Basal Ganglia

Basal Ganglia - The Brain's Grand Central

• Key Structures & Roles:
  • Striatum (Caudate + Putamen): Main input station.
  • Globus Pallidus (GPe, GPi): GPi is a major output nucleus.
  • Subthalamic Nucleus (STN): Modulatory role.
  • Substantia Nigra (SNc, SNr): SNc (dopamine); SNr (output).
• Core Functions: Motor control (initiation/suppression of movement), procedural learning, habit formation, cognition.
• Circuitry: Forms loops with cerebral cortex and thalamus.

⭐ Parkinson's Disease: Caused by loss of dopaminergic neurons in Substantia Nigra pars compacta (SNc).

Basal Ganglia - The Circuit Show

• Function: Modulates motor commands; crucial for movement initiation, suppression, procedural learning, and habit formation.
• Key Pathways:
  • Direct (GO): Cortex ➔ Striatum (D1) ➔ GPi/SNr ➔ Thalamus ➔ Cortex.
    • Effect: Disinhibits thalamus, leading to increased cortical excitation → ↑Movement.
    • SNc (Dopamine via D1): Stimulates this pathway.
  • Indirect (NO-GO): Cortex ➔ Striatum (D2) ➔ GPe ➔ STN ➔ GPi/SNr ➔ Thalamus ➔ Cortex.
    • Effect: Further inhibits thalamus, leading to decreased cortical excitation → ↓Movement.
    • SNc (Dopamine via D2): Inhibits this pathway.
• Net Dopamine Effect (SNc via nigrostriatal pathway): Facilitates movement by enhancing Direct & suppressing Indirect pathways.

⭐ Lesion of the Subthalamic Nucleus (STN) results in contralateral hemiballismus (violent, flinging movements).

Basal Ganglia - Movement & More

• Core Function: Regulates voluntary movement, procedural learning, habit formation, cognition, emotion.
• Motor Control:
  • Initiates & scales desired movements.
  • Suppresses unwanted/competing movements.
  • Maintains muscle tone.
• Key Pathways & Dopamine's Role:
  • Direct (D1 receptors): Facilitates movement (GO!). Dopamine excites.
  • Indirect (D2 receptors): Inhibits movement (NO-GO!). Dopamine inhibits.
• Beyond Motor: Involved in reward processing, motivation, working memory, and eye movements.

⭐ Lesions cause movement disorders: Parkinson's (hypokinetic, ↓Dopamine) & Huntington's (hyperkinetic).

Basal Ganglia - Disorder Drama

• Parkinson's Disease (PD): Substantia Nigra pars compacta (SNc) degeneration → Dopamine (DA) ↓.
  • Symptoms: TRAP (Tremor, Rigidity, Akinesia/Bradykinesia, Postural instability). Lewy bodies present.
• Huntington's Disease (HD): Autosomal dominant. CAG repeat expansion (HTT gene).
  • Striatal atrophy (Caudate & Putamen). GABA ↓, Acetylcholine (ACh) ↓, relative DA ↑.
  • Symptoms: Chorea, dementia, psychiatric disturbances.
• Hemiballismus: Subthalamic Nucleus (STN) lesion (often vascular).
  • Contralateral violent, involuntary, flinging movements of limbs.
• Wilson's Disease: ATP7B gene mutation → Copper accumulation.
  • Hepatolenticular degeneration. Kayser-Fleischer rings in cornea.
• Dystonia: Sustained or intermittent muscle contractions causing abnormal movements/postures.
• Tourette Syndrome: Multiple motor tics and at least one vocal tic.

⭐ In Huntington's Disease, atrophy of the caudate nucleus and putamen (striatum) is a hallmark pathological finding, particularly affecting medium spiny neurons projecting to the GPe (indirect pathway).

Basal Ganglia - Arteries & Messengers

• Arteries:
  • Caudate/Putamen: Lenticulostriate As. (MCA); Caudate (head): Heubner's A. (ACA).
  • Globus Pallidus: Ant. Choroidal A., Lenticulostriate As. (MCA).
  • Subthalamic Nucleus (STN): PCA, PCoA.
  • Substantia Nigra (SN): PCA, Basilar A. perforators.
• Messengers:
  • Striatum: GABA, ACh; Substance P (direct pathway), Enkephalin (indirect pathway).
  • SNc: Dopamine (modulatory).
  • STN: Glutamate (excitatory, +).
  • GPi/SNr (Output): GABA (inhibitory, -).

⭐ Lenticulostriate arteries (from MCA) are common sites for hypertensive bleeds, often termed "Charcot's artery of cerebral hemorrhage".

High‑Yield Points - ⚡ Biggest Takeaways

• Basal ganglia include Striatum (Caudate + Putamen), Globus Pallidus, Subthalamic Nucleus, and Substantia Nigra.
• Crucial for motor control: Direct pathway (D1) facilitates movement; Indirect pathway (D2) inhibits movement.
• Dopamine from SNpc excites direct (D1) & inhibits indirect (D2) pathways.
• Parkinson's Disease: ↓Dopamine (SNpc loss) causes bradykinesia, rigidity, tremor.
• Huntington's Disease: Striatal GABAergic loss (indirect pathway) causes chorea.
• Key neurotransmitters: GABA (inhibitory), Glutamate (excitatory), Acetylcholine (striatal, opposes dopamine).