Diencephalon- Thalamus

What does the diencephalon surround?

Third ventricle

What are the four main divisions of the diencephalon?

Thalamus (Dorsal Thalamus)

Hypothalamus

Epithalamus

Subthalamus (Ventral Thalamus)

Thalamus (Dorsal Thalamus)

Forms the bulk of the diencephalon along the lateral wall of the third ventricle. Connected across the midline by the massa intermedia (interthalamic adhesion)

Hypothalamus

Located anteriorly and inferiorly; includes the optic chiasm, infundibulum (pituitary stalk), tuber cinereum, and mammillary bodies (involved in memory/hippocampal connections)

Epithalamus

Located posteriorly; contains the pineal gland and habenular nuclei/commissure (integrates olfactory, visceral and somatic pathways)

Subthalamus (Ventral Thalamus)

Located inferior to the thalamus and superior to the midbrain; includes the subthalamic nucleus and connections with the globus pallidus

Lateral anatomical boundaries of diencephalon

Bound by the internal capsule

Anterior anatomical boundaries of diencephalon

Bound by the interventricular foramina (of Monro)

Inferior anatomical boundaries of diencephalon

Transitions into the midbrain

Internal Medullary Lamina (IML)

A Y shaped band of myelinated white matter that divides the thalamic nucleus into three major groups

Three major groups of Internal Medullary Lamina (IML)

Anterior
Medial
Lateral

Anterior Group of Internal Medullary Lamina

Enclosed by the Y-bifurcation

Medial Group of Internal Medullary Lamina

Dominated by the Dorsomedial (DM) nucleus

Lateral Group of Internal Medullary Lamina

Subdivided into a dorsal tier (LD, LP, Pulvinar) and a ventral tier (VA, VPL, VPM)

Neurochemistry

Approximately 75% of thalamic neurons are excitatory projection neurons that use Glutamater to communicate reciprocally with the cerebral cortex

Functional Groups of Thalamic Nuclei

Sensory Relay Nuclei
VA (Ventral Anterior) Nuclei

VL (Ventral Lateral Nuclei)
Anterior Nucleus
Dorsomedial (DM) Nucleus
Pulvinar and LP (Lateral Posterior) Nucleus
Thalamic Reticular Nucleus
Intralaminar/Centromedian Nuclei

Sensory Relay Nuclei

VPL (Ventral Posterolateral)
VPM (Ventro Posteromedial)
LGN (Lateral Geniculate Nucleus)
MGN (Medial Geniculate Nucleus)

VPL (Ventral Posterolateral) Nucleus

Relays somatosensory info (DCML and Spinothalamic) from the contralateral body to the primary somatosensory cortex (C1)

VPM (Ventral Posteromedial) Nucleus

Relays somatosensory info (Trigeminal system) and taste from the contralateral face to S1

LGN (Lateral Geniculate) Nucleus

Relays visual input from the optic tract to the primary visual cortex (V1)

MGN (Medial Geniculate Nucleus)

Relays auditory input from the inferior colliculus to the primary auditory cortex (A1/Hershl’s Gyri)

VA (Ventral Anterior) Nucleus

Receives basal ganglia output (GPi/SNr) and projects to premotor/frontal areas for motor planning and initiation

VL (Ventral Lateral) Nucleus

Receives cerebellar output (dentate nucleus) and basal ganglia input; projects to the primary motor cortex (M1) and premotor cortex for motor coordination and execution

Anterior Nucleus

Key component of the Papez circuit, critical for episodic memory and spatial orientation

Papez Circuit

Hippocampus—>Mammillary bodies—>Mammillothalamic tract—>Anterior thalamus—>Cingulate gyrus

Dorsomedial (DM) Nucleus

Connected to the amygdala, hypothalamus and prefrontal cortex; involved in emotion, executive function, decision-making, working memory, and the affective component of pain

Pulvinar and LP (Lateral Posterior) Nucleus

Connect to the parietal association cortex and superior colliculus; coordinate visual attention and visuospatial orientation

Thalamic Reticular Nucleus (TRN)

A thin shell of GABAergic (inhibitory) neurons surrounding the lateral thalamus. It receives input from the cortex and reticular formation and sends inhibitory signals to other thalamic nuclei to gate/synchronize activity. It has NO direct cortical projections

Intralaminar/Centromedian Nuclei

Receive brainstem reticular formation input; responsible for arousal, alertness, and widespread cortical activation

What typically causes Thalamic Syndrome (Dejerine-Roussy/Thalamic Pain Syndrome)?

Occlusion of Posterior Cerebral Artery (PCA) penetrating branches (such as the thalamogeniculate artery)

What characterizes Thalamic Syndrome (Dejerine-Roussy/Thalamic Pain Syndrome)?

Hemianesthesia (sensory loss) hemiparesis, sensory ataxia, and thalamic pain (severe, persistent central dysenthesia/hyperalgesia)

VPL/VPM Lesion

Contralateral body or facial sensory loss

Pulvinar/LP Lesion

Contralateral visuospatial neglect

Anterior Nucleus Lesion

Episodic memory impairment

VA/VL Lesion

Disturbances in movement control, planning and coordination