The interaction between regenerating and sprouted nerve terminals during reinnervation of neurones were tested in the parasympathetic cardiac ganglion in frogs. After partial (unilateral) vagotomy, remaining intact preganglionic vagal axons rapidly sprouted and innervated the entire ganglion. At later intervals after nerve damage, regenerating vagal axons were able to reinnervate ganglion cells despite the presence of synapses from sprouted nerve terminals. When vagal reinnervation took place after unilateral vagotomy, synaptic input from the sprouted vagus nerve declined. The presence of synapses from intact and sprouted nerve terminals in the ganglion after partial denervation measurably delayed the rate of vagal reinnervation. After complete denervation (bilateral vagotomy), ganglionic reinnervation was rapid and complete. However, cells initially received an excessive number of preganglionic inputs and an abnormal distribution of left/right vagal innervation in the ganglion. At long intervals (up to 85 weeks) after ganglionic reinnervation, some reduction of excess vagal inputs took place, indicating there was a slow re-organization of ganglionic synapses. The number of boutons per cell body as revealed by zinc iodide-osmium staining remained constant after vagal reinnervation, despite an initial excessive synaptic reinnervation and subsequent synaptic remodelling.
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