Time course of intercostal afferent termination of the inspiratory process

The influence exerted by several somatic nerves on the inspiratory off-switch mechanism has been assessed in decerebrate cats. These animals were paralyzed, artificially ventilated, and bilaterally vagotomized. Inspiratory activity was monitored by a phrenic neurogram. Brief stimulation of either the superficial radical nerve or the sciatic nerve had an inconsistent effect on both the depth of inspiration and the timing of the respiratory cycle. However, stimulation of the T6 intercostal nerve during inspiration elicited a premature phase switch to expiration. Distinct, repeatable thresholds were determined for 10 delays in 100 msec increments after the onset of inspiration. As the delay increased, the threshold current was observed to decrease in all 30 decerebrate cats studied. An increase in the end-expiratory %CO2 caused an elevation of the stimulus threshold. These results correspond to the known characteristics of the inspiratory off-switch. Also, since the intercostal afferents are not normally a major determinant of respiratory rhythmicity in eupnea, this work establishes intercostal nerve stimulation as a very useful technique in the study of inspiratory to expiratory phase switching mechanisms.