128 BIOCHIMICA ET BIOPHYSICA ACTA VOL. 12 (1953) 



THE SERIES ELASTIC COMPONENT IN MUSCLE* 



by 



DELBERT E. PHILPOTT and ALBERT SZENT-GYORGYI 



The Institute for Muscle Research, 

 Marine Biological Labovaiow, Woods Hole. Massachusetts (U.S.A.) 



The rubber-like elasticity of resting muscle fibers indicates the presence of folded 

 fibrous material which can be straightened out by stretching. The contractility of the 

 same fibers suggests the presence of fibrous proteins which can shorten and develop 

 tension b}^ their folding. The more a protein filament would be folded, the less tension 

 it could develop by its further folding. Since it is the primary function of muscle to 

 develop tension, it now becomes difiicult to see why this tension should be developed 

 by fibers which would already be folded in the resting state. The question arises as to 

 whether or not extensibilit}^ and contractility are not due to different fibrous elements 

 arranged in series. In resting muscle the one responsible for contraction would be straight, 

 while the other, responsible for extensibility, could be in the folded state. 



Levin and Wyman, working in A. V. Hills' laboratory a quarter of a century ago, 

 obtained length-tension diagrams which indicated that two different elements, a viscous- 

 elastic and a purely elastic element shunted in series, were responsible for the behaviour 

 of cross striated muscle. Recently A. V. Hill returned to the study of this problem and 

 showed that muscle contained, at the site of the contractile matter, a "series elastic 

 component" which was evidently identical with Levin and Wyman's viscous-elastic 

 material. He called it "tendon", indicating by the quotation marks that it may be 

 anything, the tendon, or even the contractile matter itself. In Germany, Reichel 

 devoted attention to this series elastic component which he expected to find in molecular 

 dimensions. 



The purpose of the present paper is to identify the series elastic component by 

 means of the electron microscope. Essentially, the technique consisted of fixing muscle 

 at different degrees of extension or contraction, treating it with an electron stain, 

 embedding, sectioning, and studying the sections by means of the electron microscope. 

 We were led to conclude that the protofibrils, running continuously along the muscle 

 fiber, are built up alternately of contractile and purely elastic material. This segmentation 

 lies in histological dimensions. Each sarcoma has in its middle on either side of the H- 

 band (which contains the M membrane) such elastic material, which in the unextended 

 state has the same density as the contractile matter. The part of the A-band which 

 contains contractile matter thus cannot be distinguished in this state from the part 

 which contains the clastic matter. If, however, the elastic portion is stretched, by 



* This research was sponsored by Armour and Company, Chicago, Illinois, the American Heart 

 Association, the Association for the Aid of Crippled Children, and the Muscular Dystrophy 

 Associations. 



References p. 133. 



