Thalamus

Overview of the Thalamus

Comprised of

• Dorsal thalamus

• Thalamic reticular nucleus (ReT)

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Functions

• Conveys information (“highway”) to cortex

  • Sensory pathways → cerebral cortex (except olfaction)

  • Lower centers → cortex

    • Basal ganglia

    • Cerebellum

    • Hypothalamus

• Sends information to:

  • Areas involved in attention

  • Executive functions

• Gates transfer of information to cortex

• Coordinates cortical arousal

Thalamus ↔ Cortex Connections

• Reciprocal connections between thalamus (except ReT) and cortex

• Via excitatory projection neurons

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Types of Projections

• Thalamocortical → to cortex

• Corticothalamic → to thalamus

  • Modulatory (affects processing)

📌 Key point:
Corticothalamic > thalamocortical projections

Specific vs Non-Specific Nuclei

Specific Nuclei

• Project to specific cortical areas

• Function: precise information relay

Stimulation →

• Rapid

• Localized response

• Ipsilateral cortex

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Non-Specific Nuclei

• Project to widespread cortex

• Function: generalized activation

Stimulation →

• Widespread bilateral activation

• Longer delay

Relay Nuclei (Specific Type)

General Features

• Inputs: subcortical structures

• Outputs: primary cortex

• Final step before cortex

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Location

• Anterior & lateral thalamus

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Major Relay Nuclei

• Ventral Posterior (VP) → somatosensory

• Lateral Geniculate Nucleus (LGN) → vision

• Medial Geniculate Nucleus (MGN) → audition

Ventral Posterior Complex (VP)

VPL

• Input: body somatosensory pathways

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VPM

• Input: trigeminothalamic (face/oral cavity)

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Projection

• Primary somatosensory cortex (S1)

  • Postcentral gyrus

  • Posterior paracentral gyrus

VPI (Ventral Posterior Inferior)

Location

• Between VPL and VPM

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Function

• Receives vestibular information

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Projections

• Parieto-insular vestibular cortex (PIVC)

• Vestibular cortex in:

  • Insula (posterior end)

  • Central sulcus (area 3a)

  • Intraparietal sulcus (area 2v)

VPMpc (VMb)

AKA

• Ventromedial basal nucleus (VMb)

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Afferents

• Solitary nucleus

• Taste (direct)

• GVA (via parabrachial nucleus)

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Efferents

• Taste → gustatory cortex (frontal operculum + insula)

• GVA → insula

📌 Representation order:

• Taste → GI → cardiovascular → respiratory (caudal progression)

VMpo

Location

• Caudal ventromedial thalamus

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Afferent

• Pain & temperature (anterolateral tract)

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Efferent

• Dorsal posterior insula

• Caudal to viscerosensory area

Interoceptive Cortex

• Dorsal insula

• Receives input from:

  • VMb (VPMpc)

  • VMpo

📌 Represents physiological state of the body

Metathalamus

• Lateral geniculate body (LGN)

• Medial geniculate body (MGN)

→ Caudal continuation of ventral thalamus

LGN (Vision)

Location

• Posterior thalamus (rostrolateral projection)

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Afferent

• Retina (both eyes)

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Efferent

• Primary visual cortex (V1)

  • Calcarine sulcus (occipital lobe)

MGN (Audition)

Afferent

• Inferior colliculus (via brachium)

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Efferent

• Primary auditory cortex (A1)

  • Heschl’s gyri

Motor Relay Nuclei

• Ventral Anterior (VA)

• Ventral Lateral (VL)

VA (Ventral Anterior)

Afferents

• Basal ganglia:

  • GPi → parvocellular VA

  • SNr → magnocellular VA

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Efferents

• Frontal eye fields (BA 8)

• Prefrontal cortex

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Function

• Motor planning

• Behavior

• Basal ganglia loops (motor, oculomotor, associative)

VL (Ventral Lateral)

Afferents

• Basal ganglia

• Cerebellum

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Subdivisions VLo (VLa)

• Input: GPi

• Output: premotor cortex, SMA

• Function: motor planning (basal ganglia loop)

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VLp

• Input: cerebellar nuclei

• Output: primary motor cortex

• Function: motor execution (limb movement modulation)

SMA = Supplementary Motor Area

Limbic Nuclei

Function

• Memory (Circuit of Papez)

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Components

• Anterior nuclear group

• Lateral dorsal nucleus (LD)

Anterior Nuclear Group

Structure

• 3 nuclei (within internal medullary lamina)

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Location

• Near interventricular foramen

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Connections

• Afferent: hippocampus (via fornix → mammillary bodies → mammillothalamic tract)

• Efferent: cingulate cortex

Association Nuclei

Function

• Higher-order relay

• Send info to association cortex

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Location

• Medial + posterior dorsal thalamus

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Properties

• Largest nuclei

• Strong reciprocal connections

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Components

• Mediodorsal (MD)

• Pulvinar

• Lateral posterior (LP)

Mediodorsal (MD)

Function

• Attention

• Decision-making

• Behavioral planning

• Emotion (via amygdala connections)

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Connections

• Afferents: amygdala, olfactory cortex, entorhinal cortex, SN, ALS

• Efferents: prefrontal cortex (reciprocal)

  • Includes FEF + anterior cingulate

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Damage →

• Impaired executive function

• Affective disturbances

FEF = Frontal Eye Field

Pulvinar + LP

Location

• Posterior thalamus (dorsal/lateral to midbrain)

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Afferents

• Superior colliculus

• Pretectum

• Visual cortex

• Parietal & temporal association cortices

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Efferents

• Multimodal association cortex

  • Posterior parietal

  • Lateral temporal

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Function

• Spatial attention

Gating Function of Thalamus

Two Modes Tonic Mode (Wakefulness/REM)

• Neurons depolarized

• Tonic firing

• Information passes to cortex

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Burst Mode

• Neurons hyperpolarized

• Burst firing

• Functional disconnection from cortex

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Sleep vs Wake

• Non-REM: rhythmic bursting

• Wakefulness:

  • Mostly tonic

  • Some arrhythmic bursts → detect novel stimuli

📌 Attention → shift to tonic mode

Non-Specific Nuclei

General

• Diffuse cortical projections

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Components

• Midline nuclei

• Intralaminar nuclei

Midline Nuclei

Properties

• Small, poorly defined

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Connections

• Afferents: hypothalamus, basal forebrain, brainstem

• Efferents: ventral striatum, amygdala

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Reciprocal with

• Limbic cortex

• Hippocampus

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Function

• Limbic roles

Intralaminar Nuclei

Location

• Within internal medullary lamina

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Caudal Group (CM + Pf)

Afferent:

• GPi

Function:

• Basal ganglia circuits

• ↑ cortical + striatal excitability

CM + Pf = the Centromedian nucleus of the thalamus + Parafascicular nucleus of the thalamus

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Rostral Group

Afferents:

• Reticular activating system

• Anterolateral system

• Cerebellum

Function:

• Arousal

• Cognitive alertness

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Efferents

• Striatum

• Widespread cortex

Reticular Nucleus (ReT)

Properties

• Derived from ventral thalamus

• GABAergic

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Afferents

• Collaterals from:

  • Thalamocortical

  • Corticothalamic fibers

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Key Difference

• ❌ Does NOT project to cortex

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Function

• Gates thalamic activity

• Controls attention/salience

• Synchronizes cortical activity in sleep

  • Sleep spindles

  • Delta waves

Thalamic Radiations

Definition

• Fibers connecting thalamus ↔ cortex

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Pathway

• Travel through internal capsule

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Types Anterior Radiation

• MD + anterior nucleus → frontal lobe, cingulate

• Via anterior limb

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Superior Radiation

• VP → parietal

• VA/VL → frontal

• Via posterior limb

• (aka central radiation)

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Posterior Radiation

• LGN → occipital (optic radiations)

• Via retrolenticular limb

• Also includes pulvinar projections

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Inferior Radiation

• MGN → temporal (auditory radiations)

• Via sublenticular limb

• Also pulvinar → temporal