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 T H 3rd ventricle

Thalamus 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.

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) S L M I

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. * Interventricular foramen.

Surfaces 4 Surfaces: Superior Inferior Medial Lateral S L M l

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 stria terminalis LV thalamo-striate vein ependyma choroid plexus

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 Hypothalamic sulcus

Internal Organization Thalamus 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

Thalamic 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.

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

Attention & recent Memory NUCLEUS FUNCTIONS Anterior Attention & recent Memory Medial dorsal Associated with mood & emotional balance Lateral Dorsal Integrates Sensory information Lateral Posterior Pulvinar Correlates auditory & visual information with sensations

Forms part of ascending reticular activating system 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 nuclei Nonspecific nuclei

Specific nuclei: Non-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 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

Classification of thalamic nuclei according to their projection They could be classified into 3 groups, each group contains 4 nuclei: 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).

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

Functional Connections Lesion: contralateral loss of pain/temp, discrim touch Globus Pallidus Substantia Nigra Mammillary Body Premotor Cortex Prefrontal Cortex Cingulate Gyrus VA GP SN Cerebellum (Dentate) Anterior VL Primary Motor Cortex (4) Supplementary Motor Cortex (5_ Cingulate LD Spinothalamic and LL/ML Sensory Cortex (3,1,2) DM Superior Parietal Cortex (5,7) VPM VPL LP , Solitary Nucleus Amygdala Hypothalamus Olfactory Cortex Sensory Cortex Lesion: contralateral loss of pain/temp, discrim touch in head; ipsilateral loss of taste Prefrontal Cortex Pulvinar LGN Right Optic Tract Lesion: memory loss (Wernicke-Korsakoff) MGN Primary visual Cortex (17) (lingual gyrus, cuneus) Lesion: Left Homonymous Hemianopsia Lesion: Sensory Aphasia Primary Auditory Cortex (41,42) LGN, Superior Colliculus Brachium of Inferior Colliculus Association areas of temporal, occipital, parietal lobes

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