1 158 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



BLOOD 

 PRESSURE 



RENAL 

 BLOOD FLOW 



Sl&NAL 



TIME -30 SEC 



FIG. 15. Adjustments of renal blood How during asphyxia. 

 A: Asphyxia for 2 min. 15 sec. with sympathetic innervation 

 and adrenals intact. B: Asphyxia for 1 min. 50 sec. with sympa- 

 thetic innervation cut. C: "Perfusing" arterial pressure lowered 

 by partial occlusion of abdominal aorta. [From Celander (51).] 



vasodilator nerves both have a spontaneous activity 

 and participate in integrating mechanisms not re- 

 v^ealed in animals under anesthesia. 



NERVOUS CONTROL OF VENOUS SYSTEM 



There are indications in the earlier literature 

 that vascular reflexes influence vasomotor tone not 

 only on the arterial but also on the venous side. Quite 

 recently Alexander (8), using the distensibility of veins 

 as the criterion of such responses, observ'ed veno- 

 constriction following evocation of pressor reflexes by 

 central vagal stimulation, carotid sinus hypotension, 

 hypercapnia and hypoxia. Dilatation of the venous 

 bed occurred with carotid sinus hypertension. Alex- 

 ander showed too that venomotor tone rises sharply 

 with hemorrhage. In reversible shock venomotor tone 

 rose commensurately with recoxery. In irreversible 

 shock, on the other hand, vcnodilatation occurred 

 which Alexander (g) concluded led to pooling of blood 

 in the venous system, an important factor in circula- 

 tory failure. 



Measurements of venomotor tone in anesthetized 



dogs revealed a reflex mechanism capable of dilating 

 the intestinal veins in response to an elevation of pres- 

 sure in the abdominal caval .system. "Acting sub- 

 ordinate to the buffer reflexes of the arterial side of 

 the circulation these \enous reflexes would serv-e to 

 adjust venous capacity to venous load so as to con- 

 tribute to homeostasis in a vascular system" [Alex- 

 ander (10)]. 



SIGNIFICANCE OF THE .ADREN.'^L MEDULL.A 

 IN CARDIOVASCUL.\R REGULATION 



The commonly used term, 'the sympathicoadrenal 

 system,' reflects the view that the .sympathetic nervous 

 system has two effector components, a direct, nervous 

 and a hormonal, adrenal. The discharge of catechols 

 from the adrenal medulla is thought to reinforce the 

 direct action of the nervous impulses in the sympa- 

 thetic nerves. This concept arose particularly from 

 the work of Cannon and his associates on responses to 

 the various emergency states. These responses have 

 been described by Cannon (49), Cannon & Rosen- 

 blueth (50) and others. 



The general subject of adrenal medullary secretion 

 is presented in Chapter \"II of this Handbook by von 

 Euler. Here there will be considered recent work by 

 Celander (51 ) throwing doubt on the validitv of the 

 concept that adrenal medullar)- i-ecretion plays any 

 significant role in cardio\ascular regulation. C^elander 

 stimulated the splanchnic nerves of cats and deter- 

 mined the rate of the resulting epinephrine and nor- 

 epinephrine secretion. Infusing the catechols intra- 

 venously at this rate, he found the vascular effects to 

 be small compared with those produced by the original 

 stimulation of the splanchnic ner\e. Again, the intense 

 vasoconstriction in the skin, the skeletal muscles and 

 the kidneys produced by asphyxia became insignifi- 

 cant after sympathectomy in spite of intact adrenal 

 innervation (fig. 15). Celander further showed that 

 the \'ascular changes produced by stimulation of the 

 sympathetic nerves to a vascular area, with optimal 

 frequency of stimuli, were 10 to 20 times greater than 

 those elicited by the adrenal catechols secreted as the 

 result of corresponding stimulation of a splanchnic 

 nerve. 



The oljservations of Celander are important since 

 the\' strongly suggest that the central va.somotor con- 

 trol is dominated by the neural component. It may be 

 questioned whether the adrenals under physiologic 

 conditions play any significant role in the central 

 vasomotor control. 



