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University of Oklahoma Health Sciences Center, Department of Physiology and Biophysics, Oklahoma City 73190.

1. Neurones in the spinal cord were tested for responses to premature ventricular contractions (PVCs), produced by controlled electrical extra stimuli, and other mechanical stimuli applied to the heart. Thirty-eight neurones were antidromically activated from the medial medullary reticular formation and/or the caudal thalamus, and twenty-four neurones did not project to these sites. 2. Only those neurones excited by electrical stimulation of the left stellate ganglion were tested for responses to PVCs. A total of twenty neurones (32%) responded to electrically induced PVCs. Three major patterns of responses occurred. Three neurones exhibited an early burst and a late burst (or bursts) during the arrhythmia, one neurone fired only an early burst, and sixteen neurones responded with only a late burst. The early bursts occurred shortly after the onset of the compensatory pause accompanying the PVC; the late bursts were usually associated with the subsequent potentiated contraction, although the stimulus eliciting the burst must often have occurred late in the compensatory pause. 3. Responses to PVCs were only seen in neurones receiving C fibre and A delta fibre input. However, there were some neurones with both A delta and C input that did not respond to PVCs. No neurones with only A delta input responded to PVCs. 4. Neurones projecting to thalamus were less likely to respond to PVCs than either spinoreticular neurones or neurones with unidentified projections. 5. Neurones responsive to PVCs were likely to exhibit a cardiac rhythmicity in their spontaneous or evoked activity. 6. A total of 42% of tested neurones responded to a rapid infusion of saline into the heart, 52% had a cardiac receptive field, and 74% responded to aortic occlusion. A given neurone might respond to one or more of these stimuli, without responding to every mechanical stimulus tested. 7. Cervical vagotomy never abolished a response to PVCs, although either the spontaneous discharge rate or magnitude of response was sometimes altered. 8. Neurones responsive to PVCs were also responsive to intracardiac bradykinin. In addition, 95% of the neurones received convergent somatic input. 9. We conclude that about a third of spinal neurones excited by electrical stimulation of the left stellate ganglion receive information regarding mechanical, presumably innocuous, events in the heart. Most responsive neurones also receive somatic input and noxious cardiac input, and this information is transmitted to the thalamus, reticular formation, and probably to other spinal segments.

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