Input‐output relationships of central neural circuits involved in respiration in cats

Abstract

Inspiratory output responses, measured as integrated phrenic activity, to hypercapnia, unilateral and bilateral carotid sinus nerve stimulation and combinations of these stimuli were determined in paralyzed, vagotomized and glomectomized cats whose end-tidal PCO2 [partial pressure CO2] was kept constant by a servocontrolled ventilator. The effect of the mechanism causing the respiratory after-discharge on these responses was determined. Above the threshold for rhythmic activity, the inspiratory response to hypercapnic stimulation of the central chemoreceptor was curvilinear, showing progressively smaller increments of output for equal increments of PCO2 as the latter became higher. The combining of stimuli from right and left carotid sinus nerves failed to show an algebraic additive effect; the response was about 70% of that predicted from a summing of the separate stimuli given alone. The response to a constant carotid sinus nerve test stimulus was progressively decreased in magnitude as the pre-stimulus level of respiratory activity was increased by hypercapnia conditioning stimulation of the central chemoreceptors, by stimulation of the opposite carotid sinus nerve or by the mechanism generating an after-discharge. There was a negative or hypoadditive interaction between the peripheral and central inputs at the level of the central respiratory controller. The response was due to the properties of a neural component of the central pathway and not a specific interaction between peripheral and control inputs. This component was common to both inputs and developed progressive saturation of its neural elements as its activity increased. The neural mechanism generating a respiratory after-discharge appeared to saturate completely at a lower level of inspiratory activity than the level at which the common pathway develops complete saturation. This mechanism apparently represents an independent input to the respiratory controller.