Intersegmental connections and interactions of myelinated somatic and visceral afferents with sympathetic preganglionic neurons in the unanesthetized spinal cat.

The aim of this study was to obtain a measure of the interactions through exclusively spinal circuits, of myelinated afferents with sympathetic preganglionic neurons. Experiments were performed on 16 unanesthetized cats rendered insensitive by bilateral vertebral and carotid occlusion, whose spinal cords had been transected at C1 6-12 h before recording. The evoked responses of 68 tonically active sympathetic preganglionic neurons were recorded from filaments dissected from the cervical sympathetic trunk. Excitation, inhibition and excitation-inhibition sequences were evoked by electrical stimulation of radial, femoral and pelvic nerve afferents. Inhibition was most often observed during pelvic nerve stimulation. Ninety percent of the sympathetic preganglionic neurons tested responded to radial, 77% to femoral and 85% to pelvic nerve stimulation. These differences in percentage of units responding to the three nerves were statistically insignificant. Thus, in the acute spinal cat, the fraction of tonically active sympathetic preganglionic neurons whose activity can be influenced by myelinated afferents is independent of the length of the intraspinal pathway which conveys the input. A main difference between the long pathway (mediating the responses to femoral or pelvic nerve) and the short pathway (mediating the responses to radial nerve) seems to be the efficacy of their connections. While single shocks reliably evoked responses from the radial nerve, trains (200 Hz, 20 msec) were usually necessary to elicit responses from femoral or pelvic nerve, indicating a requirement for summation in the long pathway.