AXON BRANCHING OF MEDULLARY EXPIRATORY NEURONS IN THE LUMBAR AND THE SACRAL SPINAL-CORD OF THE CAT

Intraspinal axon collaterals of expiratory (E) neurons in the caudal n ucleus retroambigualis extending their descending spinal axons to the lower lumbar (L6-L7) and the sacral (S1-S3) segments were investigated in anesthetized cats. To search for axon collaterals of single E neur ons in the lumbar segments, the spinal gray matter was microstimulated from the dorsal to the ventral sites at 100 mu m intervals with an in tensity of 150-250 mu A at 1 mm intervals rostrocaudally along the spi nal cord, and effective stimulating sites of antidromic activation in axon collaterals were systematically mapped. In addition, the detailed trajectory of collaterals in the upper lumbar (L1-L3), the middle lum bar (L4-L5), and the sacral (S1-S3) spinal cord was examined by micros timulation at a matrix of points 100-200 mu m apart with a maximum sti mulus intensity of 50 mu A. The trajectory of axon collaterals was rec onstructed on the basis of the location of low-threshold foci and the latency of antidromic spikes. Virtually all E neurons examined had 1-7 collaterals at widely separated segments of the lumbar cord. Many axo n collaterals were found in the upper lumbar spinal cord as compared t o the middle and the lower lumbar spinal cord. The locations of axon c ollaterals in the upper lumbar spinal cord overlapped with those of ab dominal motoneurons. Axon collaterals in the sacral gray matter were f ound in 3 of 9 E neurons. Axon collaterals were found within the nucle us of Onuf, in the region dorsal to the nucleus of Onuf, and in the in termediate region. The functional significance of the divergent distri bution of multiple axon collaterals of single E neurons in different s pinal levels of the lumbar and the sacral spinal cord is discussed in relation to the respiratory function of E neurons and other spinal mot or activities.