Response of stretch receptors to static inflations and deflations in an isolated tracheal segment

It has been proposed on the basis of differing responses to static lung inflations that there are two types of pulmonary stretch receptor which are organized in series or in parallel with airway smooth muscle. However, in prior studies, in which we examined the responses of a few receptors in a number of animals, we were unable to confirm this finding. In the present study we sought to definitively address this question by examining the response of a reasonable sample of receptors (greater than or equal to 5) from a single airway with a controlled mechanical environment. We examined the firing patterns of slowly adapting receptors in a tracheal segment, isolated in vivo, during inflations and deflations to different static levels of pressure between -20 and +30 cm H2O. The relationship between afferent firing and intra-tracheal pressure under static conditions was determined. We did not observe any evidence to support the existence of two distinct subtypes of receptors. During these studies we also looked for abrupt changes in the variability of afferent firing during the period of adaptation after the end of an inflation or deflation. Such changes have been described for other receptors with branched endings capable of generating spike potentials. These changes occur when the region generating the spike train seen in the parent fiber switches from one branch to another (pace-maker switching) and reflect the difference in the intrinsic variability of each branch. In the majority of fibers studied we saw no evidence of such switching and thus of the presence of multiple encoders in the receptor structure.