Organization of the tonically active pathways through the superior cervical ganglion of the rabbit.

Responses of the neurons of the rabbit superior cervical ganglion to stimulation of the fascicles of preganglionic nerve fibres dissected from the cervical sympathetic trunk, and tonic activity of these neurons were recorded with intracellular electrodes in vitro and in vivo, respectively. It was found that on average 8.5 +/- 1.2 preganglionic fibres converge on each ganglion cell. In each preparation a single input spike could be evoked in a ganglion cell by stimulation of only one of all tested fascicles. The interspike intervals for single and multiple preganglionic inputs in tonic activity of ganglion neurons displayed Gaussian and Poisson distributions, respectively. The frequencies of spikes showing a Gaussian distribution correspond in most cases to pulse and respiratory frequencies. These data suggest that (1) single input is formed by a single preganglionic fibre which evokes EPSP of an amplitude high enough to trigger a postsynaptic spike, and that (2) regular activity of the ganglion neuron triggered by single input is probably due to a regular afferent drive. A cross-correlation analysis between the intracellular tonic activity and the tonic activity simultaneously recorded from the postganglionic nerve was made. The results of the analysis were compared with those obtained when intracellular spikes were evoked by stimulation of one particular neuron. From this comparison it has been concluded that about 100 ganglion cells, on the average, discharge synchronously within the 20-ms range during their tonic activity.