■44 



HANDBOOK OK PHYSIOLOGY 



NEUROPHYSIOLOGY II 



circumspection is required in evaluating the general 

 validity of the results. 



Gernandt et al. (97) found a pronounced rise of the 

 splanchnic fiber firing in cats during asphyxia and 

 anoxia. Inhalation of pure oxygen decreased the 

 spontaneous activity. After section of all buffer nerves 

 the splanchnic discharge no longer reflected changes 

 of oxygen tension in the blood. On the other hand., 

 an elevation of the carbon dioxide tension in the blood 

 still caused an increase in splanchnic nerve activity. 

 Dontas (66) observed in cats an increase in the 

 splanchnic discharge after injection of chemoceptor 

 activating drugs. 



Aflferent stimulation of the sinus and vagus nerves 

 has been reported to produce sometimes depressor 

 effects, sometimes pressor effects, the response vary- 

 ing with the parameters of stimuli and the anesthesia 

 [Douglas et al. (67)]. Some evidence emerged that 

 there were three groups of efferent fibers, one group 

 of depressor fibers of large diameter, an intermediate 

 group of fine pressor chemoceptor fibers and, lastly, a 

 third group of depressor fibers of small diameter. 

 Neil and co-workers (167, 168) found that stimula- 

 tion of the carotid sinus nerve in cats elicited a de- 

 pressor response if the animals were under chloral 

 hydrate or pentobarbital or were decerebrate, but 

 that the response was mainly pres.sor in animals under 

 chloralose. 



The well-known pressor response to occlusion of 

 the common carotids has usually been attributed to 

 the abrupt reduction of arterial pressure in the oc- 

 cluded sinus region, the subsequent reduction of the 

 baroceptor activity and the consequent increase in 

 vasoconstrictor discharge, von Euler & Liljestrand 

 (212) have been more or less alone in their opinion 

 that the pressor response to carotid occlusion is due 

 to activation of the chemoceptors since this response 

 was dependent on the oxygen tension in the arterial 

 blood. During inhalation of pure oxygen the pressor 

 respon.ses virtually disappeared. Landgren & Neil 

 (146) found in cats that the electrical activity in the 

 chemoceptor fibers from the carotid body increases 

 during the pressor response to occlusion of the com- 

 mon carotids but not if oxygen is inhaled. 



The activity of the aortic chemoceptors has been 

 described by Landgren & Neil (147). The conduction 

 rate of chemoceptive fibers was reported by Paintal 

 (172) to be 10 it 2 m per sec. 



() Cardiac and pulmonary receptors and afferent fibers. 

 As was first shown by von Bezold (41 , 205), von Bezold 

 & Hirt (206) and Jarisch (132), injection of mistletoe 

 extracts and veratrum alkaloids produces a reflex 



fall of arterial pressure and heart rate. Several identi- 

 cal or similar cardiovascular reflexes produced by a 

 number of drugs arise from the heart and lungs. The 

 afferent impulses run in the vagus nerves, but the 

 fibers mediating these impulses have so far not been 

 identified with certainty, which is not surprising 

 since it seems very probable that the fibers are small 

 in diameter and hence difficult to record from. Ana- 

 tomical studies of the cardiac Ijranches of the vagi 

 consistently show that there are few if any fibers above 

 10 jx in diameter and that the numljers rapidly in- 

 crease with decreasing fiber diameter. Probably there 

 are also a large number of unmyelinated fibers. 



If we except aortic baroceptive and chemoceptive 

 fibers, there are among the cardiovascular afferent 

 fibers, according to Whitteridge (222), the following 

 groups. 



/) Afferent fibers from venous side, type .A. These 

 fibers, first described by .Amann & Schaefer (13), have 

 been related to the cardiac cycle and to the pressure 

 waves in the \enous pulse. Each of the three pressure 

 waves, a, c, v, in the venous pressure curve may be 

 accompanied by a volley of impul.ses [Walsh & Whit- 

 teridge (216), Jarisch & Zotterman (133), Schaefer 

 (188)]. There is a linear relation between pressure 

 and impulse frequency. Due to the fairly rapid adapta- 

 tion rate the receptors corresponding to the A fibers 

 are jjelieved to respond to rises of pressure but not 

 to the absolute level of pressure in the atrium [Strup- 

 pler (ig8)]. They behave as if they lie in series with 

 the muscle fibers. They undoubtedly arise from the 

 atria at the openings of the great \eins [Jarisch & 

 Zotterman (133), Paintal (170)]. Their conduction 

 rate is 20 ± 5 m per sec. 



2) "Venous fibers, type B. These fibers show a late 

 systolic volley roughly related to the v wave. They 

 arise from endings in the atrial walls; their conduc- 

 tion rate is 13 ± 5 m per sec. [Paintal (170)]. Their 

 behavior suggests that they lie parallel with the atrial 

 muscle fibers. 



j) Small fibers. These fibers can be excited by 

 pinching the ventricles and are probably stimulated 

 by veratrin, according to Jarisch & Zotterman (133). 



Fibers with a brief discharge during the isometric 

 contraction phase only, perhaps arising in the ven- 

 tricles or septum, were detected by Whitteridge (221), 

 Dickin.son (63) and Pearce (173). Paintal (171) ob- 

 served in the cat fibers firing in response to pheinl- 

 diguanide. Their conduction rate was 6 m per sec. 



So far no analysis has been made with an electro- 

 physiologic technique of the efferent cardiovascular 

 discharge patterns accompanying the Bezold-Jarisch 



