Reflex effects of stimulation of sympathetic afferents on the triangularis sterni

The purpose of this study was to determine whether contralateral inhibition of the triangularis sterni is produced by stimulation of intrathoracic sympathetic afferents. Dogs were anesthetized with sodium pentobarbital and placed on positive pressure ventilation. The chest was opened through a mid-sternal incision. Diaphragm and left and right triangularis sterni EMGs were recorded, post-vagotomy, before and during electrical stimulation of the left ventral ansa subclavia (VA), vagosympathetic trunk, ventrolateral and ventromedial cardiac nerves and, when present, the stellate cardiac nerve. Peak of the phasic diaphragm EMG and expiratory time were not significantly affected by stimulation of the VA. A significant decrease in inspiratory time was observed. Ipsilateral excitation and contralateral inhibition of the left and right triangularis sterni EMGs, respectively, were produced by stimulation of the VA. Stimulation of the other intrathoracic nerves produced a similar pattern of results. Conduction velocity determinations suggested that the afferents which produced the reflex responses are, at least in part, small A fibers.