Vagal Reflex Influences on Central Respiratory Co‐ordination

Abstract

A variety of vagal afterent endings presumably possessing the coiiimon property of being activated by changes iji lung volume, are found in the epithelia of the lung structures. The relationship of the afferent discharge frequencies of these endings to the phases of respiration suggests that during the inspiratory period the vagi conduct pulmonary afferent impnlses of increasing frequency which summate centrally and contribute in part to the initiation of expiration; the ovei'all eff' ect being one of shortening both the inspiratory and expiratory phases of respiration (Adrian, 1933;Partridge, 1933Partridge, , 1935Rijlant, 1933).Tlie Hering-Brener concept of respiratory control as elaborated by Head (1889) requirea two vaga] reflexes. One is inspiratory inhibitory on lung inflation, the otlier expiratory inhibitory on lung deflation, and although Kuowltoii and Larrabee (1946) have confirmed the earlier observations on two vagal flbre tyjjes concerned in respiration, the fibre groups having different thresholds and adaptations, there does not seem to be any agreement as to the origin or mechanism of the accelerating effect of lung deflation (Partridge. 1939; Hammoiida and AVilson, 1932 to 1933; Steffensen ct al., 1937). Siu-tion deflation, at best an nnphysiological procedure, may stimulate endings not normally associated with the Hering-Breuer reflex, Histologically, the lungs contain endings of varied apiiearance possibly capable of activating several different reflexes, unless they all respond to the common stimulus of lung deformation (Elftman, 1943). The fact that low frequency stinmlation of the central vagal strniip will produce acceleration of respiration while liigli frequoiicies are inhibitor (Rice, 1938), has been taken up by Wys.s (1946a). He has shown tliat the frequency conditioned reflex reversal is quite similar to, and even simpler than, that described for spinal reflexes, in that internuncial nenrones mediate their excitor or inhibitor effects ujion the respiratory motor neurones by virtue of differences in summative capacity. These effects are relayed by discrete regions within the medullii in the tractus solilarius and adjacent i)arts of the lateral reticular formation (Wyss, ]94(3b), wliilo tlie extent of their actions on the respiratory rhythm is moderated by a system which extends into the upper pons (Meier and Buclier, 1941;Bucher, 1942), since serial sectioning of the brain stem caudad from the lower portion of the inferior collieuli releases first the excitor then, at a lower level, the inhibitor actions of the vagal reflexes so that they become more exaggerated.