The Basal Ganglia

A set of deep nuclei — striatum, globus pallidus, subthalamic nucleus and substantia nigra — that select actions, form habits and learn from reward, through 'go' and 'no-go' pathways.

The basal ganglia are a group of interconnected nuclei deep in the forebrain and midbrain. Their main parts are the striatum (made up of the caudate nucleus and putamen, plus the nucleus accumbens at its lower front), the globus pallidus (with external and internal segments), the subthalamic nucleus, and the substantia nigra in the midbrain.

A loop for choosing actions

The basal ganglia sit in a loop: the cortex sends input to the striatum, the basal ganglia process it, and their output returns via the thalamus to the cortex. Their output nuclei (the internal globus pallidus and part of the substantia nigra) are inhibitory and fire constantly, keeping a brake on the thalamus and therefore on movement. To act, that brake must be selectively released.

Go and no-go pathways

In the classic model, the direct pathway ("go") inhibits the output nuclei, releasing the brake on a chosen action. The indirect pathway ("no-go"), via the external globus pallidus and subthalamic nucleus, strengthens the brake, suppressing competing actions. A hyperdirect pathway from cortex to subthalamic nucleus can slam on an emergency stop. Dopamine from the substantia nigra tips the balance: it boosts the direct pathway (through D1 receptors) and dampens the indirect one (through D2 receptors), making action more likely. The real circuitry is more intertwined, but this model has been very useful clinically.

Learning and habits

Dopamine signals in the striatum carry reward prediction errors, reinforcing actions that turned out well. With practice, control of a behaviour shifts from goal-directed circuits toward habit circuits in the dorsolateral striatum — why routines become automatic.

Disorders

In Parkinson's disease, loss of dopamine weakens "go" and strengthens "no-go", making movement slow and hard to start. In Huntington's disease, early loss of indirect-pathway neurons weakens "no-go", producing unwanted, dance-like movements (chorea). Basal ganglia circuits are also implicated in OCD, Tourette's syndrome and addiction, and they are the main targets of deep brain stimulation.