Structure of Smooth Muscle. 531 



VIII. The Behavior of the Nuclei During Contraction. 



1. The form of the contracted nucleus. 



Two ideas have been advanced as to the form of the contracted 

 nucleus. The first is that during contraction there is an active 

 shortening and thickening of the nucleus, so that it changes from 

 a rod-shaped to an oval-shaped structure. This is maintained by 

 Henneberg, Heiderich and Soli, among recent workers. The 

 other is that during contraction the nucleus is passively or perhaps 

 actively folded or twisted into a spiral. This type was described 

 by Forster and Schlater. 



In all the smooth muscle studied, during contraction the nuclei 

 are drawn closer together in the contraction waves than in the 

 uncontracted areas. They undergo distinct decrease in length and 

 increase in thickness. Figs. 22, 37. This can be observed as well 

 in fresh as in fixed material. In fact when living smooth muscle 

 is stimulated to contraction under the microscope the contraction 

 of the nucleus can be observed. The nuclei (with the possible 

 exception of the extremely long nuclei in arteries) do not undergo 

 folding or spiral twisting during contraction. However, in normal 

 contraction, when an uncontracted fiber is caught between fibers 

 that have contracted, both it and its nucleus may be passively twisted 

 or folded. 



Frequently in the walls of contracted arteries spiral nuclei were 

 observed, Figs. 81-84, from the carotid of ox. But Figs. 60, 61, 

 78-80, show conclusively that shortening and thickening of the 

 nuclei do occur. It is probable that when both nucleus and fiber 

 contract at equal rates there occurs only the type of contraction 

 shown in Figs. 78-80. If, however, as undoubtedly may happen in 

 muscle with extremely long nuclei, the fiber contracts more rapidly 

 than does the nucleus, various forms of folding or twisting of the 

 nucleus will result. Figs. 81, 82, show nuclei, which are shorter 

 and thicker as well as spirally wound. Of course, this might be 

 due to passive shortening of a partly contracted nucleus. Many of 

 the spiral nuclei belong to fibers passively contracted. However, 

 they do occur in the contraction nodes of actively contracted fibers. 



