Central respiratory neurons of the adult rat regrow axons preferentially into peripheral nerve autografts implanted within ventral rather than within dorsal parts of the medulla oblongata.

A great number of severed central nervous system (CNS) neurons of the adult rat have the capillary to regrow axons into peripheral nerve autografts. In the present experiment, autologous segments of the peroneal nerve were inserted perpendicularly to the dorsal surface of the medulla oblongata more or less laterally within either the ventral respiratory group or the so-called dorsal respiratory group (ventrolateral grafts, n = 5; dorsolateral grafts, n = 5). From 2 to 4.5 months after the graft implantation, spontaneous unitary activities (n = 197) were recorded within all the grafted nerves: they were found to arise from both central respiratory (R, n = 60) and non-respiratory (NR, n = 137) neurons which were giving off regenerated axons along the nerve grafts. The graft reinnervation by respiratory axons was found to be significantly more abundant within the medullary ventrolateral grafts than within the dorsolateral ones. The low rate of axonal regeneration from respiratory neurons observed within the dorsolateral grafts provides further evidence that the number of the respiratory neurons in the dorsal respiratory group, if present at all, is much smaller than that of the ventral respiratory group in the rat.