THE NEURAL CONTROL OF RESPIRATION 



The discussion carried on for many years over the 

 significance for respiration of the vagal fibers from the 

 lungs seems thus to have more or less come to a close. 

 In principle, the vagus can exert an inhibitory as well 

 as a facilitating influence on inspiration. Depending 

 on the animal species or the type of narcosis used, the 

 one or the other effect predominates. In the cat, dog 

 and monkey the inhibitory effect is dominant. In the 

 rabbit the facilitating effect on inspiration is clearly 

 in evidence, and in the guinea pig it is actualK' 

 indispensable for the maintenance of the respiratory 

 rhythm (144). 



VAGAL CHEMORECEPTiVE CONTROL. The importance of 

 the aortic, pulmonary and cardiac chemoreceptors 

 for respiratory and circulatory regulation has recently 

 been discussed by Dawes & C'omroe in such a com- 

 prehensive review article (51) that only the more 

 generally accepted concepts will be mentioned here. 

 After bilateral dener\ation of the carotid bodies, 

 the excitation of the aortic chemoreceptors by carbon 

 dioxide, anoxia, lobeline or piperidine leads to a 

 reflex hyperpnea which is abolished by bilateral 

 vagotomy (74, gq, roo). Experiments in which the 

 \'arious intrathoracic fibers of the vagus are inter- 

 rupted have demonstrated that the afferent fibers 

 from the aortic body run predominantly in the right 

 depressor nerve in the cat (138), while in the rabbit 

 they are found principally in the trunk of the vagus 



(103)- 



Small amounts of 0.3 .A^ acetic acid, injected di- 

 rectly into the aorta, block the aortic chemoreceptors 

 but not the pressoreceptors. Through this means, as 

 well as through denervation of the aortic chemo- 

 receptors, it lias been demonstrated that these re- 

 ceptors are much less important than the glomus 

 caroticum for the control of respiration. 



One must differentiate the chemoreceptors proper 

 (aortic chemoreceptors and glomus caroticum) from 

 receptors in the lung which are also subject to chemi- 

 cal influence. Their exact location has not yet been 

 determined, but a close association with the pulmo- 

 nary blood vessels seems probable. These pulmonary 

 'chemoreceptors' can be stimulated by a great variety 

 of substances; they produce a reflex apnea (in the cat 

 and dog in expiration, in the rabbit occasionally in 

 inspiration) followed by hyperpnea (50). For further 

 research in this direction it would be advantageous, 

 according to Dawes & Comroe (51), to define more 

 sharply the concept 'chemoreceptor,' and to classify 

 those substances which are able to influence respira- 

 tion over pulmonary receptors into three groups. 



according to their mechanism of action : a) substances 

 which excite the Hering-Breuer inflation reflex: 

 \eratrine, veratridine and other veratrum alkaloids; 

 /)) substances which excite the pulmonary respiratory 

 chemoreflex: phenyl diguanidine, diphenhydramine, 

 mepyramine, 5-hydroxytryptamine; c) unclassified 

 substances, e.g. various other amidines and anti- 

 histamines, serum, nicotine, phosgene. 



Ru/f of the Carotid Body in Respiratory Control 



Since the first descriptions of the carotid sinus as a 

 chemoreceptive reflex zone (46, 98, 170, 171), the 

 investigations by Comroe & Schmidt (44) and by 

 Heymans & Bouckaert above all (97) have con- 

 tributed to the fact that we now differentiate betw-een 

 pressoreceptors in the wall of the sinus caroticum and 

 chemoreceptors in the glomus caroticum (carotid 

 body). Both receptor regions are connected with the 

 medullary centers through the Hering nerve, a 

 branch of the glossopharyngeal nerve. A differentia- 

 tion of the influences of the pressoreceptors from those 

 of the chemoreceptors is relatively simple in the dog. 

 The glomus caroticum can be cut off from its blood 

 supply by ligation of the occipital artery at its exit 

 from the external carotid artery, and the sinus region 

 is easily denervated. In the cat and in the rabbit a 

 separation of the two receptor regions is more difficult 

 to accomplish. Moreover, the anatomical position 

 (52) and the blood supply (42) of the carotid body 

 vary from species to species. 



Of the two types of receptors, the chemoreceptors 

 are of primary importance for the reflex control of 

 respiration. Stimulation of the afferent fibers from the 

 carotid body leads — after a certain latent period — to 

 a marked increase in the respiratory amplitude and, 

 to a lesser extent, in the respiratory frequency. The 

 following have been demonstrated, in numerous ex- 

 periments, to act as physiological stimuli : decrease in 

 the oxygen saturation of the arterial blood to below 

 85 per cent, increase in tlie arterial pCO_> and lowering 

 of the pH in arterial blood. Views are divergent only 

 over the relative importance of the central and the 

 reflex stimulation of the respiratory centers by carbon 

 dioxide. The Heymans school (25, 97) considers the 

 reflex activation of the respiration by carbon dioxide 

 to be dominant and assigns a subordinate role to the 

 direct effect of carbon dioxide on the respiratory cen- 

 ter; but other authors (59, 173, 174) have demon- 

 strated in the dog that the increase in respiratory 

 minute \olume produced by carbon dioxide is almost 



