Afferent projections of the rat major occipital nerve studied by transganglionic transport of HRP.

The central projection fields of cutaneous neurons of the rat's major occipital nerve have been investigated using the method of transganglionic transport of horseradish peroxidase (HRP), with tetramethylbenzidine according to Mesulam (1978) as the chromogen. Furthermore, the course of the nerve, diameter distribution of myelinated axons, and diameter distribution of HRP-labeled perikarya of spinal ganglion cells belonging to this nerve, diameter distribution of myelinated axons, and diameter distribution of HRP-labeled perikarya of spinal ganglion cells belonging to this nerve have been studied. Following HRP application to the proximal stump of the cut nerve, labeled structures were found ipsilaterally in the cervical spinal cord and in the medulla oblongata. In the spinal cord, reaction product was mainly concentrated in the lateral parts of laminae I-III of the dorsal horn in segments C2 and C3. In C1, primary afferent terminals were more sparsely distributed and restricted to laminae I and II. Reaction product was also seen in the tract of Lissauer in segments C1-C4. In the medulla oblongata HRP labeled structures were observed in the medial cuneate nucleus, in the rostral part of the external cuneate nucleus, and in the nucleus of the spinal tract of the trigeminal nerve. A possible somatotopic arrangement of central terminals of cutaneous neurons within the cervical dorsal horn, as well as differences between the projection fields of muscle and skin afferents within the upper cervical cord and caudal medulla are discussed.