PATTERNS OF THE A-V PATHWAYS 



92 7 



muscle as a component of their walls exhibit marked 

 active vasomotion. Nicoll and Webb describe the con- 

 traction as sharp, and one that sweeps along the vein 

 as a peristaltic wave in a central direction. Each wave 

 of contraction seems to originate at a distal valve 

 and die out at the next valve central to it. Since the 

 majority of valves are located at the confluence of 

 tributaries, valve action and blood flow may seem 

 unrelated due to asynchronous waves in two segments 

 which are separated by their valves. 



Two major tributaries which form a vessel may con- 

 tract alternately. One tributary may empty into a 

 segment ahead while the other tributary is relaxed. 

 Irregular flow results when the frequency of contrac- 

 tion of the two tributaries is not coordinated. Single 

 tributaries empty into a segment of the central vessel 

 during its period of relaxation. 



Nicoll and Webb adopt the concept that vascular 

 smooth muscle cells possess an inherent ability to 

 change their tonus or exhibit sudden contraction in 

 response to changes in their immediate environment. 

 In order to determine what environmental changes 

 affect vascular smooth muscle, they observed changes 

 in vasomotion in response to nerve stimulation, various 

 gas mixtures, and temperature changes. They found 

 that arteries and large arterioles responded to nerve 

 stimulation with intense constriction. The smaller 

 vessels, arcuate and terminal arterioles, precapillary 

 sphincters, veins, and venules never showed initiation 

 or modification of active vasomotion as a direct re- 

 sponse to central impulses (129). Changing the local 

 environment by flow of constant current between a 

 single fluid electrode on the wing surface and an 

 indifferent electrode produced alternate areas of 

 marked constriction and dilatation on arteries and 

 arterioles. Reversal of the current caused previously 

 constricted areas to dilate and previously dilated 

 areas to constrict. Nicoll & Webb (89) believe that this 

 observation should be taken into account when inter- 

 preting responses to direct excitation of nerves with 

 microelectrodes. Inhalation of carbon dioxide in a 

 specific concentration proved to be a powerful stimu- 

 lus of the contractile phase of active vasomotion. 

 Variations in temperature showed the frequency of 

 active vasomotion to vary directly with the tem- 

 perature. 



Changes in internal pressure of vessels have marked 

 effects on vasomotion. Slow changes in pressure caused 

 a vessel to adjust its tone gradually. Sudden increases 

 in pressure, however, first caused a vessel to be dis- 

 tended mechanically and then to contract with great 

 intensity. The contraction then spread along the 



vessel as a peristaltic wave. Nicoll and Webb suggest 

 that rhythmical variations in small arterial vessels 

 may originate from sudden internal pressure changes 

 at their origins from parent vessels. 



Spontaneous changes in vascular tone, resulting 

 from a rise or fall in internal pressure, were demon- 

 strated. After blood flow to an area had been stopped 

 by occlusion of a small supplying artery and was then 

 allowed to resume, the vessels were first distended as 

 they filled and then were seen to contract as a response, 

 presumably, to the distention. Thus, blood was forced 

 along to the next branches. Another example of ad- 

 justment of tone to a change in internal pressure is seen 

 following denervation. The resulting dilatation of the 

 main arteries probably raises internal pressure in the 

 arterioles and increases their tone, sometimes reducing 

 the flow through the arterioles to the capillary beds 

 due to the reduction in lumen of the arterioles. 



Nicoll and Webb express the opinion that the ul- 

 timate result of active vasomotion in terminal arteri- 

 oles is to establish flow through capillary beds, the 

 muscle cells of the terminal arterioles being the prin- 

 cipal targets of changes in the local environment. 



Active vasomotion in venules and veins may repre- 

 sent the adaptation of an inherent property of vascu- 

 lar smooth muscle to aid venous return. Nicoll and 

 Webb suggest that this activity may be more wide- 

 spread in vascular systems than is currently recog- 

 nized. It may be more prominent in the veins of the 

 bat wing than in small veins in other mammals due to 

 the structure of the wing. Pressure within the veins 

 seems to be the principal stimulus for the action. 



Experimental evidence in confirmation of this pro- 

 posal appears in the investigations by Wiedeman 

 (135), in which veins in the bat wing were observed 

 during elevations in venous pressure. Both diverting 

 excess blood into a vein by ligating other venous path- 

 ways and infusing dextran to increase total volume 

 caused a significant increase in cycles of venous vaso- 

 motion. Similar results were obtained when venous 

 pressure was elevated by direct infusion with saline 

 (136). 



Although venous vasomotion is most prominent in 

 the bat wing and shows a definite rhythmicity (fig. 24), 

 spontaneous changes in pressure which are unrelated 

 to arterial pressure or respiration have been demon- 

 strated in small veins in hind legs of dogs (59, 1 37) 



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fig. 24. Rhythmical variations in the pressure in a vein 

 resulting from alternate contraction and relaxation. 



