CHARACTERISTICS OF SYMPATHETIC PREGANGLIONIC NEURONS IN THE LUMBAR SPINAL-CORD OF THE CAT

1. In anaesthetized cats extracellular recordings have been made from antidromically identified sympathetic preganglionic neurones, located in the 2nd and 3rd lumbar segments, with axons projecting into the left lumabar sympathetic chain beyond the L4 ganglion. Sympathetic preganglionic neurones have been characterized with respect to: axonal conduction velocities, firing patterns in relation to ECG and phrenic nerve activity, responses to noxious stimuli applied to the ipsilateral hindlimb and ionophoretically applied 5-HT. 2. Two hundred and ninety-seven sympathetic preganglionic neurones were studied. Their axonal conduction velocties (0.5-13.9 m/s) were in the B and C fibre range. Sixty-eight had on-going activity and the remainder were quiescent. Of the 229 quiescent sympathetic preganglionic neurones, 111 were activated by the ionophoretic application of glutamate. 3. Of the 100 sympathetic preganglionic neurones analysed for an ECG-related pattern of discharge, forty-nine had no, and fifty-one had an ECG-related pattern of discharge. Both sympathetic preganglionic neurones with on-going activity and glutamate activated cells exhibited ECG-related patterns of discharge. 4. Only six of fifty sympathetic preganglionic neurones had a respiratory-related activity pattern. Three had maximal discharge during expiration and three during inspiration. 5. Forty-one sympathetic preganglionic neurones were examined for their response to noxious stimulationof the ipsilateral hindlimb. Ten had their activity decreased (seven glutamate-activated, three with on-going activity), seven had their activity increased (four glutamate-activated and three with on-going activity) and twenty-four were unaffected. These results demonstrate that both sympathetic preganglionic neurones with on-going activity and glutamate-activated neurones can be influenced by noxious input. Ten sympathetic preganglionic neurones had properties consistent with them having a skin vasoconstrictor function and three with muscle vasoconstrictor function. 6. Ionophoretic application of 5-HT in the vicinity of fifty-one sympathetic preganglionic neurones caused increases in the discharge of 53%, decrease in the firing of 12% and did not affect the discharge of 35%. 7. Sympathetic preganglionic neurones which had excitatory responses to 5-HT showed only decreased discharge or no response to noxious stimulation of the ipsilateral hindlimb. Conversely, sympathetic preganglionic neurones which had discharge decreased by 5-HT had primarily excitatory responses to noxious inputs. 8. It is concluded that lumbar sympathetic preganglionic neurones consist of a heterogeneous population with respect to their physiological properties and their responses to ionophoretically applied 5-HT: both may be related to function.