124 



HANDBOOK OF PHYSIOLOGY 



nei;rophysiology ii 



as great after elimination of the chemoreceptors as 

 before. 



Action potential in\-estigations on the intact and 

 frayed Hering nerve in the cat have also disclosed the 

 particular sensitivity of the glomic chemoreceptors to 

 a deficiency of oxygen or an excess of carbon dioxide. 

 A reduction in the oxygen saturation of arterial blood 

 below 95 per cent Hb (lo, 193), or an increase in the 

 arterial pC02 above 30 mm Hg (15, 193), leads to an 

 enhancement of electrical activity. Moreover, the 

 discharge frequency has been shown to run almost 

 parallel to the arterial pCOj. A large loss of blood 

 (119), a reduction of the systemic blood pressure be- 

 low 40 to 50 mm Hg, or the stimulation of the cranial 

 part of the cervical sympathetic trunk (47, 68) gives 

 rise to a continuous discharge of the chemoreceptors 

 as a result of the consequent local ischemia in the 

 carotid body. The glomus caroticum can also be 

 stimulated by lobeline, nicotine, acetylcholine and 

 cyanide. The respiratory activation produced by these 

 substances is often utilized as a means of determining 

 whether or not the chemoreceptor reflex is func- 

 tioning. 



When both the glomus caroticum and the aortic 

 chemoreceptors are eliminated, the respiratory minute 

 \-olume is reduced by about 30 per cent. Thus one 

 can, in general, assign to the aortic, and particularly 

 the sinus chemoreceptors, a tonic facilitating influence 

 on the respiratory center (74). Through their inter- 

 vention or their suppression can be explained, for 

 example, the hyperpnea following clamping of the 

 carotid arteries (168, 171, 192), the reduction in the 

 respiratory minute volume occasionally observ-ed 

 during inhalation of pure oxygen and the stimulation 

 of respiration observed in oxygen dcficiencv states 



(21). 



sinus, the supply of oxygenated blood to the glomus 

 caroticum may be impaired by the operative pro- 

 cedure. Furthermore, an increase in pressure within 

 the carotid sinus not only causes a stimulation of the 

 sinus nerve but can also lead to a higher flow rate 

 through the carotid body. The decrease in chemo- 

 receptive respiratory activation, resulting from better 

 oxygenation of this organ, could simulate a presso- 

 receptive respiratory inhibition. 



An effect of the pressoreceptors on respiration can, 

 therefore, only be demonstrated when secondary 

 compensatory reflexes have been eliminated. This has 

 been attempted through the introduction of a blind 

 sack in the sinus, through the automatic compensa- 

 tion of variations on the systemic blood pressure, or by 

 allowing the experimental animal to breathe pure 

 oxygen (23). \Vith these procedures on cats and on 

 dogs stimulation of the carotid sinus receptors pro- 

 duces only slight changes in respiration (22, 74) ; but 

 even in these experiments one must consider the 

 possibility of secondary compensatory reflexes since a 

 strong expiratory inhibition is observed with the same 

 stimulation following vagotomy (22, 23). This dififer- 

 ence in the respiratory efTects of carotid sinus stimula- 

 tion in the vagotomized animals could be due to a 

 concomitant bronchoconstrictor efl"ect mediated 

 through the vagus ner\e (48, 201 ). .^n increase of 

 resistance in the air passages leads, through vagal and 

 other proprioceptive reflexes, to compensatory 

 changes in the respiration. Therefore, it is possible 

 that a secondary activation of respiration due to 

 bronchoconstriction is able to mask the inhibitory 

 effect of carotid sinus stimulation as long as the vagus 

 nerves remain intact. The inhibition can only tnani- 

 fest itself when, as a result of vagotomy, the broncho- 

 constriction fails to take place.' 



Pressorecejilor Influence on Resjiiralion 



The influence of the pressoreceptors on respiration 

 is less marked than that of the chemoreceptors. In 

 the dog, electrical stimulation of the aortic depressor 

 nerve (113) or the physiological stimulation of the 

 stretch receptors in the carotid sinus through increase 

 in the intravascular pressure leads to a decrease in 

 the respiratory frequency and amplitude or to respira- 

 tory arrest in expiration (98, 170). In general, there- 

 fore, an inhibitory influence of the pressoreceptors on 

 respiration has been assumed (11). However, this 

 assumption is not necessarily valid. For example, 

 Swedish authors point out that even when experi- 

 ments are carried out on the isolated perfused carotid 



Proprioceptive and Protective Respiratory Reflexes 



Under proprioccpti\e respiratory reflexes (in the 

 narrower sense) are included those reflexes which 

 have their origin in the respiratory muscles, the affer- 

 ent pathways for which do not run in the vagus nerve. 

 Tachographic in\estigations on man (64) and on dogs 

 and sheep (65) have disclosed a number of compensa- 

 tory respiratory reflexes against sudden changes of 

 resistance in the air passages. An increase in resistance 



' The findings by Winder (,201) should not be interpreted as 

 an inhibition of central respiratory activity, since the plethys- 

 mographic methods used allow merely a measurement of the 

 How resistance to the artificial respiration. 



