The spinally mediated 10-Hz rhythm in the sympathetic nerve activity of cats

To examine the origin of the so-called '10-Hz rhythm' in the sympathetic nerves, the mass discharges of the white ramus of the third thoracic segment (T3WR) and the inferior cardiac nerve (ICN) and the activities of single postganglionic neurons in the stellate ganglion were recorded in spinal cats. During the chemical or electrical stimulation of the spinal cord, the time of peak of discharges in the sympathetic nerves was analyzed. Both intrathecal administration of N-methyl-D-aspartic acid (NMDA; 3-10 mM) and continuous high frequency (80-200 Hz) electrical stimulation of the dorsolateral funiculus at the second cervical level increased activity of the sympathetic nerves in a similar fashion. In these conditions, modes of the inter-peak interval histograms (IPIH) were about 100 ms (range; 90-130 ms), the inverse of about 10 Hz, but no correlation was observed in autocorrelograms of these peaks of discharges. Therefore, this 100-ms interval activity might have some significance for the 10-Hz rhythm. In order to make this point clear, we stimulated the dorsolateral funiculus with intermittent trains of electrical pulses (0.2-ms duration, 10-35 pulses of 80-200 Hz frequency, in every 300-800 ms). While intermittent trains of pulses were applied, multiple peaks of discharges were evoked in the sympathetic nerves. IPIHs of the nerves were multimodal. The first mode (shortest interval) was about 100 ms. The first mode depended on none of the stimulus parameters but the probability of the about 100-ms interval activity depended on the interval of trains of pulses and the stimulus intensity. With this intermittent stimulation, the autocorrelogram of the peaks revealed the 100-ms interval rhythm. To confirm that the peak of discharges in the ICN was composed of synchronized spikes of postganglionic fibers, single neuronal activities of postganglionic neurons were recorded during the intermittent stimulation. Inter-spike interval histograms showed almost same profile as the IPIHs of the ICN. These results can be explained if the following two assumptions are valid; (i) There are mechanisms that limit minimum firing interval of most preganglionic neurons to about 100 ms. (ii) Simultaneously a interneuron in the spinal cord resets the spike generation of multiple preganglionic neurons. Similarity of the spike activities of the sympatho-excitatory reticulospinal neurons to the intermittent stimulation can explain the 10-Hz rhythm in the peripheral sympathetic nerves in intact spinal cord animals. It is not necessary to postulate the specific 10-Hz rhythm generator in the brain stem for the sympathetic nervous system.