Sprouting and regeneration of synaptic terminals in the frog cardiac ganglion

Synaptic connections are capable of a remarkable proliferation after nervous tissue has been damaged and some synapses have been destroyed. This proliferation, or 'sprouting', of remaining, intact, synapses within partially denervated tissue has been found to occur in the peripheral, autonomic and central nervous systems. As yet, however, the functional ability of these sprouted synaptic contacts has not been extensively tested and the persistence of the sprouted synapses over long periods has not been studied. The authors have been investigating these problems in the parasympathetic cardiac ganglion in the frog. Earlier studies have shown that when the nerve supply to the ganglion is partially destroyed by crushing one vagus nerve, axons in the other vagus nerve sprout and synapse with the denervated neurones. Here the author describes experiments in which the persistence of the sprouted synapses in the cardiac ganglion was tested by allowing the damaged axons to regenerate into the tissue. The results indicate that regenerating vagal fibres are able to re establish some, but not all, of their synaptic connections and that innervation from the sprouted nerve decreases.