Afferent innervation and receptors of the canine extrathoracic trachea

The aim of this study was to establish the cranio-caudal distribution of slowly (SAR) and rapidly (RAR) adapting receptors of the extrathoracic trachea (ETT) as well as their innervation and response to water solutions of different compositions. Experiments were carried out on anesthetized dogs breathing spontaneously through a low cervical tracheostomy. Eighty percent of SARs and 76% of RARs with fibers in the superior laryngeal nerve (SLN) were found in the cranial third of the ETT. Fifty-seven percent of SARs and 45% of RARs with fibers in the cervical vagus and/or recurrent laryngeal nerve (RLN) were localized in the caudal third of the ETT. Instillation of water into the tracheal lumen had no effect on the activity of any SAR tested, but stimulated 41% of the RARs with fibers in the SLN and 23% of the RARs with fibers in the cervical vagus. Some of the RARs with fibers in the SLN (24%), but none of those with fibers in the cervical vagus/RLN, responded also to iso-osmotic dextrose solutions. Trachealis muscle contraction failed to stimulate the RARs tested. The blocking temperature for SAR and RAR fibers was similar and well within the range of myelinated fibers. We conclude that the SLN provides the innervation of the cranial ETT while the RLN has fibers for the caudal ETT with some overlap in the middle. The responses to water solutions indicate that tracheal RARs constitute a more heterogeneous group than laryngeal RARs.