Quantitative assessment of the effect of chronic phrenicotomy on the induction of the crossed phrenic phenomenon.

The present study was carried out to determine if chronic peripheral phrenicotomy has a functional influence on the plasticity that is normally demonstrated by phrenic motoneurons in the spinal cord following spinal cord injury. Young adult female Sprague-Dawley rats were divided into an experimental and a control group. Left intrathoracic phrenicotomies were carried out at 1, 2, 3, and 4 weeks prior to induction of the crossed phrenic phenomenon and crossed phrenic nerve activity recording in the experimental group. Control animals were not subjected to chronic phrenicotomy. In each animal the crossed phrenic phenomenon was induced by left C2 spinal cord hemisection and turning off the ventilator. The reflex-induced activity in the phrenic nerve ipsilateral to hemisection is defined as "crossed phrenic nerve activity." All animals were subjected to spinal cord hemisection 24 h before crossed phrenic nerve activity recording. The results showed that there is a transient but statistically significant depression of crossed phrenic nerve activity at 2 weeks postphrenicotomy and a recovery to the normal activity level at 4 weeks postphrenicotomy. One control experiment was carried out to assess the effects of phrenicotomy on respiratory activity that is normally present in the phrenic nerve (i.e., not reflex-induced). This "primary respiratory nerve activity" is different from crossed phrenic nerve activity in that the phrenic motoneurons are driven by different bulbospinal respiratory pathways. The results indicated a marked decrease in primary respiratory nerve activity at 1 week after phrenicotomy with no significant recovery by the 4th week after phrenicotomy.(ABSTRACT TRUNCATED AT 250 WORDS)