THE NEURAL CONTROL OF RESPIRATION 



III9 



is high enough, an active expiration results and the 

 respiratory volume is increased, as seen, for example, 

 in dyspnea. 



A further influence on respiratory activity, always 

 ascertainable in the intact animal, is of cortical origin. 

 These psychic influences are of a manifold nature. 

 Thus it has long been known that in the course of 

 intellectual work the respiratory frequency increases, 

 and the amplitude tends rather to be reduced (20, 53, 

 185, 194). Emotional changes are also often first 

 detected by the observer through changes in the type 

 or frequency of respiration (26, 211). These changes 

 in the respiration can, at times, be so pronounced that 

 they may be interpreted as an indication of certain 

 psychic disorders (34, 63). 



The influence of the diencephalic and mesen- 

 cephalic centers on respiration has been discussed in 

 detail in the first section of this chapter. Apparently 

 we are here concerned with the intervention of a 

 superimposed regulatory system, in connection with a 

 general increa.se or decrease in the activity of the 

 organism as a whole. For this purpose it seeins ex- 

 pedient that wlicn the metabolism, circulation and 

 muscle tone are altered, respiration is also influenced 

 in a corresponding manner (90, 92). 



Respiratory Neural Discharge 



The possibility of investigating ner\ous and muscu- 

 lar activitv througii the measurement of action po- 

 tentials allows us to gain a more intimate insight into 

 the innervation patterns of the respiratory muscles. 

 Up to the present, the investigation of the innervation 

 of the most important inspiratory muscle, the dia- 

 phragm, has been particularly thorough. For this 

 purpose it is sufficient to record the electrical activity 

 of the phrenic nerve, for — apart from a delay of about 

 10 msec. — practically synchronous potential waves 

 are found in the phrenic nerve and in the diaphragm 



(72). ... 



Action potentials from single fibers of a phrenic 



nerve root (3, 159), or from isolated fibers in the 



frayed nerve trunk (152, 153), exhibit most easily the 



pattern of the diaphragmatic innervation. During 



inspiration in eupnea, impulse series of relati\'ely low 



frequency (10 to 30 per sec.) are found. The frequency 



of the action potentials usually increases slightly from 



the beginning to about the middle of inspiration and 



then remains approximately the same until the end of 



inspiration. With the beginning of expiration, the dis- 



charge salvo is, as a rule, suddenly discontinued. 

 Under dyspneic conditions (3), or with stimulation of 

 the inspiratory center in the medulla (1.52), the dis- 

 charge frequency can increase to 100 or more per sec. ; 

 in fact, maximal frequencies as high as 400 per sec. 

 have been recorded (159). With stimulation in the 

 inspiratory center, moreover, the separate inspiratorv 

 phases are lengthened. On the other hand, stimulation 

 in the expiratory region of the medulla shortens the 

 duration of the salvos or blocks the neurons com- 

 pletely. 



The action potentials present a more complicated 

 picture when they are recorded from the central stump 

 of the intact phrenic nerve. In eupnea a certain base 

 activity is often seen during expiration (206) which 

 would correspond to the tonic innervation of the dia- 

 phragm and on which the inspiratory activity is 

 superimposed. Under the influence of carbon dioxide 

 in addition to the increase in frequency in the indi- 

 vidual nerve fibers already described, a recruiting of 

 hitherto inactive neurons can be demonstrated, in the 

 frayed as well as in the intact nerve (153, 155). Thus, 

 the regulation of the depth of inspiration occurs 

 through two mechanisms: an increase in the dis- 

 charge frequency of the individual neurons, and an 

 increase in the number of discharging neurons. More- 

 over, in extreme dyspnea (resulting from collapse of 

 the lungs after bilateral vagotomy, or inhalation of 

 20 to 40 per cent carbon dioxide in oxygen), a marked 

 synchronization can be observed. This is expres.sed in 

 the electrical pattern through the occurrence of vva\es 

 which are absolutely synchronous in both phrenic 

 nerves and — whenever present — also in the vagal 

 efferents (209). 



A rhythmic activity can be demonstrated not only 

 in the plirenic nerve and diaphragm, but also in the 

 other respiratory muscles and the nerves supplying 

 them. Thus Bronk & Ferguson (29) were able to 

 demonstrate in decerebrate cats, that those intercostal 

 nerve fibers which run to the external intercostal 

 muscles (with the exception of the interchondral 

 portion) discharge only during inspiration. On the 

 other hand, fibers which innervate the internal inter- 

 costal muscles discharge only during expiration. 



Electromyographic studies in man (35-37, 106, 145) 

 have demonstrated that during quiet respiration, in 

 addition to the diaphragm, only the scalene muscles 

 show an increase in activity during inspiration, while 

 expiration occurs passively. But during voluntarily 

 forced or dyspneic respiration, the muscles listed in 

 the following table may be innervated. 



