The substantia nigra lies in the ventral midbrain, dorsal to the cerebral peduncle and near the red nucleus. “Compacta” and “reticulata” name adjacent cellular territories rather than two sharply insulated organs. SNc is enriched in dopamine-producing neurons, many containing neuromelanin, while SNr is composed mainly of tonically active GABAergic output neurons. The bilateral mesh provides a useful gross location but cannot show nigrosomes, cell populations, or the interdigitating borders needed for microscopic or patient-specific interpretation. Nigrosome 1 contributes to the normal dorsolateral “swallow-tail” appearance on high-resolution iron-sensitive MRI. Loss of that sign is common in Parkinsonian degeneration, but acquisition technique, reader experience, and overlap with other parkinsonian disorders limit its value as a standalone diagnostic test.

SNc projections modulate the caudate and putamen through dopamine receptors expressed across striatal cell populations. Their activity influences action selection, movement vigour, habit learning, reinforcement, salience, and motivation on multiple time scales. Reward-prediction-error responses are an influential account of some phasic dopamine signals, but dopamine neurons are heterogeneous and do not all encode one scalar teaching signal. Likewise, the direct and indirect pathway model is a useful starting point, not a complete description of basal-ganglia dynamics.

SNr is one of two major basal-ganglia output nuclei, alongside the internal globus pallidus. It integrates inhibitory striatal input and excitatory subthalamic input, then projects to thalamic and brainstem targets. Tonic SNr inhibition of the superior colliculus helps suppress unwanted orienting responses; patterned reductions and increases in that inhibition contribute to selecting saccades. SNr also sends potent inhibitory output toward the pedunculopontine nucleus and wider mesencephalic locomotor region. Excess basal-ganglia inhibition of these brainstem systems is one proposed contributor to gait freezing and postural instability in parkinsonian disorders, alongside distributed cortical, striatal, cerebellar, and brainstem mechanisms. These circuits run in parallel motor, associative, and limbic loops, so assigning the whole substantia nigra only to a motor network is a simplification.

Parkinson’s disease features preferential, regionally patterned loss of SNc dopamine neurons and depletion of striatal dopamine. This strongly contributes to bradykinesia and rigidity, but the disease is not a focal substantia-nigra lesion: pathology and symptoms extend across multiple brain and peripheral systems. Published estimates of neuronal loss at motor onset vary with method and sampled nigral territory, so a single fixed percentage is misleading. Levodopa improves many motor symptoms by restoring dopamine synthesis but does not recreate normal phasic circuit signalling or explain every motor and non-motor feature.

Rare unilateral infarcts or haemorrhages affecting the substantia nigra or nigrostriatal pathway can produce contralateral hemiparkinsonism. That focal syndrome should be distinguished from idiopathic Parkinson’s disease. Weber and Benedikt syndromes, by contrast, are defined by wider midbrain involvement such as the cerebral peduncle, oculomotor fascicle, or red-nucleus region; they are not specific lesion syndromes of the substantia nigra itself.