Organization of the ascending striatal afferents in monkeys.

Horseradish peroxidase was injected in various parts of the caudate nucleus and the putamen of monkeys to ascertain the relative locations of striatal projecting cells in the mesencephalon. The nigrostriatal component, as expected, is the greatest but numerous cells of the mesencephalic raphe system also project to the striatum. The projections from the pars compacta are organized in a topographical manner in all principal planes. The rostral two thirds of the substantia nigra are related to the head of the caudate nucleus. Nigral neurons projecting to the putamen are more posteriorly located and display an anteroposterior topography. The medial two thirds of the pars compacta send efferents to the head of the caudate nucleus from ventromedial to laterodorsal regions, reflecting a mediolateral topographical relationship. An inverse relationship exists dorsoventrally between nigra and caudate so that ventral compacta cells project to dorsal caudate and the dorsally situated neurons project to ventral-ventro-medial caudate regions. The dorsal and lateral parts of the putamen are more intimately related to the lateral and posterior nigra; by contrast, the ventral and ventromedial putamen receives more afferents from medial and central regions of the substantia nigra. A large group of cells in the tegmentum dorsal and medial to the medial lemniscus shows continuity with the pars compacta, and has similar connections with the striatum. This cell group should be included with the pars compacta. Significant overlap exists between the projection fields in all planes, making the nigrostriatal topographical organization seem less than precise. This apparent lack of point-to-point reciprocity may be due to the considerable size difference between the striatum and the substantia nigra. The raphe nuclei project to the greater part of the striatum but more significantly to its ventral and medial regions. The paranigral cell group sends its efferents mainly to the ventral striatum.