i8 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



PTC 



A PC 



FIG. 4. Functional scheme of respiratory center. The primary 

 inspiratory mechanism is shown in black. Vertical hatching indi- 

 cates the inspiratory-facilitating component of intrinsic control 

 (including in the pons the assumed apncustic center, APC). 

 Horizontal hatching indicates the inspiratory-inhibitin,^ (expira- 

 tory) component of intrinsic control (including in the pons the 

 assumed pneumotaxic center, PTC). C'ro.ishatching indicates the 

 extrinsic control (solitary system). A' designates the vagus 

 nerve. The horizontal broken line separates the pontine level (P) 

 from the bulbar level (M). [From Wyss (208).] 



coeruleus, in which the pneumotaxic center ha.s 

 recently been localized, produces predominantly in- 

 spiratory reactions (16, 105), while expiratory effects 

 are more easily obtained from deeper lying struc- 

 tures. The validity of the postulation of a pneumo- 

 taxic center would seem, therefore, to require further 

 clarification. 



A third mechanism, which can also lead to a trans- 

 formation of tonic inspiratory activity into a rhythmic 

 one, is seen in the vagal control reflexes (see the later 

 section on extrinsic control). Therefore, in the sche- 

 matic representation appearing in figure 4, the two 

 closely associated central control mechanisms and the 

 vagal-proprioceptive mechanism are presented as a 

 functional unity. Since three different mechanisms 

 guarantee respiratory rhythmicity, it is understand- 

 able that an isolated interruption of the pontine con- 

 trol need not always lead to a disturbance of the 

 respiratory rhythm and that, after bilateral vagotomy, 

 the respiration in most animals mav continue rliythmi- 

 cally. However, in such cases, the medullary centers 



are much more easily deranged. For example, in deep 

 narcosis and under morphine (28, 102, 175), in 

 oxygen lack, and in states in which the arterial pres- 

 sure is reduced (31, 32), respiratory patterns of the 

 Cheyne-Stokes or Biot type can easily occur, in man 

 as well as in experimental animals, and under certain 

 circumstances a sudden respiratory failure is seen. 



An interpretation varying from the above-de- 

 veloped conceptions has been advocated l>y Rijiant 

 (163, 164). On the basis of his stimulation experi- 

 ments in the medulla oblongata and in the spinal 

 cord, with simultaneous registration of phrenic action 

 currents, he believes that one must locate the origin 

 of inspiratory tonus in a pontine center (the apneustic 

 center of Lumsden). The continuous activity from 

 this center, which of itself does not lead to a manifest 

 inspiration (respiration ocai/le), would then be modu- 

 lated, i.e. physicallv furthered or inhibited, from the 

 medullary region (cnitre bulhaire modidaleur). 



Intrinsic Mechanisnis Leading to Modification 

 of Basic Respiralorv Rhythms 



The basic rhythm de\eloped by the bulbopontine 

 respiratory centers is modified through \arious in- 

 fluences. The most important of these, affecting the 

 depth and rhythm of respiration, is the partial pres- 

 sure of carbon dio.xide in the blood. Its increase 

 produces, either directly or through a reflex mecha- 

 nism, a general activation of the meduUarx', pontine 

 and spinal respiratory neurons. The mechanism of the 

 direct action on the respiratory center is still open to 

 discussion. In general, it is assumed that carbon 

 dio.xide in molecular form exerts a direct effect on 

 the nerve cells in the center (43, 140, 172). Other 

 authors postulate an indirect effect on the cell ac- 

 ti\ity through a change in the pH of the intercellular 

 fluid (14, 80, 202-204) or through a change in the 

 intracellular hydrogen ion concentration (75, 76). 

 However, since the pCOo and the extra- and intra- 

 cellular pH are closely related to one another, the 

 discussion has more academic than practical interest, 

 especially since, up to the present, no central pH 

 receptors have been disco\'ered. 



An increase in the activity of the inspiratory center 

 under the influence of carbon dioxide leads, at first, 

 to an increase in the depth of inspiration, and at the 

 same time also to a stronger and more rapid excitation 

 of the pneumotaxic and expiratory centers. This 

 causes a more rapid change-over to expiration and, 

 therei^y, an increase in the respiratory frequency. 

 When the degree of excitation in the expiratory center 



