Response of dog tracheal stretch receptors to high frequency oscillations

In eight dogs, anaesthetized and spontaneously breathing, we recorded from thin vagal filaments action potentials from eight slowly adapting receptors of the extrathoracic trachea. The extrathoracic trachea was cut longitudinally and the cut-ends of the cartilage corresponding to the site of the receptor in the trachealis muscle were stretched sinusoidally at frequencies from 0.5 to 39.5 Hz. The number of spikes per cycle decreased with increasing frequency. At the highest frequency (40 Hz), stretch receptors still responded to each oscillation, with a single action potential. The number of action potentials per second increased with increasing frequency. A decrease in smooth muscle tone, following vagotomy, decreased the firing of these endings at rates of oscillations below 20 Hz. Vice-versa, an increase in smooth muscle tone, due to histamine applied topically, increased the receptor discharge at rates of oscillations below 8 Hz. These results are consistent with an "in-series" arrangement with the muscle fibers. The increase in tracheal stretch receptor activity per second as the oscillatory frequency increases might explain the inhibitory influences on respiration observed with high frequency ventilation.