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HANDBOOK OF PHYSIOLOCV 



NEUROPHYSIOLOGY II 



able, after severance of the spinal cord at a high level, 

 to maintain — spontaneously or after previous treat- 

 ment with strychnine — a rhythmic, although ventila- 

 torily inadequate, respiration (120, 197). The presence 

 of spinal respiratory centers which are capable of 

 originating rhythmic impulses (24, 86) and of re- 

 acting to stimulation by carbon dioxide (198) was 

 therefore assumed, particularly in young animals. In 

 older animals, however, the bulbar centers take over 

 the respiratory regulation, so that respiration gen- 

 erally ceases after high severance of the spinal cord. 



Some of the respiratory tracts which connect the 

 pontine and bulbar centers with the motoneurons of 

 the diaphragm and intercostal muscles, according to 

 experiments in which the cervical cord was blocked 

 (158, 187) or stimulated (162, 165), are located in the 

 lateral column and end, for the most part, on the 

 homolateral side. However, in view of the so-called 

 'crossed phrenic phenomenon,' a crossing to the con- 

 tralateral side at the spinal level must be assumed 

 (158). Porter (158) demonstrated in dogs the ces- 

 sation of contractions in the left half of the diaphragm 

 after ipsilateral hemisection of the cord at the level 

 of the fourth cervical segment. When, following this, 

 the right phrenic nerve was severed so that the right 

 side of the diaphragm became paralyzed, strong con- 

 tractions of the left side immediately appeared. This 

 is evidently the manifestation of an influence from the 

 bulbar respiratory centers which descends in the right 

 side of the spinal cord and crosses over to the op- 

 posite side. Why this innervation comes into play 

 only when the phrenic nerve on the side contra- 

 lateral to the hemisection is severed is not quite clear. 

 The phenomenon certainly depends in part on the 

 absence of the phrenic afferent impulses. This subject 

 has been reviewed by Dolivo (58). 



Descending respiratory tracts are also found in the 

 anterior column, for a complete respiratory paralysis 

 occurs only when, after severance of the anterior part 

 of the lateral columns, the anterior columns are also 

 severed (151, 187). In the dog, Rothmann (167) 

 believed that the paths to the phrenic motoneurons 

 run in the anterior part of the lateral column (the 

 ventrolateral column) and that the anterior columns 

 contain the efferent paths to the intercostal muscles. A 

 separation into inspiratory and expiratory efferent 

 paths was attempted by Pitts (151) in the cat; lesions 

 were placed either in the inspiratory or in the ex- 

 piratory center in the medulla and the degeneration 

 of the axons in the descending fiber bundles was fol- 

 lowed with the Marchi method. In these experiments 

 degenerated fibers were found in the above-mentioned 



columns, but a separate path for the inspiratory and 

 the expiratory fibers could not be demonstrated. After 

 destruction of the ventral quadrants of the cervical 

 cord, cell degenerations were found in the inferior 

 reticular nucleus of the medulla, i.e. in the region of 

 the respiratory centers. 



INTRINSIC CONTROL OF RESPIR.^TORY .'\CTIVITY 



The question of the origin and mechanism of the 

 rhythmic alternation between inspiration and ex- 

 piration has intrigued investigators for many years 

 and, according to the direction of the investigation 

 undertaken, either humoral or nervous regulating 

 mechanisms have been postulated. Since this section 

 is concerned with the nervous regulation of respira- 

 tion, it will be necessary to forego a discussion of the 

 so-called chemical control of respiration. But, even 

 with this limitation, the literature on the subjects is so 

 extensive that, for a re\iew of the older literature, the 

 reader is referred to the summaries by Hess (89) and 

 by Cordier & Heymans (46). 



Assumed Mechanisms Leading to Rhythmic Respiration 



Most investigators in attacking the problem proceed 

 from the fact that in eupnea only the inspiratory 

 phase is active, while expiration occurs passively. 

 Eupneic respiration can be explained through two 

 possible mechanisms; either the inspiratory center 

 possesses the capability of sending out impulse salvos 

 in periodic intervals, or a continuously active in- 

 spiratory center is rhythmically blocked by a neighbor- 

 ing or superimposed inhibitory center. The first possi- 

 bility could be admitted if with quiet respiration the 

 neuronal activity in the medulla oblongata occurred 

 predominantly during the inspiratory phase. As a 

 matter of fact, neurons which discharge during in- 

 spiration have been demonstrated (56, 189), but one 

 finds just as many neurons which discharge continu- 

 ously; on the other hand, only a few are active during 

 expiration. A concentration of points showing ac- 

 tivity synchronous with inspiration has been reported 

 in the neighborhood of the nucleus hypoglossi and the 

 caudal part of the nucleus ambiguus (i), and in the 

 vicinity of the bulbar trigeminal nucleus (189). Of 

 these regions, ho\ve\er, neither the nucleus hypo- 

 glossi nor the caudal nucleus ambiguus belongs to the 

 respiratory center proper since they are the point of 

 origin for efferent motor fibers. Thus it would seem 

 that the in\cstigations of electrical acti\itv do not 



