Central projection of nuchal group I muscle afferent fibers of the cat.

To investigate the central projection of nuchal group I afferent fibers of nerves which supply the complexus (COM), biventer cervicis (BV), splenius (SP), and occipitoscapularis (OCC) muscles, experiments were conducted on cats anesthetized with chloralose-urethane. A total of 246 neurons was found to respond to electrical stimulation of those nerves. Of these neurons, 99.2% were located in the ipsilateral external cuneate nucleus (ECN) and 0.8% in the ipsilateral main cuneate nucleus. Ninety-seven neurons responded only to the BIV, 88 neurons only to the SP, and 56 neurons to the OCC alone. The number of monosynaptically activated neurons from group I fibers were found to be 26 in 97 neurons which responded to the BIV, 20 in 88 neurons responded to the SP, and 8 in 56 neurons which responded to the OCC. The ECN contains these neurons organized in a somatotopic manner. Neurons from distal muscle (the OCC) project to medial parts and from proximal muscle (the BIV) to more lateral parts of the nucleus. Forty-three of the 143 neurons in the ECN (30.1%) were activated antidromically to electrical stimulation of the ipsilateral inferior peduncle and anterior lobules IV and V of the cerebellum. Thalamic responses from nuchal muscle afferent fibers were recorded in a very narrow region of nucleus ventralis posterolateralis (VPL) which is situated dorsolaterally to the forelimb muscle afferent (deep radial nerve) projection area. The cortical evoked potentials from these nerve stimulations were observed in the anterior suprasylvian gyrus, areas 2 or 5, which we regarded as a transitional area between the second somatosensory and association areas, and the postcruciate dimple (PCD) or area 3a. Cortical potentials in the PCD were reduced after lesion of the VPL, where the focal potentials evoked by nuchal muscle afferent stimulation were recorded.