Evidence against a central stimulus for collateral reinnervation of oro-facial tissues in the cat

Experiments in adult cats have shown that when the inferior alveolar nerve is sectioned and regeneration prevented, there is collateral reinnervation from neighbouring ipsilateral and contralateral nerves. Trigeminal rhizotomy prevents nerve impulses from entering the central nervous system and causes central degeneration but leaves the trigeminal ganglion and peripheral nerve intact. This investigation was designed to determine whether these central changes provide the stimulus for collateral reinnervation. In 5 adult cats, under general anaesthesia, a unilateral trigeminal rhizotomy was carried out via a temporal craniotomy. Evidence for the development of collateral innervation was sought by testing, at 3-week intervals, for the return of the jaw opening reflex evoked by electrical stimulation of the lower teeth, skin, mucous membrane and tongue, on the side of the rhizotomy. In addition, 12 or 15 weeks after the rhizotomy, recordings were made from ipsilateral and contralateral branches of the trigeminal nerve to determine whether collateral fibres were present. There was little evidence for the return of reflex responses to stimulation of tissues on the side of the rhizotomy and at the terminal experiment recordings revealed only a few collateral fibres. These results show that trigeminal rhizotomy does not stimulate collateral innervation and so suggest that the stimulus for sensory nerve sprouting is peripheral rather than central.