Spinal inhibition of phrenic motoneurones by stimulation of afferents from peripheral muscles.

1. Phrenic nerve responses to stimulation of calf muscle receptors or their afferents were studied in two groups of cats. One consisted of paralysed, vagotomized and functionally glomectomized animals with intact central nervous systems. The other included paralysed high (C1) spinal animals whose phrenic nerve activity was either spontaneously tonic or phasic, or evoked by activation of the intercostal-to-phrenic reflex. In both groups, end-tidal PCO2 was maintained at a constant level by means of a servo-controller. 2. Physical stimulation of calf muscles in animals with intact central respiratory controller and a generally facilitatory effect on frequency, with appropriate changes of both inspiratory and expiratory durations, and on peak magnitude of phrenic (neural tidal) activity. However, for the first few sec after onset of the stimulus, neural tidal activity was inhibited. 3. Physical stimulation of calf muscles or electrical stimulation of the tibial nerve in high spinal animals uniformly caused inhibition of spontaneous phrenic activity and that evoked by facilitatory conditioning stimuli. The degree of inhibition gradually decreased as muscle stimulation continued. Following offset of muscle stimulation, post-stimulus augmentation of phrenic activity occurred, with subsequent gradual return to control level over a period of 20-25 sec. 4. We conclude that stimulation of muscle afferents in the leg has a predominantly facilitatory respiratory effect when acting through brain stem controller mechanisms, but also has a purely inhibitory effect on phrenic motoneurones when acting via spinal mechanisms. 5. In addition, the findings are consistent with (1) progressive accommodation of phrenic motoneurones during continued inhibitory input, and (2) with a large and prolonged post-inhibitory rebound of excitability.