Responses of upper airway muscles to gastrocnemius muscle contraction in dogs.

We studied electromyographic (EMG) responses of the alae nasi (AN) and the posterior cricoarytenoid (PCA) muscles, which act as upper airway dilators, during contraction of gastrocnemius muscle in six chest-intact anesthetized dogs with spontaneous breathing and in four thoracotomized, phrenicotomized and mechanically ventilated dogs with right thoracic and left cervical vagotomy. Muscle contraction was phasically induced by electrical stimulation of the intact gastrocnemius nerve or the distal cut end of this nerve for 20-30 sec. Stimulation intensity was determined as twice the motor threshold in each dog. In chest-intact animals, phasic contraction induced by intact nerve stimulation produced initial rapid increases in upper airway muscle activity, but stimulation of the distal cut end of the nerve did not show the rapid increase in upper airway muscle activity. Furthermore, stimulation of the proximal cut end did not produce any transient response with the stimulation intensity used in this study. In chest-open and vagotomized animals with artificial ventilation, responses of the upper airway muscles to contraction during the intact nerve stimulation were observed. These results suggest that the contraction of the gastrocnemius muscle activates upper airway dilating muscles via reflex mechanisms.