1. Inspiratory output responses, measured as integrated phrenic activity, to hypercapnia, to carotid sinus nerve stimulation, to unilateral and bilateral stimulation of calf muscles and to combinations of these stimuli were determined in paralysed, vagotomized and glomectomized cats whose end-tidal PCO2 was kept constant by means of a servo-controlled ventilator. 2. Confirming an earlier report (Eldridge, Gill-Kumar & Millhorn, 1981), the inspiratory response to progressive hypercapnic stimulation of the central chemoreceptors was not linear, and the responses to a constant carotid sinus nerve test stimulus were progressively decreased in magnitude as the pre-stimulus level of respiratory activity was increased by hypercapnia. 3. In contrast, the response to a test stimulus from calf muscles remained the same at all pre-stimulus levels of respiratory activity, whether conditioned by hypercapnia or by carotid sinus nerve stimulation. 4. Unlike the findings with chemoreceptor inputs, the combining of stimuli from right and left muscles exhibited an algebraically additive effect on output. 5. We suggested before that the decreasing responses to identical chemoreceptor inputs were due to progressive neuronal saturation of a common central pathway between these inputs and the respiratory controller. The absence of such behaviour with muscular afferent input indicates that this input does not travel to the respiratory controller by the same pathway as that common to the chemoreceptors.
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