Central respiratory effects of carbon dioxide, and carotid sinus nerve and muscle afferents

1. In anaesthetized and paralysed cats, artificially ventilated and with the vagi and carotid sinus nerves cut, the effects of (a) changing end-tidal CO2, (b) stimulation of carotid sinus nerves, and (c) squeezing the calf muscles, on neural respiratory output have been studied. 2. The peak value of integrated phrenic activity, which is the neural equivalent of tidal volume (VTn), the rate of rise of integrated phrenic activity (Slope), inspiratory duration (TI), expiratory duration (TE), and total cycle length (Ttot) were measured. It was found that Slope-VTn, VTn-Ttot and VTn-TE relationships were similar whether respiration was stimulated by carotid sinus nerve stimulation or by increasing end-tidal CO2. However, when respiration was stimulated by squeezing the calf muscles, a given Slope was associated with a smaller VTn and a given VTn was associated with smaller Ttot and TE values than obtained with the other two inputs. 3. It is concluded that these results are compatible with the hypotheses that (1) inputs from central and peripheral chemoreceptors have similar actions at various sites in the respiratory centre complex, and (2) afferents activated by squeezing the limb muscles have additional facilitatory actions on mechanisms terminating inspiration and expiration.