870 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



below). The smooth muscles of the aorta and the 

 pulmonalis are almost exclusively tension muscles. 

 But the proportion of tension muscles diminishes 

 toward the periphery. The smallest arteries and 

 arterioles have almost no tension muscles. They may 

 appear again on the venous side of the circulatory 

 system, but not in as large numbers as in the elastic 

 arteries [Grau (34)]. 



The ring muscles are connected with each other. 

 How they are related to the elastic and collagen tissue 

 is not certain, but it is very likely that they have slack 

 connections with both tissues. Since smooth muscles 

 are almost completely surrounded by reticular fibers, 

 it is possible that these fibers bind them together. The 

 ring muscles form the greatest part of the wall in the 

 muscular vessels, where they form a helical arrange- 

 ment [Fischer (23), Schultze-Jena (84)]. Arterioles 

 and precapillary sphincters consist mostly of ring 

 muscles. 



Smooth muscles have the general quality of sponta- 

 neous activity and self-conduction [see Biilbring (17)]. 

 Bozler (16) has concluded from this behavior that 

 they form a syncytium, which would mean that the 

 individual muscle cells are interconnected by proto- 

 plasmic bridges. But in reality they form a network 

 in which every muscle fibril is surrounded by its own 

 membrane, so that there are double membranes at 

 places where the cells are in contact with each other 

 [see Prosser (68)]. This network acts like a functional 

 "syncytium," since an excitation can be conducted 

 over the double membranes. This double membrane 

 has a high resistance, and therefore the conduction in 

 smooth muscles is much slower than that in nerve 

 fibers. The conduction can be propagated over the 

 whole organ, as in the uterus or the ureter (single-unit 

 smooth muscles), or it can be limited to a certain area, 

 as in the intestine [Bozler (16), Greven (35), Biilbring 

 (17)]. The limitation results from the presence of a 

 higher resistance of the double membranes at certain 

 places. The resistance can be changed so that the area 

 which responds to a stimulus can be increased or 

 decreased. Another characteristic of the syncytium 

 is its response to stretch [Biilbring (17)]. If smooth 

 muscles of the intestine are stretched, the membrane 

 depolarizes and spikes are produced (fig. 4). Contrac- 

 tion occurs and tension rises in direct proportion to the 

 increase in spike frequency. But in addition to having 

 independent conductivity and excitability, smooth 

 muscles also receive innervation from both the sympa- 

 thetic and parasympathetic nerve system. There are 

 ganglion cells around the adventitia [Leontowitsch 

 (55)] and nerve fibers in the media [Boeke (15)] of the 



15 



o 



uu 



1/1 

 IO p 



— 

 in 



c 111 



O 2 4 6 8 



fig. 4. Graph showing correlation between membrane 

 potentials, spike frequency, and tension during spontaneous 

 pendular rhythm recorded for 10 min. [Biilbring (17).] 



blood vessels [see also Staubesand (87)]. The auto- 

 nomic nervous system can change the spontaneous 

 activity of the smooth muscles by changing their 

 membrane potentials. 



In addition to the syncytium-like smooth muscles, 

 there are also multiple-unit smooth muscles which are 

 neither self-conducting nor spontaneously active. They 

 receive extensive innervation and appear to be or- 

 ganized in some tvpe of motor-unit plan [see Prosser 

 (68)]. 



Little work has been done on the problem of the 

 excitation and conduction of vascular smooth muscle. 

 Therefore it is hard to say whether it represents a 

 multiple-unit system or a syncytium. Monnier (62) 

 has shown that the conduction of excitation in the 

 mesenteric artery of cattle (an artery of the muscular 

 type) is very slow (only about 2 mm/sec). This is 

 much slower than the conduction of any known nerve, 

 and in a range similar to other syncytium-like smooth 

 muscles [see Biilbring (17)]. The mesenteric artery 

 also responds to stretch with a contraction. It may be 

 assumed therefore that the muscles in the peripheral 

 arteries behave as a syncytium. This is likely, in view 

 of the relationship of blood pressure to flow. For in- 

 stance, Thurau & Kramer (91) have shown that the 

 flow in the kidney becomes constant if the pressure is 

 raised above 90 mm Hg. This special flow-pressure 

 relationship is due to an increase of resistance, effected 

 by contraction of the smooth muscles in the pre- 

 glomerular arteries. Similar behavior in the arteries of 



