Neuroplasticity of primary afferents in the neo-natal cat and some results of early deafferentation of the trigeminal spinal nucleus.

The plasticity of cervical primary afferents has been investigated in young adult cats in which the trigeminal root, together with most of the ganglion, had been excised via a new approach to these structures during the first week after birth. Once degeneration debris had disappeared, bilateral ganglionectomies of the upper three cervical dorsal roots were done in five animals and the IXth and Xth roots transected on the side of the chronic trigeminal denervation in one instance; the degeneration pattern on the chronically denervated side was compared to that on the normal side. There was mild evidence of increased degeneration of the cervical afferents in the C1 segment and in the medulla (subnucleus interpolaris of V) on the chronically denervated side. The proliferation was more apparent in the kitten operated at three days of age and was only vestigial in animals operated at six or seven days; it had no appreciable tendency to extend into the large contiguous pool of denervated trigeminal neurons. There was no evidence of sprouting of the IXth and Xth nerve afferents. These observations indicate that primary afferents of the upper cervical roots and of the glossopharyngeal and vagus nerves have little neuroplastic potential even at early stages. The literature on neuroplasticity is controversial and a brief review of this together with suggestions regarding the reasons for some of these conflicts is presented. Some structural and functional effects of chronic trigeminal denervation are briefly described.