Basal Ganglia

Anatomy and Components

The basal ganglia are a collection of gray matter nuclei located deep within the white matter of the cerebral hemispheres. The main components are:

Structure	Subdivision	Notes
Striatum	Caudate nucleus + Putamen	Receives virtually ALL inputs to the BG; "neostriatum"; cellular bridges give striated appearance
Globus pallidus	Internal (GPi) + External (GPe)	Primary output nucleus
Subthalamic nucleus (STN)	-	Diencephalon; only excitatory output nucleus within BG
Substantia nigra	Pars compacta (SNc) + Pars reticulata (SNr)	SNc = dopaminergic; SNr = inhibitory output

Associated structures include the nucleus accumbens and ventral pallidum (limbic/reward circuits), and the ventral anterior/ventral lateral thalamus (relay back to cortex). The internal capsule passes through the space between the caudate and putamen, carrying corticospinal fibers.

Fig. 57.9 - Anatomical relations of the basal ganglia to the cerebral cortex and thalamus (Guyton & Hall)

Basal ganglia in situ - striatum (blue), globus pallidus (red), subthalamic nucleus (yellow) (Neuroanatomy through Clinical Cases)

Inputs and Outputs

Inputs (afferents): Almost all areas of the cerebral cortex project topographically onto the striatum, with a critical input from the motor cortex. The substantia nigra pars compacta sends a major dopaminergic input back to the striatum.
Outputs (efferents): The striatum projects to the globus pallidus and substantia nigra. The GPi and SNr project (inhibitory) to the thalamus, which then projects excitatory signals back to the motor cortex. The BG do NOT project directly to the spinal cord or brainstem motor neurons.

Neuronal Circuitry: Direct vs. Indirect Pathways

The basal ganglia modulate thalamo-cortical output via two opposing pathways. Their net balance controls the level of motor activity.

Fig. 57.11 - Putamen circuit through the basal ganglia for subconscious execution of learned motor patterns (Guyton & Hall)

Direct Pathway (facilitatory - net excitatory effect on cortex)

Cortex → Striatum → GPi/SNr → Thalamus → Cortex

Cortex excites the striatum (glutamate)
Striatum inhibits GPi/SNr (GABA)
GPi/SNr inhibition is reduced, so thalamus is released from inhibition
Thalamus excites the cortex (glutamate)

Net effect: increased motor activity

Indirect Pathway (inhibitory - net inhibitory effect on cortex)

Cortex → Striatum → GPe → STN → GPi/SNr → Thalamus → Cortex

Cortex excites the striatum
Striatum inhibits GPe (GABA)
GPe inhibition is reduced, releasing the STN from inhibition
STN excites GPi/SNr (glutamate)
GPi/SNr strongly inhibits the thalamus (GABA)
Thalamic excitation of cortex is suppressed

Net effect: decreased motor activity

The direct pathway is excitatory and the indirect pathway is inhibitory. They are carefully balanced - a disturbance in one causes either an increase or decrease in motor activity, which is the basis of basal ganglia diseases. - Costanzo Physiology 7th Edition

Dopamine's Role (Nigrostriatal Pathway)

The substantia nigra pars compacta (SNc) sends dopaminergic projections to the striatum. Dopamine has opposing effects in the two pathways:

D1 receptors (direct pathway): Dopamine is excitatory - activates the direct pathway → more movement
D2 receptors (indirect pathway): Dopamine is inhibitory - suppresses the indirect pathway → less inhibition of movement

So dopamine, overall, facilitates movement by simultaneously enhancing the direct (go) pathway and suppressing the indirect (no-go) pathway.

Two Major Functional Circuits

1. Putamen Circuit - Execution of Learned Motor Patterns

Inputs: premotor cortex, supplementary motor area, somatosensory cortex
Path: Cortex → Putamen → GPi → Thalamus (VA/VL) → Primary motor cortex
Function: Controls complex, subconscious motor patterns (writing, throwing, vocalization, eye movements)
Damage: athetosis (globus pallidus lesion), hemiballismus (subthalamic lesion), chorea (putamen lesions)

2. Caudate Circuit - Cognitive Control of Motor Sequences

Inputs: association cortex (frontal, parietal, occipital, temporal lobes)
Path: Cortex → Caudate → GPi → Thalamus (VA/VL) → Prefrontal/premotor/supplementary cortex
Function: Plans and sequences motor acts based on thought (cognitive motor planning); signals return to accessory motor areas for building sequential patterns lasting 5+ seconds
Does NOT feed directly back to primary motor cortex - feeds the planning areas

Other Functions

Beyond motor control, the basal ganglia contribute to:

Memory and habit learning - initiation, execution, and termination of learned motor tasks
Cognitive functions - attention, working memory, decision-making (frontal-striatal circuits)
Affective/emotional control - limbic circuits via nucleus accumbens (reward, motivation)
Eye movement control - via the superior colliculus
Timing and scaling - determining speed and amplitude of movements

Diseases of the Basal Ganglia

Disease	Pathology	Movement Type	Mechanism
Parkinson's disease	Degeneration of SNc dopamine neurons	Hypokinetic: rigidity, akinesia, resting tremor	Loss of dopamine → indirect pathway overactive + direct pathway underactive → excess inhibition of thalamus
Huntington's disease	Destruction of striatal GABAergic and cholinergic neurons	Hyperkinetic: choreoathetosis, dementia	Loss of indirect pathway inhibition → thalamus overactive → excess cortical excitation
Hemiballismus	Subthalamic nucleus lesion	Wild flailing movements of one limb	STN lesion → GPi not driven → thalamus released from inhibition
Athetosis	Globus pallidus lesion	Continuous writhing movements (hand/arm/face)	Loss of basal ganglia output regulation
Chorea	Multiple putamen lesions	Flicking movements (hands, face)	Disrupted striatal output

Treatment of Parkinson's: L-dopa (dopamine precursor), dopamine agonists (bromocriptine), or deep brain stimulation of the STN or GPi to reduce excess inhibitory output.

Summary of Neurotransmitters

Pathway	Neurotransmitter	Effect
Cortex → Striatum	Glutamate	Excitatory
Striatum → GPe (indirect)	GABA	Inhibitory
Striatum → GPi/SNr (direct)	GABA	Inhibitory
GPe → STN	GABA	Inhibitory
STN → GPi/SNr	Glutamate	Excitatory
GPi/SNr → Thalamus	GABA	Inhibitory
Thalamus → Cortex	Glutamate	Excitatory
SNc → Striatum	Dopamine	D1 (excitatory)/D2 (inhibitory)

Sources:

Guyton and Hall Textbook of Medical Physiology, Ch. 57
Costanzo Physiology 7th Edition, Ch. 3
Neuroanatomy through Clinical Cases 3rd Edition, Ch. 16
Bradley and Daroff's Neurology in Clinical Practice

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