ULTRASTRUCTURAL-CHANGES IN THE RAT PHRENIC NUCLEUS DEVELOPING WITHIN 2-H AFTER CERVICAL SPINAL-CORD HEMISECTION

The present study was conducted to describe the ultrastructural changes which occur in the young adult rat phrenic nucleus within 2 h after an ipsilateral C2 spinal cord hemisection. The main objective was to determine if there is a temporal relationship between specific ultrastructural changes in the phrenic nucleus and a significant augmentation of crossed phrenic nerve activity which occurs as early as 2 h after hemisection. Phrenic motoneurons were identified at electron microscopic levels by retrograde HRP labeling. Ultrastructural features in the phrenic nucleus of control and experimental rats were qualitatively analyzed and then quantitated. At 2 h posthemisection, there was a significant increase in the mean percentage of phrenic dendrodendritic appositions. In the control rats, 4.73 ± 0.18% of phrenic dendrites were in apposition, and this percentage increased significantly to 8.58 ± 0.54% at 2 h after injury. Furthermore, the mean lengths of asymmetrical and symmetrical synaptic active zones increased significantly at 2 h posthemisection from control lengths of 0.372 ± 0.009 μm and 0.404 ± 0.007 μm to 0.410 ± 0.011 μm and 0.513 ± 0.032 μm, respectively, in experimental rats. The phrenic nucleus is therefore capable of morphological plasticity as early as 2 h after spinal cord hemisection and this plasticity coincides temporally with the physiological augmentation of crossed phrenic nerve activity at 2 h. The data further suggest that these morphological changes may be part of the substrate for the unmasking of ineffective synapses during the crossed phrenic phenomenon. © 1993 Academic Press, Inc.