Supramedullary projections to the dorsal and ventral divisions of the paramedian reticular nucleus in the cat.

Injections of combined lectin-conjugated and unconjugated horseradish peroxidase were made in the dorsal (d) and ventral (v) divisions of the paramedian reticular nucleus (PRN), a precerebellar relay nucleus, of the cat. The origins of supramedullary afferent projections to the PRN were identified in the pons, midbrain and cerebral cortex using the transverse plane of section. The data indicate a segregation of input from a number of sites to the dPRN and vPRN. The interstitial nucleus of Cajal projects bilaterally to the dPRN and predominantly to the ipsilateral side. The vPRN receives only a unilateral projection from the ipsilateral nucleus of Cajal. Major afferent projections to the vPRN arise from the ipsilateral nucleus of Darkschewitsch and the intermediate layer of the contralateral superior colliculus. Neither of these sites projected to the dPRN. The raphe nuclei and medial reticular formation of the pons and midbrain contribute a moderate input to both divisions of the PRN. A moderate bilateral cerebral cortical projection arises from the first somatomotor area (SMI). The ventral coronal and anterior sigmoid gyri project mainly to the dPRN and vPRN respectively. Smaller afferent projections arise from the posterior sigmoid gyri and area 6 of Hassler and Mühs-Clement (1964) in the medial wall of the anterior sigmoid gyrus. Inputs from the accessory oculomotor nuclei, tectal regions and the first somatomotor cortex suggest a role in postural control for the PRN which may underlie its involvement in mediating orthostatic reflexes.