Basal Ganglia: Anatomy, Circuits, and Clinical Disorders

Overview

The basal ganglia are a collection of subcortical gray matter nuclei that form the core of a complex cortico-basal ganglia-thalamocortical loop. They are critically involved in motor control, procedural learning, habit formation, and reward-based behavior. The term "basal ganglia" is somewhat historical; these structures are not ganglia in the peripheral sense but deep brain nuclei.

Nuclei of the Basal Ganglia

Striatum (Caudate + Putamen + Nucleus Accumbens)

The striatum is the primary input structure of the basal ganglia:

• Caudate nucleus: C-shaped, follows the lateral ventricle; head, body, and tail. Involved in cognitive and associative loops.
• Putamen: largest component; primarily motor and sensorimotor processing
• Nucleus accumbens: ventral striatum at the junction of caudate and putamen; reward and motivation circuit
• Together, caudate + putamen = neostriatum (or dorsal striatum)

The striatum receives glutamatergic input from the cerebral cortex (corticostriatal projection) and dopaminergic input from the substantia nigra pars compacta (SNc) via the nigrostriatal pathway.

Globus Pallidus (GP)

• Internal segment (GPi): major output nucleus; GABAergic projections to thalamus and brainstem
• External segment (GPe): connects to subthalamic nucleus in the indirect pathway
• Together = pallidum (lenticular nucleus = putamen + globus pallidus)

Subthalamic Nucleus (STN)

• Located in the diencephalon, below the thalamus
• Only glutamatergic (excitatory) nucleus in the basal ganglia
• Critical node in the indirect pathway
• Target for deep brain stimulation (DBS) in Parkinson's disease

Substantia Nigra (SN)

• Located in the midbrain tegmentum
• SNc (pars compacta): dopaminergic neurons projecting to striatum (nigrostriatal pathway); contain neuromelanin (black pigment); neurons degenerate in Parkinson's disease
• SNr (pars reticulata): GABAergic output similar to GPi; projects to superior colliculus and thalamus

Claustrum and Amygdala

Sometimes included; the amygdala is linked to the ventral striatum in emotional and reward processing.

Basal Ganglia Circuits

Direct Pathway (facilitates movement)

Cortex → Striatum → GPi/SNr (inhibited) → Thalamus (disinhibited) → Cortex
Net effect: facilitates desired movements

Indirect Pathway (suppresses unwanted movements)

Cortex → Striatum → GPe (inhibited) → STN (disinhibited) → GPi/SNr (excited) → Thalamus (inhibited) → Cortex
Net effect: suppresses unwanted competing movements

Dopamine Modulation

Dopamine from SNc modulates both pathways:

• D1 receptors (direct pathway): dopamine is excitatory → facilitates direct pathway → promotes movement
• D2 receptors (indirect pathway): dopamine is inhibitory → inhibits indirect pathway → reduces movement suppression

In Parkinson's disease: loss of dopamine → overactive indirect pathway → excessive inhibition of thalamus → reduced cortical activation → bradykinesia, rigidity

Clinical Disorders

Parkinson's Disease

• Pathology: degeneration of SNc dopaminergic neurons (>60% lost before symptoms)
• Cardinal features: tremor (resting, "pill-rolling"), rigidity (cogwheel), bradykinesia, postural instability
• Lewy bodies (α-synuclein aggregates) are the pathological hallmark
• Treatment: levodopa/carbidopa, dopamine agonists, MAO-B inhibitors, DBS of STN or GPi

Huntington's Disease

• Pathology: degeneration of striatal neurons (especially indirect pathway medium spiny neurons)
• Features: chorea, cognitive decline, psychiatric symptoms
• Genetics: autosomal dominant, CAG repeat expansion in HTT gene

Hemiballismus

• Lesion: contralateral subthalamic nucleus (usually vascular)
• Features: violent flinging movements of proximal limb contralateral to lesion

Wilson's Disease

• Copper deposition in basal ganglia and liver
• Features: dysarthria, dystonia, tremor; Kayser-Fleischer rings in cornea

Blood Supply

Striatum: lateral striate arteries (lenticulostriate arteries) from MCA — these are "end arteries," explaining why hypertensive hemorrhage commonly occurs in the basal ganglia.

Study Points

• Name all five basal ganglia nuclei and identify their inputs and outputs.
• Trace the direct and indirect pathways and explain how dopamine modulates each.
• Explain the pathophysiology of Parkinson's disease using the direct/indirect model.
• Identify the STN as the target for DBS and explain why stimulating it helps Parkinson's symptoms.
• Distinguish chorea (Huntington's) from hemiballismus by their anatomical substrates.