Axonal projections from the rostral expiratory neurones of the Bötzinger complex to medulla and spinal cord in the cat.

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Abstract
Axonal projections of 84 rostral medullary expiratory neurons of the Boetzinger complex were tested using antidromic mapping techniques in anesthetized cats. A projection to the ventral respiratory neurons of the medulla (n.r.a.) was shown in 11 of 12 tested neurons. Also a spinal projection to the C[cervical]5-C6 cervical segments was evident in > 72% of tested neurons; probably near 100% project to cervical cord. These axonal projections were found bilaterally in both brain stem and spinal cord. The majority of Boetzinger complex expiratory neurons had 2-4 axonal collaterals to the ventrolateral (v.l.) nucleus of the solitary tract (n.t.s.) and/or the n.r.a. and/or the spinal cord. In 8 of 12 of the tested neurons, electrophysiological evidence of axonal arborization in > 1 of n.r.a. inspiratory, n.r.a. expiratory or v.l. n.t.s. regions was obtained. Similar evidence for the terminal arborization was found for 26% of tested neurons in the phrenic motor nucleus. The descending spinal expiratory axons of the Boetzinger complex neurons are located in the dorsal and medial parts of the lateral funiculus in C4 and C5 segments. Conduction velocity measurements indicate that these are large myelinated axons. The Boetzinger complex expiratory neurons may be a source of synaptic inhibition for n.r.a. inspiratory neurons and phrenic motoneurons.