Electrophysiological evidence for axonal branching of ambiguous laryngeal motoneurons

Laryngeal motoneurons in the nucleus ambiguous (NA) were identified antidromically by stimulation of the ipsilateral superior laryngeal nerve (S) and/or the recurrent laryngeal nerve (R). In some NA motoneurons, antidromic spikes elicited by both S and R stimulation collided with the spontaneously occurring discharges. In the same neuron, spikes evoked antidromically by stimulation of one laryngeal nerve always collided with antidromic spikes elicited by stimulation of the other laryngeal nerve. Of 105 NA neurons activated by S and R stimulation, 36 neurons satisfied the criteria, and were classified as NA neurons with branching axons (branching NA (B-NA) neurons). Those neurons activated by either S or R stimulation but not both were classified as NA neurons without branching axons (unbranched NA (UB-NA) neurons). Mean antidromic latencies of B-NA neurons were 0.79 +/- 0.20 ms to S stimulation and 1.91 +/- 0.45 ms to R stimulation and those values for UB-NA neurons were 0.84 +/- 0.17 ms to S stimulation and 2.10 +/- 0.53 ms to R stimulation respectively. None of these mean values were significantly different from one another. Conduction time in the unbranched portion of the branching axon was estimated according to the equation reported by Anderson and Yoshida. The mean conduction time for 20 B-NA neurons was 0.45 +/- 0.35 ms. The branching point in B-NA neurons was estimated on the basis of the conduction time in the unbranched stem portion and those times in two branches of a branching axon measured electrophysiologically. The results suggest that the majority of B-NA neurons bifurcate within a half axonal length.(ABSTRACT TRUNCATED AT 250 WORDS)