The Basal Ganglia

Lecture 12

What are the five major nuclei of the basal ganglia?

Caudate, putamen, globus pallidus (GPe/GPi), subthalamic nucleus (STN), substantia nigra (SNc/SNr).

What structures make up the striatum?

Caudate, putamen, nucleus accumbens.

What structures make up the lenticular nucleus?

Putamen + globus pallidus.

What is the primary input nucleus of the basal ganglia?

Striatum.

What is the primary output nucleus of the basal ganglia?

GPi and SNr.

Are basal ganglia outputs inhibitory or excitatory?

Inhibitory (GABAergic).

Which pathway facilitates movement?

Direct pathway.

Which pathway inhibits movement?

Indirect pathway.

What neurotransmitter does the substantia nigra pars compacta release?

Dopamine

What receptors does dopamine activate in the direct vs. indirect pathway?

D1 = excitatory (direct), D2 = inhibitory (indirect).

What happens in Parkinson’s disease?

Degeneration of SNc dopaminergic neurons.

What are common symptoms of PD?

Bradykinesia, rigidity, tremor, akinesia.

Name all major basal ganglia nuclei and describe their general roles.

• Caudate nucleus: cognitive/oculomotor functions; input to BG.

• Putamen: sensorimotor input; main motor-related input region of striatum.

• Globus pallidus:

  • GPe: regulates indirect pathway.

  • GPi: major output nucleus.

• Subthalamic nucleus (STN): excitatory drive to GPi (glutamate).

• Substantia nigra:

  • SNc: dopamine → modulates striatum (D1/D2).

  • SNr: output nucleus (similar to GPi).

Describe the main afferent connections into the striatum.

• Corticostriatal: from all cortical areas (glutamate).

• Thalamostriatal: from intralaminar nuclei (glutamate).

• Nigrostriatal: from SNc (dopamine).

Describe the major efferent connections of the basal ganglia.

Outputs are inhibitory (GABA) and arise from:

• GPi → thalamus (VA/VL/DM) → motor cortex.

• SNr → thalamus, brainstem gaze centers.

Describe the motor loop through the basal ganglia.

Motor/association cortex → striatum → GPi/SNr → VA/VL thalamus → motor cortex (M1/SMA).
Basal ganglia do NOT initiate movement, but select, reinforce, or suppress motor programs.

Describe the direct pathway (step-by-step).

• Cortex excites striatum (glutamate).

• Striatum inhibits GPi/SNr (GABA).

• GPi/SNr inhibition → releases thalamus from inhibition (“disinhibition”).

• Thalamus excites motor cortex → movement promoted.

Describe the indirect pathway (step-by-step).

• Cortex excites striatum.

• Striatum inhibits GPe.

• Reduced GPe activity → less inhibition of STN.

• STN becomes more active → excites GPi/SNr.

• GPi/SNr increases inhibition on thalamus.

• Thalamus cannot excite cortex → movement inhibited.

Explain how dopamine affects the direct & indirect pathways.

• D1 receptors (direct): dopamine excites → increases movement.

• D2 receptors (indirect): dopamine inhibits → reduces indirect pathway → increases movement.
  Net effect = promotes movement.

List the four basal ganglia circuits and their functions.

• Motor loop: learned movements and execution.

• Cognitive loop: planning, working memory.

• Limbic loop: emotional/motivational aspects of movement.

• Oculomotor loop: voluntary saccadic eye movements.

Describe the basal ganglia changes in Parkinson’s disease.

• Loss of dopaminergic neurons in SNc.

• Decreased direct pathway activity (less D1 stimulation).

• Increased indirect pathway activity (less D2 inhibition).

• Increased GPi/SNr output → excessive thalamic inhibition.

• Hypokinetic symptoms: bradykinesia, rigidity, resting tremor, shuffling gait.

What disorders arise from excessive movement (hyperkinesia)?

• Chorea: rapid, jerky movements (Huntington’s).

• Athetosis: slow, writhing movements.

• Ballismus: violent flinging of limbs (STN lesion).

• Tremor: resting or intention.