Neuronal mechanisms responsible for ongoing activity of rabbit superior cervical ganglion neurons

Abstract

The action potentials evoked by stimulation of single preganglionic nerve fibres or of single neurons of the ganglion were recorded from postganglionic nerves of rabbit superior cervical ganglion. The averaging technique was used to improve the nerve signal-to-noise ratio. It was found by dividing the area by the area that single preganglionic nerve fibre discharges, as an average, 15 neurons of the ganglion. Action potentials were also recorded from preganglionic nerves, either appearing synchronously with the intracellular spikes recorded during ongoing activity of the ganglion neuron, or being evoked by stimulation of a single preganglionic nerve fibre. It was found by dividing the area by the area that each ongoing spike in the ganglion neuron is preceded by firing, as an average, of three preganglionic nerve fibres. Thus, 45 ganglion neurons are expected to be discharged by a preganglionic volley during their ongoing activity. This number is much lower than that found in our previous experiments (about 100 neurons). The difference suggests that in the majority of the ganglion neurons the ongoing discharges need summation of excitatory effects produced by a few converging preganglionic nerve fibres (multiple input) rather than being evoked by a single preganglionic nerve fibre (single input). Methacine, a selective muscarinic antagonist (0.3–0.5 mg/kg, i.v.), decreased by about 10% the rate of the multiple input-induced ongoing spikes while not affecting the single input-induced ongoing spikes in the ganglion neurons. This effect is probably due to the increase in the duration of after-hyperpolarization observed after the application of methacine. The results obtained suggest that muscarinic receptors of the ganglion neurons are permanently activated during the ongoing activity maintaining the rate of firing at a somewhat increased level.