Overview

The substantia nigra is the largest nucleus of the midbrain and extends its full length as a curved plate. It has two parts:
• The pars compacta (SNc) contains dopaminergic neurons that are dark with neuromelanin, a by-product of dopamine oxidation. Their axons form the nigrostriatal pathway to the caudate and putamen. There they facilitate the direct pathway through D1 receptors and inhibit the indirect pathway through D2 receptors, so dopamine promotes movement and reinforces learned actions.
• The pars reticulata (SNr) is a GABAergic output nucleus equivalent to the internal globus pallidus. It projects to the thalamus and the superior colliculus, gating saccadic eye movements.

The neighbouring ventral tegmental area gives rise to the mesolimbic and mesocortical dopamine pathways. Together with the SNc it is called the midbrain dopamine system.

Location: Ventral midbrain, between the crus cerebri (anteriorly) and the tegmentum with the red nucleus (posteriorly); extends from the pons to the subthalamus.

Parts & subdivisions

Pars compacta (SNc)
Dopaminergic, neuromelanin-containing neurons → striatum (nigrostriatal pathway). Degenerates in Parkinson disease.
Pars reticulata (SNr)
GABAergic output to the thalamus (VA/VL), superior colliculus and pedunculopontine nucleus; tonic inhibition released to allow saccades and movement.

Anatomy

Histology: SNc neurons are large and multipolar, with brown neuromelanin granules visible to the naked eye as a black band. In Parkinson disease they contain eosinophilic Lewy bodies (α-synuclein).

Function

Supplies dopamine that facilitates movement initiation and vigour
Reinforcement of motor learning and habits (reward-prediction signalling)
Output of the basal nuclei for eye movements (SNr → superior colliculus)

Clinical significance

Parkinson disease

Progressive loss of SNc neurons causes pallor of the substantia nigra at autopsy. Motor signs (bradykinesia, rigidity, resting tremor, later postural instability) appear once ≈50–60% of the neurons are gone. DaT-SPECT shows reduced striatal dopamine transporter, and loss of the "swallow-tail" sign (nigrosome-1) on susceptibility-weighted MRI is supportive.

Drug-induced and toxic parkinsonism

D2-receptor blockers (typical antipsychotics, metoclopramide, prochlorperazine) cause reversible parkinsonism. MPTP, a contaminant of synthetic opioids, is converted by MAO-B into MPP⁺, which selectively kills SNc neurons. Recognising this toxicity in the 1980s revolutionised Parkinson research.

Test yourself

Parkinson disease results from degeneration of:

Cholinergic neurons of the nucleus basalis
GABAergic neurons of the caudate nucleus
Dopaminergic neurons of the substantia nigra pars compacta
Noradrenergic neurons of the locus coeruleus only

Answer: Dopaminergic neurons of the substantia nigra pars compacta

Loss of nigrostriatal dopamine reduces the D1-excited direct (go) pathway and disinhibits the D2-inhibited indirect pathway. The GPi then over-inhibits the thalamus, causing bradykinesia. In Huntington disease, by contrast, the caudate (striatal) neurons degenerate.