Effects of usage of a dormant respiratory nerve pathway upon its subsequent activity

Abstract

The crossed phrenic pathway, usually dormant, was provoked into activity for 4 to 12 hours to ascertain whether it might continue functioning after the initial stimulus was removed. Spinal hemisections above the level of the phrenic nucleus were performed. Then after several days or weeks of convalescence, conduction in the phrenic nerve on the side opposite the hemisection was temporarily blocked. During this period of nerve block, the crossed phrenic pathway was clearly activated in thirteen out of fifteen dogs as judged by the enhancement of electrical activity from the hemidiaphragm previously paralyzed by the hemisection. This crossed activity persisted in twelve of the thirteen animals, albeit with diminished force, while the electrical activity of the hemidiaphragm on the unhemisected side indicated that the nerve block had worn off. The perseveration of activity exceeded the duration of nerve block by 1 or 2 days in the majority of experiments but in two instances the activity was clearly evident later at 13 and 31 days, respectively. A spontaneous return of function of the hemiplegic diaphragm was not observed in a control group under observation by electromyographic technique for a minimum of 2 months. Evidence suggested that the prolonged arousal of the crossed phrenic pathway had some effect on the irritability of the phrenic nucleus or medullary respiratory center, or possibly an effect on both sites.