Organization of spinal inputs to the perihypoglossal complex in the cat.

First- and second-order spinal afferents to the perihypoglossal complex were sought by using axonal transport of WGA-HRP. Injections in C1, 2, and 3 dorsal root ganglia resulted in axonal labeling in the nucleus intercalatus and the external cuneate nucleus, with a number of retrogradely labeled cells seen as well in the latter. A similar pattern of axonal labeling in the nucleus intercalatus as well as several retrogradely labeled cells were found after spinal cord injections at levels C1, 2, and 3. A prominent field of labeled axons was also present in the rostral main cuneate nucleus. No labeling was seen in the perihypoglossal nuclei after injections in the spinal cord or dorsal root ganglia at levels caudal to C3. After injections of HRP into the perihypoglossal nucleus we were able to identify labeled neurons within Rexed's laminae V-VIII and the central cervical nucleus. Anterograde labeling in the main cuneate nucleus was observed after C1 to C5 ganglion and C1 to C6 cord injections. The pattern and extent of labeling in the perihypoglossal nuclei and adjacent structures seen after cerebellar injections into lobules V and VI were comparable to those previously reported and permitted evaluation of the relay from dorsal root ganglia through the intercalatus to the vermis. Topography of the cervical projections to the nucleus intercalatus is considered with respect to that of the perihypoglossal-collicular projection. A discussion is offered of the apparent importance of nucleus intercalatus as a relay of cervical and vestibular afferent information to premotor structures involved in neck motor control. The perihypoglossal complex is viewed as being organized in such a fashion as to allow the nuclei intercalatus and prepositus hypoglossi to function as key structures in the integration of inputs related to neck and ocular motor control, respectively.