Respiratory and Circulatory Reflexes in Response to Water Stimuli Applied to the Nasal Cavity in Anesthetized Rabbits

Abstract

This study was aimed to identify the stimulus mechanisms and neural pathway of water-induced respiratory and cardiovascular reflexes originating in the nasal cavity in rabbits with the upper airway isolated, breathing through a tracheostomy or having artificial ventilation. The superior laryngeal nerves (SLNs) were previously sectioned. A conspicuous inhibition of breathing (apnea), bradycardia, a rise in systemic blood pressure and also a vise in left ventricular pressure followed by its fall were elicited by the instillation of distilled water (38°C), but not by saline (38°C), into the nasal cavity. These reflexes could be consistently induced in rabbits with a dennervation of the glossopharyngeal nerves together with the SLNs, indicating that the most adequate stimulus site of these reflexes is the nasal cavity and the most possible neural pathway is the trigeminal nerve. Isotonic solution of glucose (6%, 38°C) failed to elicite these reflexes, suggesting that hypoosmolarity is a main stimulus factor. Since cold saline (12°C) could also cause similar reflexes, but much weaker than that in distilled water-stimulation (38°C), the decrease in temperature is one of factors enhancing the reflexes. The bradycardia was inhibited to some extent (13% decrease in heart rate) by distilled water applied under the artificial ventilation and was able to be induced by transienly stopping of aritificial ventilation. Therefore, some bradycardia was secondarily elicited by the occurrence of apnea. Further experiments with autonomic nerve blockades demonstrated the predominant effects of the parasympathetic mechanism on the heart and the sympathetic mechanism on the peripheral vascular system.