Functionally part of the limbic system. Involved in control of instinctive drives, emotional aspect of behaviour and in memory

Mammillary body of hypothalamus via mammillothalamic tract

cingulate gyrus

Midline NucleiMidline Nuclei Located

between medial nuclear group and the ependyma of 3rd ventricle

Important in visceral functions

Brainstem reticular formation

Cingulate gyrus and hypothalamus

Intralaminar NucleiIntralaminar Nuclei Located within the

internal medullary lamina

Main nuclei: Centromedian & Parafascicular

Function as activator of the cerebral cortical mantle

Lesions reduce the perception of pain and level of conciousness

Widespread regions of cerebral cortex, caudate & putamen of the basal ganglia

Reticular formation, spinothalamic & trigeminothalamic systems

Reticular NucleusReticular Nucleus

Located between the external medullary lamina & the internal capsule

Regulates the activity of thalamus

Collaterals of both Thalamocortical & Corticothalamic fibers

Other thalamic nuclei

Functions of the thalamusFunctions of the thalamus Receives and analyses all the sensory information

(except olfactory) from the body Having extensive connections with the basal ganglia and

the motor cortices, it plays a pivot role in voluntary motor activity. Thalamotomy (VA, VL) was once used to treat basal ganglia disorders

Connections with the limbic system makes it important in the control of mood, emotional and sexual behavior, and memory

Thalamic LesionsThalamic Lesions Cerebrovascular lesions or tumors of thalamus lead to: Loss of sensation in the contralateral side of face and

body followed by distressing discomfort, & burning and diffuse pain in the anaesthetic areas (thalamic pain)

Thalamic syndrome: Abnormal voluntary movements (chorea or hemiballismus) with hemisensory disturbance

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