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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



XXXIII XXXVI XXXIX 



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FIG. 7 a: Horizontal section through 

 the medulla showing the location of 

 the vertical and horizontal planes 

 drawn in h and c, respectively, b and c: 

 Location of the vagal reflex centers in 

 the medulla of the rabbit relative to the 

 fasciculus solitarius [FS], the nucleus of 

 the hypoglossal nerve (NcXIl), the 

 inferior olive iOi), the decussation of the 

 medial lemniscus (.S"), the descending 

 pyramidal tract (Py) and the spinal tract 

 of the trigeminal nerve (T.sp.V). Relay 

 stations for vagal expiratory (£), aortic 

 depressor (£)) and vagal inspiratory (J) 

 reflexes on the medial border of the 

 solitary bundle at the level of the 

 promontorium gliosum (the abex, PG) . 

 [From Oberholzer (141).] 



— S 



the proprioceptive control of respiration only the 

 rapidly conducting components of the vagus nerve 

 are considered important. They are believed to 

 establish contact with the inspiratory as well as the 

 expiratory center through an as yet unknown manner 

 of distribution and to stimulate both centers simul- 

 taneously. Slowly conducting fibers may be significant 

 for the occurrence of a strong inspiratory reaction 

 following collapse of the lungs or a stronger stimula- 

 tion of the vagus (208), but their role in the vagal 

 control of respiration is not yet clear. Evidence for the 

 simultaneous action of the vagus fibers on both 

 respiratory centers is seen in the appearance of re- 

 bound phenomena after expiratory reactions produced 

 by stimulation of the vagus with higher frequencies. 



In the rabbit it has also been possible to localize 

 the relay centers for the vagal respiratory reflexes in 

 the medulla. Through high-frequency electrocoagula- 

 tion in the region of the caudal end of the tractus 

 solitarius, a small zone can be eliminated following 

 which electrical stimulation of the ipsilateral central 

 stump of the vagus yields only the expiratory vagal 

 reflex, but not the inspiratory reflex (142). Conversely, 



an expiratory region has been described lying 2 to 3 

 mm cranial from the obex and also belonging to the 

 system of the tractus solitarius, the destruction of 

 which causes solely the loss of the vagal expiratory 

 reflexes (8). But these regions should not be desig- 

 nated as respiratory centers since their bilateral 

 destruction causes only slight changes in the respira- 

 tion of animals previously subjected to bilateral 

 vagotomy. They have, therefore, been designated as 

 relay stations for vagal respiratory reflexes. Their 

 approximate location in relation to the nucleus of the 

 hypoa;lossus nerve, the obex and the inferior olive, as 

 well as to the relay station for the aortic depressor 

 reflexes, is diagramed in figure 7 (141). The actual 

 respiratory centers lie, according to what has been 

 said earlier, in the reticular substance of the medulla. 

 They are connected with the reflex zones of the soli- 

 tary tract through secondary neurons. The caudally 

 and medially located inspiratory center would seem 

 to be closely related to the inspiratory reflex center; 

 the less sharply circumscribed, more cranially and 

 laterally located expiratory center to the expiratory 

 reflex center. 



