Basal ganglia

Components - The Brain's Gatekeepers

• Corpus Striatum: The primary input zone, receiving signals from the cerebral cortex.
  • Caudate Nucleus: C-shaped structure involved in cognitive functions.
  • Putamen: Outer part of the lentiform nucleus; regulates movements.
• Globus Pallidus: Main output nucleus.
  • Internal (GPi) & External (GPe) segments.
• Subthalamic Nucleus (STN): Key target for deep brain stimulation in Parkinson's disease.
• Substantia Nigra: Midbrain structure.
  • Pars Compacta (SNc): Provides dopamine to the striatum.
  • Pars Reticulata (SNr): An output nucleus.

⭐ The lentiform nucleus (putamen + globus pallidus) is supplied by the lenticulostriate arteries, which are prone to hemorrhage in chronic hypertension.

Motor Pathways - Go vs. No-Go

• Function: The basal ganglia fine-tunes motor commands by balancing two key pathways originating from the striatum (caudate + putamen).

• Dopamine's Role (from Substantia Nigra pars compacta):

  • Binds to D1 receptors → stimulates the Direct pathway → ↑ movement.
  • Binds to D2 receptors → inhibits the Indirect pathway → ↑ movement.
  • 📌 Mnemonic: Dopamine 1=Direct; Dopamine 2=Indirect.

⭐ Parkinson's Disease: Characterized by the loss of dopaminergic neurons in the substantia nigra. This leads to ↓ stimulation of the direct (Go) pathway and ↓ inhibition of the indirect (No-Go) pathway, resulting in bradykinesia and rigidity.

Clinical Correlates - When Circuits Break

Imbalance between direct (GO) and indirect (NO-GO) pathways causes movement disorders.

• Hypokinetic (↓ Movement): Parkinson's Disease

  • Lesion: Loss of dopaminergic neurons in substantia nigra pars compacta (SNc).
  • Mechanism: ↓ direct pathway activity & ↑ indirect pathway activity.
  • Signs: Bradykinesia, resting tremor, rigidity. 📌 TRAP mnemonic: Tremor, Rigidity, Akinesia, Postural instability.

• Hyperkinetic (↑ Movement):

  • Huntington's Disease:
    • Lesion: Atrophy of GABAergic neurons in the caudate/putamen.
    • Mechanism: ↓ indirect pathway inhibition → chorea, athetosis.
  • Hemiballismus:
    • Lesion: Contralateral subthalamic nucleus (STN).
    • Mechanism: ↓ indirect pathway activity → wild, flinging movements.

⭐ Huntington's disease is an autosomal dominant disorder caused by CAG trinucleotide repeats on chromosome 4. "Hunting 4 CAGs."

High‑Yield Points - ⚡ Biggest Takeaways

• The basal ganglia are key for motor control, acting to initiate and smooth voluntary movements.
• The direct pathway (D1) facilitates movement by disinhibiting the thalamus; the indirect pathway (D2) inhibits it.
• Dopamine from the substantia nigra excites the direct and inhibits the indirect pathways, promoting movement.
• Parkinson's disease: ↓ Dopamine leads to a hypokinetic state (bradykinesia, rigidity, tremor).
• Huntington's disease: Caudate degeneration leads to a hyperkinetic state (chorea).
• Subthalamic nucleus lesion causes hemiballismus (contralateral flinging movements).