Adaptation of tracheal stretch receptors

The adapting properties of airways slowly adapting stretch receptors have been generally measured as their declining response to maintained inflation only within the first few seconds. We measured the adaptation of tracheal stretch receptors for periods of 30 min or more and related this property to the mechanics of the structure containing them, i.e. the trachealis muscle. We recorded action potentials from thin filaments of the right vagus originating from slowly adapting stretch receptors in the extra-thoracic trachea of spontaneously breathing dogs. A transversal elongation was applied at constant speed to the posterior region of the extra-thoracic trachea and maintained for at least 30 min. The receptor discharge declined rapidly at first and progressively more slowly afterwards reaching a steady value after approximately 3 min. A similar time course was measured for the decay in force developed by the trachealis muscle subjected to a comparable elongation. The rapid decay in receptor discharge was affected by the rate at which the elongation was introduced. The slower decrease in activity resembles the stress relaxation process of the trachealis muscle. Antidromic stimulation of the endings did not modify the time course of their adaption. From these results we infer that the adaptive processes of these tracheal receptors are essentially dependent on the visco-elastic properties of the trachealis muscle.