Receptors sensitive to inflation and receptors sensitive to deflation of the lungs during spontaneous respiration in the guinea pig

Abstract

Afferent activity in the vagus nerve during spontaneous breathing was studied by means of the single fibre technique in 155 guinea-pigs anaesthetized with urethane. The impulses were recorded from the left vagus sectioned below the nodose ganglion. The animáis were placed in a body-plethysmograph and submitted to variations of extrathoracic pressure ranging from —6 to +14 cm H2O. The respiratory effects were evaluated by correlating the following parameters: duration of inspiration (In), dura- tion of expiration (Es), In/Es quotient and respiratory frequency, with the extrathoracic pressure and the stretch receptor and deflation receptor activity recorded. The results can be summarized as follows:

The pulmonary stretch receptors during spontaneous breathing discharge regular action potential sequences at each inspiration. The discharges are inversely proportional to the extrathoracic pressure, i.e., the greater the extrathoracic pressure, the smaller the number of impulses per sec during each inspiration, and vice versa.

An increase of stretch receptor activity produces a decrease of inspiratory and an increase of expiratory duration. As a result, the respiratory 'cycle is lengthened and a slight diminution of respiratory frequency occurs. A decrease of stretch receptor activity produces the reverse effect i.e., increase of inspiratory and decrease of expira­ tory duration. The result is a weak inspiratory effect characterized by modérate increase of the In/Es quotient and slight augmentation of respiratory frequency.

Extrathoracic pressure raised above +5 cm H=O or induction of a unilateral pneu- mothorax corresponding to the tidal volume of the lung concemed, induces excitation of the deflation receptors during the expiratory phase of breathing. The discharges are directly proportional to the extrathoracic pressure.

Deflation receptor activity produces marked decrease of expiratory duration and a modérate decrease of inspiratory duration. As a result, a marked inspiratory' effect characterized by a marked increase of the In/Es quotient and of the respiratory fre­ quency occurs.

A survey of the data presented suggests that variations of the In/Es quotient are related to both stretch receptor and deflation receptor activity. Changos in respira­ tory frecuency, however, appear to be dependent upon deflation receptor action. De­ flation receptor effects, when they do occur, predomínate over the concomitant stretch receptor action.

A paradoxical excitation of the deflation receptors may occur during inspiration as a result of over-large inflations of the lungs. A decisive factor appears to be the State of the lungs preceding the overinflation, i.e., the number of terminal units closed. It is suggested that the sighs, i.e. complementary cycles of respiration, are the result of deflation receptor stimulation. The purpose of this reflex would be to reopen the alveoli which slowly cióse during normal breathing. Deflation receptor excitation should be regarded as a defence mechanism against pulmonary collapse.