MORPHOLOGICAL-STUDY OF LONG AXONAL PROJECTIONS OF VENTRAL MEDULLARY INSPIRATORY NEURONS IN THE RAT

The aim of this study was to examine medullary and spinal axonal proje ctions of inspiratory bulbospinal neurons of the rostral ventral respi ratory group (VRG) in the rat. A direct visualization of long (9.8-33 mm) axonal branches, including those projecting to the contralateral s ide of the medulla oblongata and the spinal cord, was possible due to intracellular labeling with neurobiotin and long survival times (up to 22 h) after injections. Seven of the nine labeled neurons had bilater al descending axons, which were located in discrete regions of the spi nal white matter; ipsilateral axons in the lateral and dorsolateral fu niculus, contralateral in the ventral and ventromedial funiculus. The collaterals issued by these axons at the mid-cervical level formed clo se appositions with dendrites of phrenic motoneurons, which had also b een labeled with neurobiotin. None of these collaterals crossed the mi dline. The significance of this finding is discussed in relation to th e crossed-phrenic phenomenon. Additional spinal collaterals were ident ified in the C, and T, segments. Within the medulla, collaterals with multiple varicosities were identified in the lateral tegmental field a nd in the dorsomedial medulla (in the hypoglossal nucleus and in the n ucleus of the solitary tract). These results demonstrate that inspirat ory VRG neurons in the rat have some features which have not been prev iously described in the cat, including frequent bilateral spinal proje ction and projection to the nucleus of the solitary tract. In addition , this study shows that intracellular labeling with neurobiotin offers an effective way of tracing long axonal projections, supplementing re sults previously obtainable only with antidromic mapping, and providin g morphological details which could not be observed in previous studie s using labeling with horseradish peroxidase.