substantia nigra

An anatomical

illustration from the 1908 edition of Sobotta"s Anatomy Atlas

The substantia nigra is one of the brainstem nuclei and part of the extrapyramidal system. While other nuclei such as the red nucleus are as 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.

Gross anatomy

They are situated in the anterior midbrain and mark the transition point of the tegmentum and cerebral peduncles.

Function

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)
putamen
precentral cortex

Nigrosomes

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 .

Radiographic features

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 .