Thalamic Nuclei, Subthalamus, and Epithalamus Connections

Neothalamus Part II

Ventral Anterior Nucleus (VA)

Function: Regulates motor pathways.

Ventral Lateral Nucleus (VL)

• Inputs: Globus pallidus, Substantia nigra
• Outputs: Motor and premotor cortex

Anterior Thalamic Nuclei

Function: Limbic system relay, memory, behavior.

• Inputs: Mammillary nuclei (via mammillothalamic tract), Hippocampus (via fornix)
• Outputs: Cingulate gyrus and frontal cortex

Dorsomedial Nucleus (DM)

Function: Affect, cognition, judgment, memory, integration.

• Inputs: Hypothalamus, Globus pallidus, Amygdala, other thalamic nuclei
• Outputs: Prefrontal cortex (including frontal pole)

Subthalamus

Function Overview

Involved in facilitating automatic movements and inhibiting involuntary movements through basal ganglia loops.

Basal Ganglia Pathways & Loops

• Striato-pallidal pathway
• Subthalamo-pallidal pathway
• Nigro-pallidal pathway
• Thalamo-striato-pallidal loop
• Pallido-cortico-striatal loop
• Subthalamo-cortico-pallidal loop
• Motor loops involving thalamus (e.g., Ansa lenticularis, Fasciculus lenticularis)

Subthalamic Nucleus (STN)

Part of the subthalamo-pallidal fasciculus.

• Connections to midbrain and pontine motor nuclei (Pallidotegmental fasciculus)

Nucleus Subthalamicus (of Luys)

Function: Modulates motor output. Lesions can cause contralateral hemiballismus (abrupt, involuntary movements).

• Inputs: Cortex, Globus pallidus (external segment), Substantia nigra
• Outputs: Globus pallidus (internal segment), Substantia nigra

Subthalamus Part II: Associated Structures

Forel's Fields H

Fiber tracts associated with globus pallidus and subthalamic nucleus outputs.

• H1 (Thalamic fasciculus): Fibers from globus pallidus (internal) and cerebellum to thalamus.
• H2 (Lenticular fasciculus): Fibers from globus pallidus (internal) arching over subthalamic nucleus.

Zona Incerta

Acts as a relay and integration center.

• Inputs: Motor cortex, Sensory pathways (e.g., Medial lemniscus - proprioception)
• Outputs: Motor cortex, Superior colliculus, Pretectal area, Thalamic nuclei

Epithalamus

Function: Regulates circadian rhythms and endocrine secretion, influenced by light.

Habenula

Relays information from the limbic system to the midbrain.

• Inputs: Amygdaloid nucleus (via stria terminalis), Hippocampus (via fornix), Septal nuclei, Paraolfactory cortex, Preoptic and supraoptic areas (hypothalamus), Superior colliculus, Ascending serotonergic and adrenergic fascicles.
• Outputs: Interpeduncular nucleus (via Habenulo-interpeduncular tract / Fasciculus retroflexus of Meynert), Tegmental nuclei (periaqueductal gray, reticular formation), Superior colliculus. Connects to brainstem autonomic centers (e.g., via Dorsal longitudinal fasciculus of Schütz). Influences pineal gland activity indirectly.

Pineal Gland (Epiphysis Cerebri)

An endocrine gland involved in circadian rhythms.

• Afferent/Efferent connections via Habenular commissure and Posterior commissure.
• Receives sympathetic innervation via Nervus conarii from the Superior cervical ganglion.

Light Regulation Pathway (Indirect):

1. Retina detects light levels.
2. Signal travels via Optic Nerve to the Suprachiasmatic Nucleus (SCN) of the Hypothalamus.
3. SCN signals the Paraventricular Nucleus (PVN) of the Hypothalamus.
4. Descending autonomic pathways project to the Intermediolateral cell column (spinal cord T1-T2).
5. Preganglionic sympathetic fibers ascend the sympathetic chain to the Superior Cervical Ganglion.
6. Postganglionic fibers travel via the Internal Carotid Plexus and Nervus Conarii.
7. Innervation of the Pineal Gland regulates melatonin synthesis and release (inhibited by light).

Hormonal Output:

Secretes Melatonin, particularly during darkness.

Melatonin Targets:

• Influences various forebrain structures.
• Modulates activity in the ventral thalamus.
• Provides feedback to the hypothalamus (e.g., SCN).