Reinnervation of a striated muscle by vagal sensory axons

Fibres of the sterno-cleido-mastoid (s.c.m.) muscle normally innervated by effects of the accessory nerve have been reinnervated by afferent fibres of the vagus nerve after supranodose vagal-accessory nerve anastomoses or direct implantation of the vagus nerve into the s.c.m. in 58% of the rabbits, 60% of the cats and 75% of sheep in which experiments were performed. Afferents of the vagus growing from cell bodies of the nodose ganglion after severance of central connections can replace the efferent of motor supply to the muscle. Evidence that there was reinnervation of the s.c.m. muscle by vagal afferent fibres was provided from the observations that: (i) electrical stimulations of the anastomosed cervical vagus nerve elicited potentials in the s.c.m. muscle which were abolished by local anaesthesia or final section of the nerve proximal to the site of stimulation; (ii) discharges recorded as bursts of electromyographic potentials occurred during spontaneous movements of larynx, respiratory tract, oesophagus and stomach and on their mechanical or evoked stimulation; and (iii) horseradish peroxidase injected into the reinnervated s.c.m. muscle was detected in somata of ipsilateral nodose ganglia cells. The afferent fibres contributing to the reinnervation were confirmed to be cholinergic as transmission was blocked by gallamine and histochemical evidence obtained of cholinergic motor end-plates. Factors which may have limited the small extent of reinnervation--only one vagal sensory axon out of 600 is able to form functional connections--are discussed.