Laminar organization of the substantia nigra pars reticulata in the macaque monkey, with special reference to the caudato-nigro-tectal link.

A laminar organization of the substantia nigra pars reticulata (SNr) was revealed in the macaque monkey by using an semihorizontal section plane parallel to the long axis of the SNr in the frontal plane. After injecting horseradish peroxidase conjugated to wheat germ agglutinin into the caudate nucleus (Cd) or putamen, anterogradely labelled fibres and axon terminals in the SNr were observed to form parallel bands in the anteromedial-posterolateral direction. After injecting Fast Blue into the superior colliculus, a cluster of retrogradely labelled neuronal cell bodies also formed a single band arranged in the similar anteromedial-posterolateral direction. The cluster was observed along the anterolateral margin of the SNr. This SNr region containing nigrotectal neurons appeared to overlap with one of the bands containing terminals of striatonigral fibres arising from the body of the Cd. The caudato-nigro-tectal link via the anterolateral marginal area might be involved in the control of saccadic eye movements.