Afferent vagal fibres with an expiratory discharge in the rabbit

1. An investigation was undertaken in the spontaneously breathing, anaesthetized rabbit to observe the response of vagal sensory endings to inflation and deflation stimuli which were sufficient to produce Hering-Breuer reflexes.2. When artificial changes in lung volume were produced by applying step changes in intratracheal pressure, activity in pulmonary stretch receptors increased with inflation and decreased with deflation.3. A small number of vagal afferent fibres were encountered which exhibited a discharge with a respiratory modulation consisting of an increase in activity during the phase of expiration. This pattern of discharge was accentuated when the animal was artificially ventilated with the chest either open or closed.4. These expiratory receptors were stimulated by step changes in intratracheal pressure of -5 and +15 mm Hg to produce a high frequency, slowly adapting discharge which persisted for the 10 sec period of the pressure application. The endings were localized in the extrapulmonary bronchi and lung parenchyma and the afferent fibres were myelinated with conduction velocities of 10.6-29.5 m/sec.5. The pronounced effect of deflation stimuli on the endings indicates that they probably act as one of the afferent mechanisms in the Hering-Breuer deflation reflex.