Functional Subdivisions

(forms 80% of diencephalon)

Archithalamus:

• Oldest;
• Animals with rudimentary cortex;
• Sense of smell;
  • Afferents -- reticular formation & limbic system.
  • Efferents -- cortical connections & basal ganglia
  • Functions as a diffuse thalamic activation system.
    • Produces cortical desynchronization.
    • Critical for perception of stimuli.
    • Decreased activity during sleep.

Paleothalamus:

Preliminary processing of sensory information to cortex.
Two way connections

Neothalamus:

• Largest division of nuclei
• Projects to association areas of the cortex.
• Larger in animals with well-developed cortex.
• Intrinsic nuclei -- input from other thalamic nuclei.

Archithalamus

• Oldest division of thalamus; animals with rudimentary cortex; sense of smell.
• Functions as a diffuse thalamic activation system
• Produces cortical desynchronization
• Critical for perception of stimuli
• Decreased activity during sleep
• Afferents and Efferents
  • Afferents-- reticular formation & limbic system
  • Efferents--cortical connections & basal ganglia

Archithalamic Nucleus:

• Internal medullary lamina - divides medial and lateral segments

• Hypothalamus, basal ganglia, amygdala, orbital gyri (diffuse fibers to association cortex)
• Intralaminar (in internal medullary lamina)
  • Largest (centrum medianum)
  • Receives diffuse spinal and RF connections
  • Diffuse cortical projections
• Reticular- lateral to external medullary lamina
  • Diffuse efferent system to cortex
• Anteroventral nucleus -- from mammilothalamic tract to cingulate gyrus

Paleothalamus

Description

• Preliminary processing of sensory information to cortex; two-way cortical connections

Paleothalamic Nuclei:

1. Anterior nuclei--project to geniculate bodies
   • Vision and audition
   • Have connections with pulvinar
   • Accessory arcuate nucleus--taste

2. Ventral lateral nucleus
   • Fibers from brachium conjunctivium
   • Efferents-- internal capsule to motor cortex of precentral area
   • Brings cerebellar influence to bear on voluntary motor intentions

Neothalamus

Description

• Largest division of nuclei
• Projects to association areas of cortex
• Larger in animals with well-developed cortex
• Intrinsic nuclei--input from thalamus

Neothalamic Nuclei:

1. Medial nucleus--projects frontal cortex (non-motor)
   • Afferents--from ventral nuclei, hypothalamus
   • Dorsal lateral nucleus & posterior lateral -- parietal projection nuclei

2. Nucleus lateral posterior--parietal and temporal
   • Temporal pole area has no thalamic connections
   • Receives medial geniculate connections
   • Projects to audio-psychic zone of temporal lobe

3. Nucleus lateralis dorsalis
   • Afferents--somatic sensory nerves
   • Efferent--area 7 of parietal lobe

4. Pulvinar
   • Afferent--lateral and medial geniculate bodies
   • Two-way connection with visuopsychic area

5. Ventral anterior nucleus
   • From globus pallidus to limbic areas and back

Functions of the Thalamus

• Thalamus and cortex have area-to-area connections
• These connections are usually two-way
• If the thalamus were only a relay, it would be an inconvenient relic
• Transmission of sensory message does not concern the neothalamus
• May play role as a level of sensory integration
• Attention

Terms to Know