A quantitative study of the projections of the gracile, cuneate and trigeminal nuclei and of the medullary reticular formation to the thalamus in the rat.

Following injection of horseradish peroxidase into the thalamus of one side, the numbers of labelled neurons in the nuclei of the dorsal funiculi and in the trigeminal sensory complex were counted. A comparative study was made of the pattern of labelling after a range of survival times, and animals surviving for 72 h after injection were used to provide detailed quantitative information about the patterns of distribution of labelled cells. The principal sensory nucleus of the trigeminal nerve (8683 labelled neurons) and the nucleus of the spinal trigeminal tract, pars interpolaris (1920) label heavily after thalamic injection. Pars oralis of the spinal nucleus labels more sparsely (524 labelled neurons), while the pars caudalis (260 labelled neurons) shows a laminar labelling pattern which continues across the spinomedullary junction into the upper cervical segments. The gracile (2152 labelled neurons) and cuneate (2339) nuclei also show rostrocaudal variation in labelling density: the middle one-third of each nucleus contains 66% of labelled gracile and cuneate cells. The findings are correlated with known features of the arrangement of the ascending sensory projections from these nuclei in various species, and are compared with previous findings on the distribution of thalamically-projecting cells in the upper cervical segments of the spinal cord.