The influence of normal innervation upon abnormal synaptic connections between frog parasympathetic neurons

Abstract

Synaptic connections form between parasympathetic neurons in the frog cardiac ganglion when their normal preganglionic innervation is removed. The synapses between the ganglion cells are inappropriate in the sense that they are absent in normally innervated ganglia. These novel synapses form slowly; they are readily detected by intracellular recording only after 1–2 months of preganglionic denervation. After 3 months of extrinsic denervation two-thirds of the ganglion cells receive physiologically detectable postganglionic inputs, yet the efficacy of these synapses remains substantially less than that of native, preganglionic synapses. This reduction in efficacy is caused at least in part by the location of the postganglionic synapses; electron microscopy reveals them to be situated on the axons of ganglion cells, whereas preganglionic axons terminate primarilly on the cell bodies. The intrinsic synapses that form in the absence of extrinsic innervation are no longer detectable physiologically after ganglion cells are reinnervated by preganglionic axons. This suppression of connections between ganglion cells is nearly complete (90%) and is a consequence of competition with preganglionic axons: it does not occur in the absence of preganglionic reinnervation. The data indicate that the presence of preganglionic inputs to the ganglion cells discourages the formation and maintenance of synaptic connections between them.