AN ELECTROPHYSIOLOGICAL AND MORPHOLOGICAL-STUDY OF ESOPHAGEAL MOTONEURONS IN RATS

Previous anatomic studies have shown that motoneurons supplying the st riated musculature of the esophagus form a tightly grouped cluster in the rostral portion of the nucleus ambiguus, known as the compact form ation. This study, conducted in anesthetized rats, presents the first in vivo intracellular and extracellular recordings from this group of motoneurons, which were identified by antidromic stimulation directly from the esophagus (latency 7-68 ms). The motoneurons were silent at r est, and those impaled intracellularly (n = 44) showed no respiratory modulation of their membrane potential. Intracellular labeling with Lu cifer Yellow (n = 3) or Neurobiotin (n = 15) revealed multipolar somas with longitudinally oriented dendritic trees mainly confined to the c ompact formation. No axon collaterals were found. When swallowing-like activity was induced by muscarine applied to the dorsal medullary sur face, the motoneurons displayed bursting activity, with the majority o f bursts occurring during expiration. These results show that antidrom ic stimulation of esophageal motoneurons with an electrode inserted in to the esophagus provides a simple way of identifying these motoneuron s. In the absence of pharmacological stimulation, these motoneurons re ceive no respiratory-modulated synaptic input, in contrast to adjacent motoneurons in the semicompact formation (supplying the upper airways ), which are known to display respiratory activity. However, some sync hronization of respiratory and swallowing-like activity was observed a fter pharmacological activation of the swallowing pattern generator in the dorsal medulla.