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

Abstract

Phrenic nerve responses to stimulation of calf muscle receptors or their afferents were studied in 3 groups of cats. One consisted of paralyzed, vagotomized and functionally glomectomized animals with intact CNS. The other included paralyzed high (C1) spinal animals whose phrenic nerve activity was spontaneously tonic or phasic, or evoked by activation of the intercostal-to-phrenic reflex. In both groups, end-tidal PCO2 [CO2 partial pressure] was maintained at a constant level by means of a servo-controller. Physical stimulation of calf muscles in animals with intact central respiratory controller had a generally facilitatory effect on frequency, with appropriate changes of inspiratory and expiratory durations and on peak magnitude of phrenic (neural tidal) activity. For the 1st few s after onset of the stimulus, neural tidal activity was inhibited. Physical stimulation of calf muscles or electrical stimulation of the tibial nerve in high spinal animals uniformly caused inhibition of spontaneous phrenic activity and activity 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 s. Stimulation of muscle afferents in the leg apparently has a predominantly facilitatory respiratory effect when acting through brain stem controller mechanisms, but has a purely inhibitory effect on phrenic motoneurons when acting via spinal mechanisms. Progressive accommodation of phrenic motoneurons during continued inhibitory input, and a large and prolonged post-inhibitory rebound of excitability apparently occur.