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

The ability of native (sympathetic preganglionic) and foreign (vagal) nerve fibres to re-innervate neurones of the guinea-pig superior cervical ganglion, either alone or in competition with each other, has been studied by means of intracellular recording and electron microscopy. 1. Native fibres make synaptic contacts with nearly all ganglion cells within one month of cervical trunk section; within 6 months the degree of innervation, judged by measurement of excitatory post-synaptic potential (e.p.s.p.) amplitude and electron microscopical synapse counts, approaches normal. However, even after 15 months innervation was weaker than in normal control ganglia. 2. Vagal fibres are less successful during re-innervation. Although a similar number of foreign fibres grown into denervated ganglia and make contact with nearly all ganglion cells within a month, after 6-12 months e.p.s.p. amplitudes in response to foreign nerve stimulation remain relatively small, and counts of synapses are only about 60% as great as in ganglia re-innervated with the native nerve. 3. When both native and foreign fibres are allowed to re-innervate ganglion cells simultaneously, about half the neurones in the ganglion receive synapses from both sources after 1 month. The proportion of dually invervated cells remains roughly constant for at least 14 months. Neither set of preganglionic fibres dominates or displaces the other, although neurones generally are re-innervated more effectively by native than foreign fibres, as is true during non-competitive re-innervation. 4. Thus during re-innervation of mammalian sympathetic neurones native fibres are preferred to foreign ones only in the sense that roughly the same number of native fibres 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 re-innervation of mammalian autonomic neurones.