A Light Microscopic HRP-Degeneration Study of Corticospinal Input to the Spinal Respiratory Centers
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Although detailed analysis of the involuntary respiratory system has been done, not much work has been done on the morphology of the voluntary respiratory system. This study was carried out to determine if there is corticospinal input to the spinal respiratory center (phrenic nucleus) of the rat. The phrenic nucleus was labeled by retrograde transportation of horseradish peroxidase (HRP) after transecting the nerve in the neck or injecting a conjugated cholera toxin-HRP into the diaphragm. The corticospinal tract was labeled by cortical ablation of the areas of the cortex known to contain corticospinal neurons followed by a Fink-Heimer staining technique which labeled anterograde degenerated neurons. The results at light microscopic levels indicate that there was no direct corticospinal input into the phrenic motorneurons. However, possible corticospinal degeneration patterns were seen in the contralateral and ipsilateral dorsolateral funiculi (DLF). In addition, distal dendrites of the phrenic nucleus show movement towards the DLF but none were seen actually entering into the DLF. These results indicate the need for a better labeling method for distal dendrites of the phrenic nucleus and a more detailed electron microscopic analysis to determine the exact nature of the degeneration patterns seen.