Competitive and non‐competitive re‐innervation of mammalian sympathetic neurones by native and foreign fibres.

Abstract

The ability of native (sympathetic preganglionic) and foreign (vagal) nerve fibers to reinnervate neurons of the guinea-pig superior cervical ganglion, either alone or in competition with each other, was studied by means of intracellular recording and EM. Native fibers make synaptic contacts with nearly all ganglion cells within 1 mo. of cervical trunk section; within 6 mo. the degree of innervation, judged by measurement of excitatory post-synaptic potential (epsp) amplitude and EM synapse counts, approaches normal. Even after 15 mo. innervation was weaker than in normal control ganglia. Vagal fibers are less successful during reinnervation. Although a similar number of foreign fibers grow into denervated ganglia and make contact with nearly all ganglion cells within a month, after 6-12 mo. epsp amplitudes in response to foreign nerve stimulation remain relatively small, and counts of synapses are only about 60% as great as in ganglia reinnervated with the native nerve. When both native and foreign fibers are allowed to reinnervate ganglion cells simultaneously, about half the neurons in the ganglion receive synapses from both sources after 1 mo. The proportion of dually innervated cells remains roughly constant for at least 14 mo. Neither set of preganglionic fibers dominates or displaces the other, although neurons generally are reinnervated more effectively by native than foreign fibers,as is true during non-competitive reinnervation. During reinnervation of mammalian sympathetic neurons native fibers are preferred to foreign ones only in the sense that roughly the same number of native fibers form many more synapses on ganglion cells than do vagal axons. A foreign synapse, once formed, is as stable as a native one, and shows no tendency to be replaced by native terminals. These findings are discussed in relation to other evidence which has suggested specificity and selectivity during reinnervation of mammalian autonomic neurons.