Projections and terminations of single respiratory axons in the cervical spinal cord of cat.

Position, divergence, branching, and termination patterns of single, respiratory axons were studied in cat cervical spinal cord by injecting horseradish peroxidase (HRP) intra-axonally. We stained 12 axons which were characterized by their firing patterns and by electrical stimulation. Five axons discharged during inspiration (I); the remaining 7 discharged during expiration (E). No injected axon was evoked by stimulating ipsilateral phrenic nerve roots while 7 (4 I, 3 E) of 12 were excited at a short latency from stimulating at a medullary site (on the midline, 1-2 mm rostral to the obex, approximately 3 mm below the dorsal medullary surface) where many bulbospinal respiratory axons decussate. All injected stem axons were located in the ventral and ventrolateral funiculi, traversed in a rostrocaudal direction, and were stained for lengths ranging from 3.6 to 12.4 mm. Mean axonal diameter was 2.9 microns. In 6 axons (4 I, 2 E), 14 collaterals were stained: 1 on each E axon, 2 on one I axon, 3 each on 2 others and 4 on another I axon. Collaterals emerged perpendicularly from the descending stem axon and projected directly to the ventral horn. The average distance between neighboring collaterals was 1.0 mm (n = 7). Collaterals did not arborize until they were near or within the ventral horn. Both en passant and terminaux types of presynaptic boutons were found primarily within the rostrocaudal cylinder that defined the phrenic motor column. In addition, some boutons were located dorsomedial to the phrenic motor column. We conclude that I axons, presumably of medullary origin, have multiple collaterals which terminate primarily in the phrenic motor column. However, the same axon can have terminals in different regions of the ventral horn, which are known to contain dendrites of phrenic motoneurons.