Studies conducted during the past year were concerned primarily with the effects of neonatal nerve damage upon the peripheral projections of axotomized and undamaged ganglion cells. An initial series of electrophysiological experiments demonstrated that neonatal transection of the infraorbital nerve resulted in dramatic changes in the distribution of response properties of the ganglion cell that contributed axons to this trigeminal branch. There was a marked increase in the number of cells that responded only to noxious stimuli, and many neurons developed split or usually large receptive fields. The existence of ganglion cells with split fields suggested the possibility that individual primary afferent neurons in neonatally nerve damaged rats may send collaterals into two or more trigeminal branches. We tested that possibility in series double labelling experiments which did, in fact, demonstrate that this was the case. In this study, we also used sequential double labelling techniques to show that both collateral sprouting by undamaged ganglion cells and the development of multiple peripheral collaterals by neonatally axotomized primary afferents contributed to this reorganization. We examined the issue of reorganization within the regenerate infraorbital nerve using both retrograde tracing and electron microscopic techniques. Neonatal infraorbital nerve damage disrupted the normal organization of the ganglion cells that contributed axons to the different vibrissae follicles and it also substantially reduced the number of myelinated axons in vibrissal nerves.
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