The sequence of events during reinnervation of the cardiac ganglion in the frog following interruption of the vagosympathetic nerve supply was studied with both electrophysiological and morphological techniques. When cardiac ganglia were denervated by crushing the vagosympathetic nerve supply to the heart all synaptic endings on parasympathetic ganglion cells degenerated. Vacated post-synaptic densities were detected on denervated neurones for periods of at least 7 weeks. The earliest signs of reinnervation were subthreshold responses evoked by stimulating the regenerating vagosympathetic trunks 2 1/2 -3 weeks after crushing the cardiac branches of the vagus nerves. Analysis of the reversal potentials of these responses indicated that these synapses were distant from the cell body. At slightly longer times (4-5 weeks), regenerating synapses could be recognized on post-ganglionic axons; no synapses were detected on the neuronal perikarya at these times. By 6-7 weeks following denervation, vagal synapses reinnervated neuronal perikarya as well as post-ganglionic axons. At the same time, vacated post-synaptic densities declined in number. Furthermore, vagal stimulation at this stage evoked large, suprathreshold post-synaptic potentials. These studies indicate that post-ganglionic axons are the initial sites for reinnervation of parasympathetic neurones in the heart. Only some time later are neuronal perikarya reinnervated and ganglionic transmission completely restored.
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