CIRCULATION IN SKELETAL MUSCLE 



'3 6 5 



uons. Raising the legs increases forearm blood flow 

 but it has no effect on blood flow in the hand, which 

 has very little muscle. The increase in forearm flow 

 is accompanied by increase in oxygen saturation of 

 the blood draining from the forearm muscles but 

 there is no change in the oxygen saturation of the 

 blood draining from the forearm skin. Therefore 

 the vasodilatation in the forearm must be in the mus- 

 cles. As mentioned previously, the reflex is mediated 

 by the sympathetic nerves as it is absent in sympa- 

 thectomized forearms and after deep nerve block. 

 Atropinization of the forearm does not weaken it. 

 Therefore vasodilator fibers do not seem to be impli- 

 cated, and it is probably due to decrease in the dis- 

 charge frequency in the vasoconstrictor fibers (167). 

 This low pressure receptor reflex in skeletal muscle 

 vessels may function so as to reduce the effect of 

 alterations in venous pressure upon the arterial 

 blood pressure. The effect of an increase in venous 

 pressure and cardiac output on arterial pressure 

 would be reduced because of reflex vasodilatation in 

 the muscles. Conversely reflex constriction in skeletal 

 muscles would tend to maintain arterial blood pres- 

 sure after a fall in venous pressure and cardiac output. 

 After major operations forearm blood flow is de- 

 creased for several days (1 10). This may be due to a 

 low pressure receptor reflex induced by hemorrhage 

 and decreased venous pressure. The importance of 

 low pressure receptor reflexes in man may be related 

 to the upright posture. 



IMPULSE FREQUENCY IN SYMPATHETIC VASOCONSTRIC- 

 TOR fibers. Folkow (90) has also investigated the 

 impulse frequency in the vasoconstrictor fibers to 

 skeletal muscle — postganglionic C-fibcrs. He con- 

 cluded that whereas in somatic fibers the maximum 

 discharge frequency may reach 50 per sec the maxi- 

 mum frequency in the vasoconstrictors to muscle 

 hardly ever exceeds 6 to 10 per sec, and normal 

 sympathetic tone is maintained at a discharge fre- 

 quency of only about 1 per sec. The experiments 

 forming the basis of this statement are most elegant. 

 The preparation is seen in figure 1 1 and a typical 

 result in figure 12. Atropine was given to block the 

 action of the vasodilator fibers. Venous outflow from 

 the isolated muscles of one cat's leg was recorded 

 before, during, and after i-min periods of stimulation 

 of the abdominal sympathetic chain at frequencies 

 increasing stepwise from 0.5 to 20 per sec. As the 

 frequency was increased the reductions of the blood 

 flow became greater and were maximal at a fre- 

 quency of 16 per sec. After stimulations at frequencies 



BLOOD REINFUSED 



STIMULATION 



OF 

 SYMPATHECTIC 



VENOUS DRAINAGE 

 OF MUSCLES 



OCCLUDE CAROTIDS 



BLOOD PRESSURE 



PARTIAL OCCLUSION 

 OF AORTA TO KEEP 

 PERFUSION PRESSURE 

 CONSTANT 



BLOOD PRESSURE 



fig. 1 1 . Preparation used by Folkow to investigate the 

 impulse frequency in sympathetic vasoconstrictor fibers. [After 

 Folkow (90).] 



below 7 or so per sec the blood flow returned rapidly 

 to its initial value, but its restoration became more 

 and more delayed after stimulations at progressively 

 higher frequencies. Folkow then turned his attention 

 to the venous outflow from the leg muscles of the 

 opposite side of the cat, the side on which the abdomi- 

 nal sympathetic chain was still intact; maximal 

 physiological stimulation of the vasoconstrictor fibers 

 (only) was induced by clamping both carotid and 

 vertebral arteries. The vagi had been cut. The possi- 

 bility of adrenaline secretion had been eliminated 

 and arterial inflow pressure into the leg was kept 

 constant by tightening a screw clip on the abdominal 

 aorta. The reduction in flow during carotid and 

 vertebral occlusion and the rate of its restoration 

 afterward were then compared with the reductions 

 in flow and subsequent rates of restoration that had 

 been obtained during and after electrical stimulation 

 of the peripheral end of the cut abdominal sympa- 

 thetic chain. The comparison showed that maximal 

 physiological stimulation of the vasoconstrictors (by 

 occlusion of the arterial supply to the head) caused 

 changes in muscle blood flow closely resembling those 

 recorded during and after electrical nerve stimulation 

 at 6 to 8 per sec. Blood flow during stimulation at 6 

 to 8 per sec was reduced by 80 per cent of the maxi- 

 mal reduction recorded during maximum electrical 

 stimulation at 16 per sec. 



