Basal Ganglia Overview and Pathways

Overview

This lecture reviews the basal ganglia's anatomy, functions, and pathways, and explains their clinical relevance, especially in movement disorders.

Basal Ganglia Anatomy

• The main components are the caudate nucleus, putamen, globus pallidus (internal and external), thalamus (ventral anterior and lateral nuclei), subthalamic nucleus, and substantia nigra.
• The caudate nucleus and putamen together form the striatum.
• The putamen and globus pallidus together form the lentiform nucleus.
• The substantia nigra has two parts: zona compacta (dopaminergic neurons) and zona reticularis.

Functions of Basal Ganglia

• The basal ganglia regulate starting, stopping, and modulating voluntary motor movements.
• They receive motor plans from the cortex, modify them, and send them back for precise movement control.

Direct Pathway

• The direct pathway increases/stimulates motor activity and helps initiate movement.
• Sequence: Cortex → Striatum → Globus Pallidus Internus → Thalamus → Cortex.
• Glutamate (excitatory) and GABA (inhibitory) are key neurotransmitters; less inhibition of the thalamus results in increased motor activity.

Indirect Pathway

• The indirect pathway decreases/inhibits unwanted or undesired motor activity.
• Sequence: Cortex → Striatum → Globus Pallidus Externus → Subthalamic Nucleus → Globus Pallidus Internus → Thalamus → Cortex.
• Increased inhibition of the thalamus leads to decreased motor activity.

Nigrostriatal Pathway (Modulation)

• The nigrostriatal pathway (substantia nigra to striatum) modulates direct and indirect pathways using dopamine.
• Dopamine stimulates the direct pathway (via D1 receptors—excitatory) and inhibits the indirect pathway (via D2 receptors—inhibitory).
• Damage to dopamine-producing neurons, as in Parkinson’s disease, reduces movement initiation.

Neurotransmitter Mechanisms

• GABA opens channels for negative ions, causing hyperpolarization (inhibition, IPSP).
• Glutamate allows positive ions in, causing depolarization (excitation, EPSP).
• D1 dopamine receptors use Gs proteins to increase cAMP (stimulation).
• D2 dopamine receptors use Gi proteins to decrease cAMP (inhibition).

Clinical Relevance

• Parkinson's disease: Loss of nigrostriatal dopamine, impaired direct pathway, difficulty initiating movement.
• Huntington's disease: Damage to indirect pathway, unwanted movements.
• Wilson’s disease and Sydenham’s chorea: Basal ganglia involvement, movement disorders.
• Drug-induced (antipsychotics): Extrapyramidal symptoms affect D2 receptors, causing dyskinesias.

Key Terms & Definitions

• Striatum — Caudate nucleus plus putamen.
• Lentiform nucleus — Putamen plus globus pallidus.
• Substantia nigra (zona compacta) — Midbrain structure with dopaminergic neurons.
• Direct pathway — Basal ganglia circuit that facilitates movement.
• Indirect pathway — Basal ganglia circuit that inhibits unwanted movement.
• Nigrostriatal pathway — Dopaminergic modulation from substantia nigra to striatum.
• D1 receptor — Dopamine receptor that stimulates the direct pathway.
• D2 receptor — Dopamine receptor that inhibits the indirect pathway.

Action Items / Next Steps

• Review diagrams of basal ganglia pathways.
• Study neurotransmitter effects (glutamate, GABA, dopamine) in each pathway.
• Read about clinical conditions related to basal ganglia dysfunction (Parkinson’s, Huntington’s, etc.).