THALAMUS

THALAMUS

• Two ovoid masses of grey matter lying close together rostrally.

• Is a collection of neurons organized into large number of well-defined nuclear masses.

– Joined by mass intermedia in midline.

– Separated only by third ventricle.

THALAMIC NUCLEI:

DIVISIONS:

1. Medial nuclear mass

• Intra-laminar nuclei.

• Centro-median nucleus.

• Dorsomedial nucleus.

• Midline nuclei.

• Medial geniculate body.

• Reticular nuclei.

2. Lateral nuclear mass.

• 2 Subtypes.

2a. Dorsal/Lateral group.

• Lateral dorsal

• Lateral posterior

• Pulvinar

• Lateral geniculate body.

2b. Ventral group:

• Ventral anterior.

• Ventral lateral.

• Ventral posterior – 

– Ventro-Posterior Lateral (VPL) &

– Ventro-Posterior Medial (VPM).

FUNCTIONAL DIVISION OF NUCLEI:

• Divided into 4 groups.

GROUP	Description	Nuclei Involved
Specific sensory nuclei	Acts as “Relay Nuclei”. Receives all sensory afferents from ascending pathways projecting to somato-sensory cortex.	Ventro-Posterior nucleus. Medial & lateral geniculate bodies.
Association nuclei	Have reciprocal connections with “Association areas” of cerebral cortex. Helps integrating different types of sensory information.	Lateral group of nuclei (Pulvinar, lateral dorsal, lateral posterior nuclei). Part of medial dorsal nucleus.
Nonspecific nuclei	Project to cortex in diffuse manner. Involved in arousal induced by sensory stimuli.	Intra-laminar nuclei. Reticular nuclei.
Motor nuclei	Relay & process messages from basal ganglia & cerebellum to motor & premotor cortex.	Ventral anterior nuclei & Ventral lateral nuclei.

FUNCTIONS OF THALAMUS:

1. Acts as “Sensory Relay Station” for all sensory pathways.

• Includes Gustatory & Olfactory tracts reaching cortex.

• Modulates sensory information relayed.

• Determines conscious awareness for sensations.

2. Integrative center –

• Connects many brain areas.

• Including cerebral cortex, basal ganglia, hypothalamus & brain stem.

• Ascending sensory fibers pass through brainstem –> Synapse in thalamus –> Relay on cerebral cortex via internal capsule.

4. Concerned with conscious interpretation of crude touch, pain & temperature.

• Final discrimination of sensory modalities occurs in sensory cortex after thalamus provides information.

5. Integrates motor activities information from cerebellum & globus pallidus with premotor cortex.

• By VA & VL thalamic nuclei.

6. Important for visual-attention & visually-guided movements-

• By Pulvinar nuclei.

7. Modulates brain wave synchronization & de-synchronization.

• Influences attention & consciousness.

• Done by Reticular nucleus

8. Role in emotions & memory –

• Due to relay station in Papez circuit.

9. Acts as “Single functional entity” with cortex.

• “Thalamo-Cortical” system.

• Prepares crude blueprint of final product achieved by cortex.

10. Output of reticular nuclei – Mainly to thalamic nuclei.

• Every thalamic nuclei sends output to different cortical areas.

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