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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



© 



fig. 7. Model for the arrangement of tension muscles and 

 elastic tissue. Stretch in vertical direction, a: Tension muscles 

 relaxed; b: tension muscles contracted. [Kapal & Bader (43).] 



fig. 7A. Baroceptor in the adventitia of the human aortic 

 ar< li a: End fiber of the aortic depressor nerve; b: network; 

 c: end network; d: neurofibrils. Method after Biclschowsky. 

 X 1100. [Abraham (1).] 



membranes, and the contraction of the smooth mus- 

 cles seems to influence only the part of the curve 

 which is ascribed to the extension of elastic tissue, 

 Kapal & Bader (43) have designed a model which is 

 similar to an arrangement which Burton (20) had 

 published in 1953 in his highly stimulating review 

 article. It shows the action of the tension muscles in 

 elastic arteries. 



In figure ~ja the smooth muscles are relaxed; in 

 yb, thev are contracted. The smooth muscles fasten 



on the elastic fibers or membranes at right angles to 

 the direction in which the elastic fibers are stretched. 

 These fibers become elongated by contraction of the 

 smootli muscles, but the circumference of the vessel 

 is not changed (fig. jb). The consequence is a rise 

 in the tension of the elastic fibers. Since the model will 

 be involved in a stretch in the direction of the elastic 

 fibers, the tension muscles do not need to develop 

 tension as great as the total wall tension, but can 

 increase the stress on the elastic tissue with relatively 

 little work. 



This model has an advantage to neurophysiologists 

 as well as to muscle physiologists. Heymans & Delau- 

 nois (37) have shown that the blood pressure decreases 

 if the smooth muscles of the carotid sinus are stimu- 

 lated by noradrenalin. Heymans et al. (38) obtained 

 the same results by elongating the carotid sinus. They 

 concluded from these experiments that the pres- 

 soreceptors located in the carotid sinus, which cause a 

 decrease of the mean blood pressure after stimulation, 

 do not respond to the blood pressure, but rather to the 

 wall tension [see Heymans & van den Heuvel-Hey- 

 mans (39)]. It is very likely that the pressoreceptors 

 situated in the aorta work in the same way. 



The pressoreceptors appear as a very fine network 

 of neurofibrils (fig. J A). They are mainly located in 

 the adventitia of the carotid sinus [Sunder-Plassmann 

 (89)] and in the adventitia and the outer part of the 

 media of the aortic arch [Seto (85)]. Stohr (88) has 

 the impression that this network of neurofibrils shown 

 in figure yA may be only a part of the whole 

 neurofibril mass of which the pressoreceptor is 

 constituted. He assumes that smaller fibrils exist but 

 are not visible because of limitations in the staining 

 method and in the optical properties of the light 

 microscope. 



There are very few clues as to how the network of 

 the pressoreceptor is related to the surrounding tissue. 

 Sunder-Plassmann (89) has shown that the media of 

 the carotid sinus is thinner than that of the nearby 

 vessel, but the membrana elastica externa is thickened. 

 The elastica externa in the carotid sinus shows a 

 sharp boundary separating it from the media, but a 

 more gradual merging with the adventitia. In this 

 diffuse zone, which shows collagen fibers and large 

 elastic membranes, the pressoreceptors of the carotid 

 sinus are located, and the neurofibril networks show 

 a certain degree of adaptation to the shape of the 

 connective tissue. Abraham (1) describes the neuro- 

 fibril networks of the aortic arch as nestling flat 

 against the vessel wall. Their position follows the direc- 

 tion of the fibrous elements. 



