Electrophysiological determination of the axonal projections of single dorsal respiratory group neurons to the cervical spinal cord of cat.

Antidromic microstimulation and orthodromic extracellular spike-triggered averaging were used to determine the axonal positions, divergence and terminations of 16 axons arising from bulbospinal, inspiratory (I) neurons. Activity from these neurons was recorded in the dorsal respiratory groups (DRG) of 12 cats. Systematic tracking was done both transversely and longitudinally in the contralateral fifth and sixth cervical segments of the spinal cord. Axonal position was determined by antidromically activating axons and by recording axonal field potentials. Thirteen axons were located in the lateral funiculus, two in the ventrolateral funiculus and one in the ventral funiculus. Axonal conduction velocity (CV) was calculated from conduction distance and conduction time, the latter defined as the interval of time from the recorded action potential in the medulla to the recorded averaged axonal potential in the spinal cord. Average (+/- S.D.) axonal CV was 52 +/- 15 m/s. Terminal potentials were recorded for 13 of these axons and were coincident with the location of evoked field potentials resulting from antidromic stimulation of phrenic motoneurons. In addition, terminal potentials from single I cells were recorded at multiple sites along the longitudinal axis of the phrenic motor column. These data indicate that axons of spontaneously active, DRG bulbospinal I cells descend predominantly in the lateral columns and diverge and terminate extensively within the phrenic motor column.