AN ELECTROPHYSIOLOGICAL DEMONSTRATION OF AXONAL PROJECTIONS OF SINGLEVENTRAL INSPIRATORY NEURONS TO THE PHRENIC NUCLEUS OF THE CAT

Axonal branching patterns of single inspiratory (I) neurons of the nuc leus retroambigualis (NRA) were studied electrophysiologically in cat phrenic nucleus (C4-C6). Experiments were performed on Nembutal anesth etized, artificially ventilated cats, and extracellular spikes of I ne urons were recorded. The cervical spinal gray matter was microstimulat ed from dorsal to ventral sites at 100 mu m intervals with an intensit y of 150-250 mu A using a glass insulated tungsten microelectrode. The stimulations were made at 1 mm intervals rostrocaudally along the spi nal cord, and effective stimulating sites of antidromic activation in axonal collaterals were systematically mapped. I neurons examined (n = 8) descending contralaterally distributed multiple collaterals in the phrenic nucleus. These collaterals were found throughout the rostroca udal phrenic nucleus. An I neuron (n = 1) descending ipsilaterally als o distributed collaterals in the ipsilateral phrenic nucleus. Axonal c ollaterals in the contralateral phrenic nucleus occupied 44.2% of the total length of the cervical spinal cord examined. To determine the de tailed trajectory of collaterals in the cervical gray matter, microsti mulation was performed in and around the collateral arborizations at t he maximum intensity of 50 mu A. The descending stem axons could be lo calized in the lateral funiculus in four I neurons and in the ventral funiculus in one I neuron. I neurons distributed axonal collaterals wi thin the phrenic nucleus. Some part of the collaterals ran to the medi al region of the gray matter, re-crossed the midline under the central canal and reached the phrenic nucleus ipsilateral to the I neuron. Re -crossed collaterals arborized in the phrenic nucleus, but did not ext end to the gray matter more lateral than the phrenic nucleus. Rostroca udal extension of the re-crossed collaterals was found to be narrow.