Degenerative and regenerative changes in central projections of glossopharyngeal and vagal sensory neurons after peripheral axotomy in cats: A structural basis for central reorganization of arterial chemoreflex pathways

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Hypoxic hyperventilation in cats is a reflex normally initiated by afferent impulses originating in the carotid body and conducted to the brain stem by the carotid sinus nerves. The reflex response is abolished acutely after section of carotid sinus nerves and excision of the carotid bodies; but, chronically, there is a chemoreflex restoration which is mediated by the aortic body via the aortic depressor nerves. The restoration is associated temporally with changes in efficacy of ventilatory reflexes elicited by electrically stimulating carotid sinus and aortic depressor nerves, and these changes are postulated to reflect a central reorganization of the reflex pathways. In the present study, histological and ultrastructural techniques were used to investigate the neuroanatomical basis of the reorganization. The brain stem of the cat was examined using the Fink-Heimer silver stain to determine if degenerating axons were present following section of the carotid sinus nerve peripheral to its sensory ganglion. Degeneration was found 4–15 days postoperatively and the distribution of the axons corresponded with that reported for central projections of carotid sinus nerves labeled by transganglionic transport of horseradish peroxidase. The fine structure of nerve terminals in nucleus tractus solitarius was then examined with electron microscopy after cutting the vagus and glossopharyngeal nerves unilaterally peripheral to the sensory ganglia. Structural changes consistent with nerve terminal degeneration were observed 4–91 days postoperatively, and presumptive axonal sprouts were seen at 56–91 days. It is concluded that central projections of the sensory ganglion cells of the vagus, glossopharyngeal and carotid sinus nerves degenerate slowly after the nerves are cut peripheral to the sensory ganglia and that degeneration is followed by renewed central sprouting. The degenerative loss of central projections of carotid sinus nerves may permit central reorganization of the chemoreflex pathway to occur by providing vacant postsynaptic sites which can be occupied by converging aortic depressor nerve projections which undergo sprouting.