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The substantia nigra (plural: substantiae nigrae) is one of the brainstem nuclei and part of the extrapyramidal system. While other nuclei such as the red nucleus are small and contained within an axial slice at the superior colliculi (see figure), the substantia nigra is seen in axial slices at both superior and inferior colliculi.
It is an important relay station in the motor system. It consists of a compact part (dark, containing melanin) and a reticular part (reddish, containing iron). Most of its axons are projected diffusely to other brain areas and not arranged into tracts. Numerous axonal tracts terminate in the substantia nigra:
- caudate nucleus (striatonigral fibers)
- anterior cerebral cortex (corticonigral fibers)
- precentral cortex
Nigrosomes are small clusters of dopaminergic cells measuring up to a few millimeters in size within the substantia nigra that exhibit calbindin D28K negativity on immunohistochemistry. Five nigrasomes have been described with the largest labeled as nigrosome-1 positioned in the caudal, medio-lateral portion of the substantia nigra. Nigrasome-1 contains the largest proportion of neurons affected in Parkinson disease 3.
High resolution T2*/SWI weighted MRI is able to identify the distribution of normal signal within healthy and diseased substantia nigra both at 7 T and 3 T field strength. The healthy nigrosome-1 produces a swallow tail appearance on axial imaging. Absence of the normal swallow tail appearance of nigrosome-1 is reported to be a reliable sign of Parkinson disease 3.