Two different inhibitory effects on respiration by thin-fiber muscular afferents in cats

In anesthetized, vagotomized, paralyzed, and artificially ventilated cats, respiratory responses to electrical stimulation of the gastrocnemius muscle nerve were studied by recording the phrenic nervous discharges. Besides intensity-dependent facilitation of respiration, electrical stimulation of the thin-fiber muscular afferents caused two different types of respiratory suppression, during and after the stimulation. The first suppressive phase ('initial suppression') was observed immediately after the start of stimulation above A-delta fiber threshold, and usually accompanied by a fall of blood pressure. The other suppressive phase ('after suppression') was observed within several minutes after the cessation of the stimulation. The intensity of stimulation required to evoke the 'after suppression' was much higher than that for the 'initial suppression', and was usually in the C fiber stimulation range. Naloxone did not affect the 'initial suppression', but abolished the 'after suppression'.