Reflex respiratory responses to continuous and intermittent stimulation of afferent fibres from muscles in cats

In anaesthetized cats, reflex respiratory responses were evoked by electrically stimulating proximal ends of cut nerves to gastrocnemius-soleus muscles of the left hindlimb. The direction of these responses, in the excitatory or inhibitory sense, depended on the types of afferent fibres stimulated and on the mode of stimulation. Continuous stimulation, delivered with stimuli of 0.1 ms at 200 or 300 Hz, in all cases induced excitatory responses, consisting of increases in pulmonary ventilation, mainly caused by increases in tidal volume. Intermittent stimulation, which consisted of brief trains of stimuli repeated at 2-3 Hz, induced excitatory responses when afferent fibres with conduction velocities higher than 20 m/s were stimulated, while it induced inhibitory responses, consisting of decreases in pulmonary ventilation, tidal volume and respiratory frequency, when fibres with conduction velocities below 20 m/s were simultaneously stimulated with the high-conduction velocity fibres.