Positive feedback facilitation of external intercostal and phrenic inspiratory activity by pulmonary stretch receptors

Both in lightly pentobarbitone anesthetized and decerebrate cats increments in lung volume (V) during inspiration caused facilitation of inspiratory activity both in phrenic (Phr) and external intercostal (EI) motoneurons. This effect had low volume threshold, well below eupnoeic tidal volumes. It was readily reduced or abolished by small additional doses of pentobarbitone. This facilitatory effect appeared with considerably greater magnitude in EI than in Phr. The response magnitude was linearly related to the corresponding increments in V but not to increments in airflow (V). Sustained elevation of V at zero V caused sustained facilitation of EI and Phr. This positive feedback facilitation which was similarly obtained in spontaneously breathing and paralysed cats occurred continuously with great regularity in every breath. It was abolished by bilateral vagotomy but could then be elicited by electrical stimulation of the central end of the vagus nerve at the same threshold strengths required to elicit a just detectable shortening of inspiratory duration. The results indicate that the slowly adapting pulmonary stretch receptors are responsible for this positive feedback facilitation prior to the negative feedback effect on the inspiratory 'off-switch' elicited by the same receptors. Clear distinctions are described between the reflex characteristics of this 'low-threshold' volume dependent facilitatory reflex and the 'high-threshold' transient excitatory reflex effects provoked by large and rapid inflations.