The projection of low-threshold muscle afferents of the forelimb to the main and external cuneate nuclei of the monkey.

The bulbar relay subserved by primary muscle afferents from the monkey's deep radial nerve was investigated by means of transganglionic labeling with wheatgerm agglutinin conjugated with horseradish peroxidase as well as by electrophysiological techniques. It was found that the "deep" input was confined to the pars triangularis of the main cuneate nucleus and to the ventromedial segment of the external cuneate nucleus. On the other hand, labeling occurring following incubation of the superficial (cutaneous) branch of the radial nerve was limited to the pars rotunda of the main cuneate nucleus. The cytoarchitectonic differentiation of the medullary relays is thus met by a clear-cut submodality segregation.