THE NEURAL CONTROL OF RESPIRATION 



through the vagal nerves: with the expansion of the 

 lungs an inhibition of inspiratory and furthering of 

 expiratory activity; with the diminution of the lung 

 volume an exactly opposite influence. 



Some insight into the mechanism of such a respira- 

 tory regulation has been afforded by action potential 

 investigations (2, 49, 112, 146-149). Distention of the 

 lungs produces an increase in electrical activity in the 

 intact trunk of the vagus, as well as in the frayed 

 nerve, which has been attriijuted to the stimulation of 

 stretch receptors in the lung. The exact location of the 

 receptors is not yet known; it has been suggested as 

 subpleural (196) or as peribronchial (199, 200). The 

 frequency of the action potentials in the isolated nerve 

 fiber, and the number of stimulated nerve elements, is 

 directly proportional to the rapidity with which the 

 increase in lung volume results (49) and to the degree 

 to which the lungs are distended. This would seem to 

 be valid, in principle, for both types of receptors in- 

 vestigated up to the present, the slowly adapting (2) 

 and the rapidly adapting stretch endings (112). 

 During inspiration the receptors discharge with an 

 impulse frequency up to 100 per .sec; during ex- 

 piration many elements cease discharging altogether, 

 others discharge with at most 30 impulses per sec. 

 It has i^een assumed that with the increase in the 

 number of afferent impulses in the vagus the inspira- 

 tory center is increasingly inhibited, so that a shorten- 

 ing of the duration and a decrease in the depth of 

 inspiration results (2, 153, 155). When this inhibition 

 is abolished through bilateral vagotomy, the inspira- 

 tions ijecome longer and deeper. 



But the inhibition of inspiratory activity cannot 

 represent the only fimction of the vagal fibers from 

 the lungs, for stimulation of the central stump of the 

 vagus (with stimuli only slightly above threshold and 

 a stimulus frequency between 20 and 40 per .sec.) 

 elicits either a more or less pronounced tonic in- 

 spiratory reaction or an acceleration of the respiration, 

 depending on the animal species. On the other hand, 

 with stimulus frequencies between 100 and 300 per 

 sec, a lengthening of the expiratory phase and a de- 

 crease in the depth of inspiration is obtained. This 

 dependence of the result of afferent vagal stimulus on 

 the frequency has up to now been demonstrated in 

 the rabbit (205), thecal (161, 180), the monkey (207) 

 and the guinea pig (143, 144). It has been particu- 

 larly well investigated in the rabbit. In this animal, 

 action potential measiu'ements on the cranial stump 

 of the electrically stimulated vagus, with simultane- 

 ous registration of a pneumogram, have demon- 

 strated that the reflex reversal appcarint; with increase 



in the stimulus frequency can be elicited by stimuli 

 just strong enough to excite the a and /3 fibers but 

 which is subliminal for the more slowly conducting 

 fibers. With stronger stimuli (three to four times the 

 threshold intensity for a fibers), a stimulation with 

 low frequencies produces a more marked inspiratory 

 effect, occasionally an inspiratory tetanus. A higher 

 frequency of stimulation can then result in a respira- 

 tory arrest in expiration with active participation of 

 the expiratory muscles. In both cases, however, the 

 more slowly conducting 5 fibers are always excited. 



On the basis of these experiments, Wyss (208) 

 believes the Hering-Breuer reflex is due to a central 

 stimulation from stretch receptors which can dis- 

 charge with a high or a low impulse frequency, de- 

 pending on the degree of distention of the lungs. The 

 assumption is then made that the inspiratory center 

 reciuires a lower degree of summation than the ex- 

 piratory center, so that it can be activated by afferent 

 stimulation of a lower frequency. An inhibition of 

 inspiration occurs only wiien, as a result of increasing 

 temporal and spatial summation, the expiratory cen- 

 ter becomes sufficiently activated to exert an inhibi- 

 tory influence on the inspiratory center (fig. 6). For 



FIG. 6. Schematic representation of weak inspiratory- 

 facilitating reflex from small lung volume {left) and strong 

 inspiratory-inhibiting (expiratory) reflex from large lung 

 volume {right). /, bulbar inspiratory center; E, bulbar ex- 

 piratory center; IS, solitary tract system. [From Wyss (208)] 



