- Striatum
- The primary input nucleus of the basal ganglia, composed of the caudate nucleus (curved, following the lateral ventricles) and the putamen (lateral, adjacent to the insula). The striatum receives massive glutamatergic input from virtually the entire cortex, as well as dopaminergic input from the substantia nigra pars compacta (SNc). Its medium spiny neurons — GABAergic, making up 95% of striatal neurons — project through the direct and indirect pathways. The ventral striatum (nucleus accumbens) is the limbic component, receiving input from the hippocampus, amygdala, and prefrontal cortex and mediating reward and motivation.
- Direct pathway ("go" signal)
- The movement-facilitating pathway: Cortex → Striatum (D1 MSNs) → Globus pallidus interna (GPi)/Substantia nigra pars reticulata (SNr) [inhibited] → Thalamus [disinhibited] → Cortex. Because the GPi tonically inhibits the thalamus, striatal inhibition of the GPi releases the thalamus to excite the cortex. Dopamine acting on D1 receptors in the striatum strengthens this pathway. The direct pathway selects and initiates the desired movement.
- Indirect pathway ("stop" signal)
- The movement-suppressing pathway: Cortex → Striatum (D2 MSNs) → Globus pallidus externa (GPe) [inhibited] → Subthalamic nucleus (STN) [disinhibited] → GPi/SNr [excited] → Thalamus [inhibited] → reduced cortical activity. This pathway suppresses competing movement programmes. Dopamine acting on D2 receptors weakens the indirect pathway. The balance between direct and indirect pathways determines which movements are selected and executed.
- Substantia nigra
- A midbrain nucleus with two distinct parts. The pars compacta (SNc) contains the dopaminergic neurons whose axons form the nigrostriatal tract — the major dopamine input to the dorsal striatum — and whose degeneration causes Parkinson's disease. The pars reticulata (SNr) is functionally similar to the GPi and serves as an output nucleus of the basal ganglia, projecting GABAergically to the thalamus and superior colliculus. The SNc's dark pigmentation (neuromelanin, from which it takes its name) is produced by dopaminergic neurons.
- Subthalamic nucleus (STN)
- A small, lens-shaped nucleus below the thalamus that is the only glutamatergic (excitatory) nucleus in the basal ganglia. In the indirect pathway, the STN drives the GPi to inhibit the thalamus. STN lesions in humans or animals produce hemiballismus — wild, flinging movements of the contralateral limb — illustrating that the STN is a necessary brake on movement. Deep brain stimulation (DBS) of the STN is one of the most effective treatments for advanced Parkinson's disease, reducing motor symptoms by modulating this critical node.
- Reward prediction error (RPE)
- The signal encoded by dopaminergic neurons in the VTA and SNc, discovered by Wolfram Schultz (1997). Dopamine neurons fire above baseline when a reward is better than expected (positive RPE), return to baseline when it matches prediction, and are suppressed below baseline when an expected reward is omitted (negative RPE). This signal is mathematically equivalent to the temporal difference (TD) error in reinforcement learning. RPE signals propagate through the mesolimbic pathway (VTA → nucleus accumbens) and nigrostriatal pathway to update striatal synaptic weights, driving learning and habit formation.
- Procedural and habit learning
- The dorsal striatum is essential for stimulus-response habit learning — the gradual acquisition of automatic behavioural routines that do not require conscious deliberation. This is dissociable from hippocampus-dependent declarative learning: patients with Parkinson's disease or Huntington's disease show impaired probabilistic classification learning while preserving declarative memory, while amnesic patients with hippocampal damage show the reverse. The distinction between goal-directed (frontal-hippocampal) and habitual (striatal) action control is fundamental to addiction, OCD, and compulsive behaviour.