Effect of graded cooling of intermediate areas on respiratory response to vagal input

The present study was undertaken to determine if the phrenic responses to vagally mediated inputs are also affected by focal cooling of the intermediate areas (IA) of the ventral medulla. Anesthetized, paralyzed cats whose vagi and carotid sinus nerves had been cut were studied. The IA were cooled focally with a thermostatically controlled thermode. When the IA were 40 degrees C, low intensity vagal stimulation caused inhibition of phrenic activity. The stimulus was also applied when IA were cooled to 30 and 20 degrees C. The magnitude of the inhibition was unaffected by the cooling. In another series of experiments, high intensity vagal stimulation was used. This led to an hyperpnea when IA were 40 degrees C. The magnitude of the response was much smaller when the test stimulus was given at lower IA temperatures. The effect of cooling IA on the phrenic response to mechanical stimulation of pulmonary stretch receptors and airway irritant receptors were also studied in cats with intact vagi. We found that the response to irritant receptor, but not to stretch receptor, stimulation was abolished by the cooling. We conclude that the intermediate areas are involved in the integration of afferent input from airway irritant receptors that reaches the respiratory controller via high threshold vagal afferents, but not involved in processing signals from pulmonary stretch receptors.