THE NEURAL CONTROL OF RESPIRATION 



during the inspiratory phase leads to a lengthening of 

 the inspiration and a development of greater strength 

 in the inspiratory muscles. The converse is seen with 

 an increase in resistance during expiration. With sud- 

 den removal of an obstruction in the air passages, the 

 inspiration (or expiration) is reflexly shortened 

 and there is a reduced development of strength in the 

 respective respiratory muscles. The reflex is elicited 

 through stimulation of stretch receptors in the dia- 

 phragm (38, 57) and in the intercostal muscles. It is 

 also seen after bilateral vagotomy but is abolished by 

 combined severance of the vagus and phrenic nerves 

 and the posterior roots. The significance of these 

 proprioceptive reflexes is seen in a reflex adjustment 

 of the strength of the respiratory muscles to the flow 

 resistance in the air passages. Moreover, they are be- 

 lie\ed to reinforce the Hering-Breuer reflex. 



.■\ number of protective reflexes are able to block 

 respiration temporarily. The Ijest known of these 

 have their origin in the mucous membrane of the nose. 

 They have been designated trigeminal protective re- 

 flexes (116). Irritating suljstances, e.g. ciiloroform or 

 ether vapors, ammonia, tobacco smoke, acrolein, 

 phosgene or hydrogen sulfide (116, 128), provided 

 they do not penetrate beyond the upper respiratory 

 passages, cause a slowing of respiration in low con- 

 centrations, a respiratory arrest in higher concentra- 

 tions. These effects can lead to a complete cessation 

 of respiratory actixity so that electrical activity can be 

 demonstrated neither in the phrenic ner\e nor in the 

 fifth intercostal nerve which is normally active 

 in expiration (133). These protecti\e reflexes are 

 generally no longer present after seserance of the 

 trigeminal nerve. The occasional observance of con- 

 siderably weakened protective reflexes can be ex- 

 plained ijy the fact that expiratory reactions are also 

 obtainable by stimulation of the olfactory mucosa 

 (4, 5). However, the inhibitory respiratory reflexes of 

 olfactory origin are not as easily elicited as the activat- 

 ing olfactory reflexes since particularly aromatic and 

 balsamic substances evoke a cortical activation of 

 respiration (sniffing) in animals (6, 19). The sneezing 

 reflex can also be elicited from the nasal mucous 

 membrane (169). It can ije obtained in the rabbit 

 and in the cat by stimulation of the anterior nares, in 

 man by touching the anterior or posterior end of the 

 middle and inferior nasal conchae and corresponding 

 parts of the nasal septum. The afferent pathways run 

 in the ethmoidal branches of the nasociliary nerve. 



A protective respiratory reflex occurring innumer- 

 able times every day takes place during swallowing. 

 It consists essentially in a closure of the nasopharynx 



by the soft palate, a closing of the glottis and a raising 

 of the larynx. The resulting respiratory arrest can 

 occur in any phase of respiration. In animal experi- 

 ments, electrical stimulation of the superior laryngeal 

 nerve (181), or of the glossopharyngeal nerve (130), 

 has led to swallowing moveiuents and inhibition of 

 respiration. Light contact with the palatinal and 

 pharyngeal mucosa in the pentobarbiialized cat leads 

 to an acceleration of respiration; stronger stimulation 

 of the mucous membrane results in respiratory arrest 

 in expiration (186). More recentlv, a reflex influence 

 on respiration during the course of swallowing was 

 demonstrated in the cat in urethane anesthesia Ijy 

 recording action potentials from neurons in the me- 

 dulla which discharge synchronously with inspiration 

 (or with expiration) (104). However, the sequence of 

 the afferent and efferent impulses in the trigeminal, 

 glossopharyngeal and vagus nerves has been so little 

 investigated tliat only conjectures can be made about 

 the central mechanism of respiratory control during 

 swallowing. 



One is probably dealing with a nociceptive reflex 

 when, in the rabbit (78), dog (17) and man (iiB), a 

 slowing of respiration or a respiratory arrest in ex- 

 piration is obtained Ijy stimulation of the afferent 

 fibers of the splanchnic ner\e. Occasionally an ex- 

 teroceptive influence on respiration has been demon- 

 strated, for example a deep inspiration, or respiratory 

 arrest, with stimulation of the skin by cold, and 

 respiratory activation (less often inhibition) with 

 painful stimuli. 



Hyperpnea Associated with Muscular Activity 



According to what has been said previously, the 

 nervous control of respiration has its origin in ponto- 

 meduUary centers located in the reticular substance. 

 Most of the neurons in the inspiratory center are 

 automatically active and exert a tonic influence on 

 the motoneurons of the inspiratory muscles. The de- 

 gree of automatic activity is dependent upon meta- 

 bolic processes; it is probably determined, above all, 

 by the partial pressure of carbon dioxide or the hy- 

 drogen ion concentration in arterial blood or both. 

 The continuous activity of the primary inspiratory 

 center is transformed into a rhythmic one through 

 medullary and pontine inhibitory processes, thereby 

 leading to the alternation between inspiration and 

 expiration. The respiratory rhythm is also controlled 

 from the lungs over vagopuliuonary (Hering-Breuer) 

 refle.xes. The activity of the medullary and pontine 

 respiratory centers does not, howexer, follow a rigid 



