Effect of graded vagal blockade and pulmonary volume on tonic inspiratory activity in rabbits

Reflex respiratory responses to increased and decreased tracheal pressure have been studied at bilateral local vagus temperatures between 30 degrees C and 0 degrees C in anaesthetized spontaneously breathing rabbits. The temperature of each cervical vagus nerve was governed by separate thermodes, which were cooled by Peltier elements and heated by transistors. In this study, the lowest activity level of the diaphragm during expiration was considered to reflect tonic inspiratory activity. As an indicator for this level, the longest interspike interval in the electromyogram of the diaphragm during a breath was used. Tonic inspiratory activity at tracheal pressures below -1.2 kPa was higher with vagi intact than after vagotomy. This observation indicates that disinhibition, due to the decreased activity of pulmonary stretch receptors, occurring during the Hering-Breuer deflation reflex, is an insufficient explanation for the facilitation of tonic inspiratory activity. When local vagus temperatures are 8 degrees C, and tracheal pressure has been increased or decreased, tonic inspiratory activity is higher than it would be with vagi at 0 degrees C or cut. Two reflex mechanisms that might explain the observed high tonic inspiratory activity are discussed.