What is the Ongoing Activity of Sympathetic Neurons?

The patterns of tonic activity were recorded with intracellular electrodes from the neurons of rabbit superior cervical ganglion whose preganglionic inputs remained intact, and were compared with the responses of the same neurons to single stimuli applied to preganglionic fibers. It was found found that each neuron received two preganglionic inputs, dominant and accessory. Dominant input is formed by one preganglionic fiber with a potent synaptic action on the neuron evoking spikes with high safety factor. Accessory input is formed by two or more converging preganglionic fibers that can evoke spikes only through summation of their EPSPs. It is suggested that preganglionic fibers evoking accessory spikes in each particular neuron are discharged by common presynaptic source, and that dominant spikes, accessory spikes and ineffective EPSPs in tonic activity of the ganglion neuron are each produced by separate central generators.