Head's Paradoxical Reflex

Abstract

Cold block of the vagus nerves of rabbits completely prevents conduction in pulmonary stretch fibres at temperatures close to or higher than those required to block the Hering-Breuer inflation reflex. These temperatures are several degrees higher than those required to block Head's paradoxical reflex. It is concluded that Head's paradoxical reflex cannot be due to pulmonary stretch fibre activity during cold block; that is, cannot be due to cold block of the vagus nerves being more effective for high-frequency discharges (during lung inflation) than for low-frequency discharges in pulmonary stretch fibres. This conclusion is supported by a consideration of the pattern of Head's paradoxical reflex, and by a summary of the published evidence that the reflex can be elicited when the nerves are not being cooled at all.IN 1889, Head showed that, when conduction in the cervical vagus nerves of rabbits was partially blocked by cold, inflation of the lungs caused a strong and long-lasting inspiratory effort (usually referred to as Head's paradoxical reflex), instead of the inhibition of breathing seen when vagal conduction was intact (the Hering-Breuer inflation reflex). Paintal [1966] has recently shown that, during cold block of the cat's vagus nerves, low impulse frequency discharges in pulmonary stretch fibres pass the block more readily than do high frequency discharges, and he has concluded that this phenomenon underlies Head's paradoxical reflex; in other words, with cold block fewer impulses in pulmonary stretch fibres pass the block during the rapid discharges of lung inflation than do during the slower discharges of spontaneous breathing, and since the impulses inhibit inspiratory activity the effect of lung inflation during cold block is a lessening of inhibition or a stimulation of inspiration. Paintal concludes that Head's paradoxical reflex is a 'physiological artifact'.An important physiological role has been ascribed to Head's paradoxical reflex, both in causing the occasional deep breaths which are thought to counteract any tendency to atelectasis in quiet breathing Reynolds, 1962;Reynolds and Hilgeson, 1965;Widdicombe, 1964] and possibly in reinforcing the gasps or inspiratory efforts whereby new-born mamnmals aerate their lungs [Cross et al., 1960; Cross, 1961]; it therefore seems worthwhile to describe experiments, hitherto only briefly reported [Widdicombe, 1959], which indicate that Head's paradoxical reflex is not a 'physiological artifact', and to emphasize some of the welldocumented evidence for the same conclusion (see Discussion). * Present address.

Muscular, bronchomotor and cardiovascular reflexes elicited by mechanical stimulation of the respiratory tract

“…Similar patterns of diaphragmatic activity were recently reported during electrical stimulation of the pharyngeal wall (Takagi et al 1966), and in medullary inspiratory neurones at the beginning of swallowing (Sumi, 1963). It is possible that, after the primary inspiratory response of the aspiration reflex, some of the later bursts of activity in the phrenic-diaphragmatic units may be evoked or potentiated reflexly from the lungs by the mechanism underlying Head's paradoxical reflex (Head, 1889;Widdicombe, 1967); this may be the same as the inspiratory exciting reflex described by Larrabee & Knowlton (1946) and the respiratory augmenting reflex studied by Reynolds (1962). However, these latter reflexes are all elicited from lung receptors by large or rapid pulmonary inflations, while the aspiration reflex persists after vagotomy .…”

The effect of vagotomy, vagal cooling and efferent vagal stimulation on breathing and lung mechanics of rabbits

“…However, at this thermode temperature (8-10°C) for the rabbit's vagus all conduction in slowly adapting low threshold pulmonary stretch fibres, mediating the Hering-Breuer inflation reflex, is blocked (Widdicombe, 1967). It is known that, with vagi at this temperature, vagal afferent pathways for reflexes other than the Hering-Breuer inflation reflex can still conduct impulses (Dawes et al 1951;Troelstra, 1960;Karczewski & Widdicombe, 1969a, b (Troelstra, 1960), Head's paradoxical reflex (Widdicombe, 1967), and the pulmonary respiratory chemoreflex (Dawes & Comroe, 1954) which is activated by intravenous injections of phenyl diguanide into the rabbit (Dawes et at. 1951).…”

“…Abolition of the inflation reflex was not assumed to mean that other afferent pathways were unaffected. However, with these criteria, it is known that vagal afferent pathways other than that for the Hering-Breuer inflation reflex can still be effective (Dawes, Mott & Widdicombe, 1951;Troelstra, 1960;Widdicombe, 1967;Karczewski & Widdicombe, 1969a, b), and that vagal efferent bronchoconstrictor (Widdicombe & Nadel, 1963;De Kock, Nadel, Zwi, Colebatch & Olsen, 1966) and cardio-inhibitor (Dawes et at. 1951) fibres can still conduct impulses.…”

Re‐evaluation of Respiratory Reflexes

“…SOME of our present concepts [cf. Aviado and Schmidt, 1955;Widdicombe, 1964; Wyss, 1964] regarding the reflex effects of cardio-respiratory and cardiovascular afferent fibres have arisen from experiments in which the vagus was cooled to various temperatures [Head, 1889;Hammouda and Wilson, 1935 a; 1935 b;Partridge, 1939; Whitteridge and Biilbring, 1944;Hammouda et al, 1943; Torrance and Whitteridge, 1948;Dawes et al, 1951; 1954 b;1959]. The conclusions in the more recent of these studies (from 1948 onwards) have rested on the tacit or stated assumption that faster conducting fibres are blocked at higher temperatures than the slower fibres.…”