Efferent connexions of the pars lateralis of the substantia nigra (SNL).

Unilateral lesions were made in the lateral nucleus of the substantia nigra in eight cats and in the zona compacta and reticularis in two cats. After a 10 days survival period, brains were perfused and stored in 10% formalin. Frozen sections were stained with the De Olmos-Ingram and Fink-Heimer silver methods for degenerating axons and terminals. Efferents from the lateral nucleus of the substantia nigra projected to the dorsolateral third of the head of the caudate nucleus, putamen, medial reticular formation, central tegmental tract, amygdala, zona compacta and reticularis of the substantia nigra, ventral anterior and ventral ateral thalamic nuclei. Projections to the amygdala, medial reticular formation and central tegmental tract were seen only in animals with lesions in the lateral nucleus of the substantia nigra. Projections to the caudate and putamen were more abundant when compared to those in the thalamus. Within the thalamus, projections to the ventral lateral nucleus were scanty compared to those in the ventral anterior nucleus. The present study compares for the first time efferent connexions of the lateral nucleus of the substantia nigra and those of the pars compacta and reticularis. The findings suggest a specialization of the different zones of the substantia nigra with regard to their projection sites.