THALAMUS

Objectives

• 1. Describe the location of the thalamus• 2. List the different parts and nuclei of the

thalamus• 3. List the afferent input to different nuclei of

the thalamus• 4. List the efferent output from the different

nuclei of the thalamus

Thalamus Part of the diencephalon (The

other parts are hypothalamus, subthalamus and epithalamus)

A midline symmetrical structure, formed of 2 oval masses of grey matter

Is the largest nuclear mass Situated between the cerebral

cortex and brainstem Together with the

hypothalamus, it forms the lateral wall of the 3rd ventricle

3rd ventricle

T

H

• Functionally considered as the great sensory gateway to the cerebral cortex

• It relays received information to the cerebral cortex from diverse brain regions.

• Axons from every sensory system (except olfaction) synapse in the thalamus as the last relay site before the information reaches the cerebral cortex.

• There are some thalamic nuclei that receive input from cerebellar nuclei, basal ganglia and limbic-related brain regions.

• Its function includes relaying sensory and motor signals to the cerebral cortex, along with the regulation of consciousness, sleep, and alertness.

Thalamus

Relations

Lateral: Posterior limb of the internal capsule (IC)

Medial: Together with hypothalamus, forms the lateral wall of the 3rd ventricle

Superior: Caudate nucleus (C) fornix (F) & lateral ventricle (LV)

Inferior: Hypothalamus (H) anteromedially & Subthalamus (ST) posterolaterally.

External Features• Ends : Anterior &

Posterior• Surfaces : Superior Inferior Medial Lateral

Medial s.: frequently connected to the thalamus of the opposite side by the interthalamic adhesion (massa intermedia)

L

I

M

S

**Interventricular

foramen.

Thalamus has 2 ends.*Anterior: Forms a projection called

anterior tubercle which lies just behind the interventricular foramen.

*Posterior: Forms a projection called Pulvinar which lies above the superior colliculus and the lateral & medial geniculate bodies.

**

Pulvinar.

Surfaces

4 Surfaces:• Superior• Inferior• Medial• Lateral

S

l

ML

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

caudate nucleus

LV

ependymachoroid plexus

thalamo-striate vein

stria terminalis

Lateral Surface

• Related to the internal capsule

Inferior Surface• Rests on the

subthalamus & hypothalamus

Medial Surface

• 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 OrganizationThalamus is composed of grey matter, interrupted by two vertical sheaths of white matter called medullary laminae. • External medullary

lamina: - Located laterally, separates reticular nucleus from the rest of the thalamic mass - Contains thalamocortical & corticothalamic fibers

• Internal medullary lamina

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

Contains: Fibers connecting

thalamic nuclei with one another

Neuronal collections called intralaminar nuclei

• Anterior nuclear group: Anterior nucleus• Medial nuclear group: Largest nucleus is medial dorsal nucleus

(MD)• Intralaminar nuclei: Lie within the internal medullary lamina• Midline nuclei: Lie deep to ependyma of 3rd ventricle• Lateral Nuclear group

Thalamic Nuclei

Lateral nuclear group is divided into Dorsal & Ventral tiers.

Dorsal tier contains: lateral dorsal n. (LD) lateral posterior n. (LP) pulvinar.

Ventral tier contains ventral anterior (VA) ventral lateral (VL) ventral posterior (VP)

nuclei, divided into lateral & medial parts

medial & lateral geniculate bodies.

Thalamic Nuclei

NUCLEUS FUNCTIONS

VPM Relay station for impulses from face, head & taste buds

VPL Relay Station for exteroceptive& proprioceptive from all body EXCEPT head & face

VA Relay station for Striatal impulses (attention & recent memory )

VL (VI) Relay station for cerebellar impulses

MGB Relay station for Auditory impulses

LGB Relay station for Visual(Optic) impulses

NUCLEUS FUNCTIONS

Anterior Attention & recent Memory

Medialdorsal

Associated with mood & emotional balance

Lateral Dorsal

Integrates Sensory information

Lateral Posterior

Integrates Sensory information

Pulvinar Correlates auditory & visual information with sensations

NUCLEUS FUNCTIONS

Reticular Forms part of ascending reticular activating system

Intralaminar (Including centromedian)

Awareness of painful stimuli at thalamic level

Functional Organization• All the nuclei of the

thalamus except reticular nucleus, project to ipsilateral cerebral cortex

• The whole of the cerebral cortex receives input from the thalamus

• All thalamic nuclei receive corticofugal fibers in a basically reciprocal fashion

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

• Specific 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 inputConnect 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

Classification of thalamic nuclei according to their projection

A)Simple sensory relay nuclei: receive well defined sensory impulses, and relay them to functionally distinct areas of the sensory cortex. 1. Ventral posterolateral nucleus (VPL).2. Ventral posteromedial nucleus (VPM).3. Lateral geniculate body (LGB).4.Medial geniculate body (MGB).

• They could be classified into 3 groups, each group contains 4 nuclei:

B) Circuit relay nuclei: receive impulses from different areas of CNS and relay them to specific areas in cerebral cortex. They include:1. Lateral ventral nucleus (projects to primary motor cortex).2. Anterior ventral nucleus (projects to premotor cortex).3. Anterior nucleus (projects to cingulate gyrus).4. Part of dorsomedial nucleus.

C) Associative nuclei: receive impulses from other thalamic nuclei and relay these impulses to the association areas of the cerebral cortex, They include:5. Part of dorsomedial nucleus.6. Pulvinar.7. Lateral dorsal nucleus.8. Lateral posterior nucleus.

Functions of Thalamus

• Sensory integration and relay station for all the sensory pathways except Olfactory

• Recognition of crude pain, temperature & touch • Influences voluntary movements by receiving

impulses from basal ganglia & cerebellum & relaying them to motor cortex , influence LMN

• Participates in maintenance of state of wakefulness & alertness through RAS

• Role in emotions & recent memory

AnteriorVA

VL

VPLVPM

LD

LP

Pulvinar LGN

MGN

DM

Functional Connections

Mammillary Body

Cingulate Gyrus

AmygdalaHypothalamusOlfactory Cortex

Prefrontal Cortex

Globus PallidusSubstantia Nigra

Premotor CortexPrefrontal Cortex

GPSNCerebellum (Dentate)

Primary Motor Cortex (4)Supplementary Motor Cortex (5_Cingulate

Superior Parietal Cortex(5,7)

Spinothalamic and LL/ML

Sensory Cortex (3,1,2)

, Solitary NucleusSensory Cortex

Right Optic Tract

Primary visual Cortex (17)(lingual gyrus, cuneus)

Brachium of Inferior Colliculus

Primary Auditory Cortex (41,42)LGN, Superior Colliculus

Association areas of temporal, occipital, parietal lobes

Lesion: memory loss (Wernicke-Korsakoff)

Lesion: Sensory Aphasia

Lesion: contralateral loss of pain/temp, discrim touch

Lesion: contralateral loss of pain/temp, discrim touch in head; ipsilateral loss of taste

Lesion: Left Homonymous Hemianopsia

Thalamic syndrome

• Due to vascular lesion (thrombosis of thalamogeniculate artery)

• Threshold for pain, touch & temperature decreased on opposite side of body

• When threshold reached then exaggerated • Emotional instability, spontaneous laughing &

crying

Thalamic hand• Opposite hand shows abnormal posture• Forearm is pronated, wrist flexed,

metacarpophalangeal joints flexed & interphalangeal joints extended

• Fingers can move actively, but movements are slow• Due to altered muscle tone in different muscle groups