PATTERNS OF THE A-V PATHWAYS 



925 



Vasomotion 



The word "vasomotion" has had extensive use 

 since its first appearance in 1944 (20). The term was 

 used at this time by Chambers and Zweifach to 

 describe the spontaneous contractions and dilations of 

 small arterioles (metarterioles) and the muscle cells 

 of their branches (precapillary sphincters) in the rat 

 mesentery, also called mesoappendix. It has subse- 

 quently come to be used to indicate observed diame- 

 ter changes of any blood vessel. 



Reports of variations in the caliber of small blood 

 vessels have been in existence for almost as long as 

 microscopic studies of them have been carried out. 

 Special interest in this phenomenon was shown during 

 the period of controversy over capillary contractility. 

 In the years following the introduction of the rabbit 

 ear chamber for microscopic observation of small 

 blood vessels, numerous papers appeared in which 

 spontaneous alterations in small blood vessels were 

 described. Clark & Clark (29) spoke of the normal 

 occurrence of spontaneous rhythmic contractions of 

 arteries down to their smallest branches. Different 

 arteries and parts of arteries were seen to contract at 

 different rates (30). Sandison (106) reported rhyth- 

 mical contractions of arterial vessels but saw no active 

 contractions of veins or venules. Clark el al. (35) be- 

 lieved that an intact nerve supply was necessary for 

 spontaneous contraction of the arterial vessels. Nu- 

 merous other investigators reported periodic altera- 

 tions of small vessel diameters (17, 57, 62, 141 ). 



Chambers & Zweifach (20) described vasomotion 

 in terminal arterioles and larger arterioles as irregu- 

 larly periodic dilatations that are slower and more 

 regular than the diameter changes seen in metarteri- 

 oles and precapillary sphincters. When metarterioles 

 were exhibiting vasomotion, they usually showed a de- 

 crease in diameter of about one-third, but were even 

 seen to reduce the diameter by one-half or more. 

 Other observations were that when a tissue was 

 hyperemic, the dilator phase was most prominent, the 

 constrictor phase dominating in ischemic tissue. No 

 synchrony in vasomotion of neighboring arterioles 

 was seen. Vasomotion was seen to continue in a metar- 

 teriole in the absence of blood flow through it. Also, 

 diminished blood flow was followed by an increased 

 dilator phase, while increased blood flow apparently 

 brought on an intensified constrictor phase. Vaso- 

 motion was affected by local environmental condi- 

 tions (irritation of the tissue caused vasomotion to 

 disappear). Vasomotion also stopped when the ani- 

 mal was deeply anesthetized. 



The recurrent vasomotion in metarterioles was 

 considered by Chambers and Zweifach to be the 

 factor which controls the rate of flow through the 

 central vessels of a capillary bed while the vasomotion 

 of precapillary sphincters controlled the flow through 

 the true capillaries. 



An extensive discussion of vasomotion bv Nicoll & 

 Webb (88) in 1946 described various types of caliber 

 changes seen in both arterial and venous vessels. They 

 suggested that the word vasomotion should be pre- 

 ceded by a suitable adjective to indicate a specific kind 

 of change in vessel diameter, e.g., if the caliber change 

 is brought about by contraction or relaxation of the 

 vascular musculature, reference should be made to 

 active vasomotion. If, on the other hand, caliber 

 changes are produced by internal or external altera- 

 tions of pressure not due to the activity of vascular 

 musculature, reference should be made to passive 

 vasomotion. Active vasomotion was further classified 

 into three groups. "Tonic active vasomotion" was 

 the term used to describe the maintained contraction 

 of arteries, considered to be a tonus response. Super- 

 imposed on tone was the rapid contraction and relaxa- 

 tion of vessels that occur in response to nerve impulses. 

 This was called irregular active vasomotion. The third 

 type of movement was called rhythmical active vaso- 

 motion and referred to a regular alternation of con- 

 traction and relaxation of the vascular smooth muscle. 



An analysis of the various types of active vasomo- 

 tion, as given by Nicoll & Webb (88), follows. That 

 arteries and arterioles possess tone, or are in a con- 

 tinuous state of active contraction, can best be demon- 

 strated by noting the marked increase in their diame- 

 ter that follows denervation. The diameters of ar- 

 terial vessels in a denervated area have been shown to 

 increase 27 to 29 per cent following nerve section 

 (134). Nicoll and Webb state that the outstanding 

 characteristics of tonic active vasomotion are its con- 

 stancy and sluggishness, and suggest it may function 

 to correlate blood vessel volume and blood fluid 

 volume. 



Irregular active vasomotion is characterized by 

 rapid changes in the caliber of arteries and arterioles. 

 The changes vary as to their magnitude and the 

 length of time they endure. Such caliber changes are 

 the direct result of impulses from the vasomotor 

 nerves, controlled by the vasomotor center. Nerve 

 section obliterates this type of activity. Nicoll and 

 Webb are of the opinion that the function of irregular 

 active vasomotion is to modify peripheral resistance 

 and also to regulate the pressure gradient in the 

 capillaries. 



