i 3 68 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



Minutes 



fig. 15. Results showing that active cholinergic vasodilator 

 nerves to human muscle contribute to the vasodilatation in the 

 forearm muscles during stress. Open circles: hand blood flow. 

 Solid circles : forearm blood flow. During the time represented by 

 the rectangle it was suggested to the subject that he was suffering 

 from severe blood loss. [Blair et al. (39).] 



that they were suffering from severe blood loss. 

 In one experiment, the result of which is illustrated 

 in figure 15, forearm blood flow rose from 8 to 50 

 ml per min, while hand blood flow was not affected. 

 In another subject oxygen saturation of blood drain- 

 ing from muscle rose from 20 to 65 per cent. In six 

 subjects the vasodilator responses to a wide variety 

 of stimuli were found to be reduced by atropinization 

 of the forearm. They concluded that activation of 

 cholinergic vasodilator nerves to human muscle con- 

 tributed to the vasodilatation in the forearm muscles 

 during stress. 



Do Posterior Root Fibers Affect Muscle Blood Flow? 



There is no important evidence of any efferent path- 

 way via the posterior roots to animal or human muscle 

 vessels. These fibers certainly play no part in the 

 arterial baroreceptor reflex which is mediated solely 

 by sympathetic vasoconstrictor fibers (101), and 

 they play no part in hypothalamic vasodilatation 

 which is mediated solely by sympathetic vasodilator 



fibers (78). In man, too, sympathectomy of the limbs 

 completely abolishes all known vascular responses 

 of central origin. 



There remains the question of whether or not 

 axon reflexes from sensory endings in muscle influ- 

 ence the vessels. If so, then stimulation of the posterior 

 roots should cause "antidromic" vasodilatation in 

 muscle. Celander & Folkow (56) investigated the 

 effect on paw flow and muscle flow of stimulation 

 of the peripheral cut ends of L5-S2. There was 

 marked vasodilatation in the paw but no effect on 

 the circulation in muscle. Nor was there any change 

 in the flow through muscle when the small C-fibers 

 were selectively stimulated by heating the sciatic 

 nerve. They concluded that axon reflexes in muscle 

 were of very little significance. 



Effect of the Temperature-Regulating Center 

 on the Circulation in Muscle 



It is well known that rise in body temperature re- 

 leases sympathetic vasoconstrictor tone in the paws. 

 However the temperature-regulating center has 

 very little influence on the circulation in muscle. 

 Folkow et al. (100) heated the cat's hypothalamus by 

 diathermy and recorded marked cutaneous vaso- 

 dilatation, but there was no change in the venous 

 return from the skinned hind parts. In man, Edholm 

 et al. (75) recorded marked increase in flow in the 

 forearm during body heating, but this was absent 

 in the opposite forearm in which the cutaneous cir- 

 culation had previously been arrested by adrenaline 

 electrophoresis. This has been confirmed by observa- 

 tions of muscle flow made with the Hensel needle 

 (15) and by measurements of the oxygen saturation 

 of blood obtained from veins draining muscle (165). 

 Body heating which causes sweating and rise in 

 mouth temperature does not increase blood flow in 

 skeletal muscle. 



Role of Sympathetic Fibers to Muscle in Exercise 



Gaskell (108, iog) at first thought that vasodilator 

 nerves were responsible for the vasodilatation In 

 muscle in exercise, but later he realized that the ac- 

 tion of metabolites was more important (107). There 

 is strong evidence that the hyperemia of exercise is 

 due to the action of a local mechanism which is 

 triggered by the process of contraction. For example, 

 Hilton (120) and others showed that the muscular 

 contractions and vasodilatation elicited by motor 

 nerve stimulation are both completely abolished by 



