MUSCLE STRUCTURE 71 



chiefly proteins called actin and myosin bound together in a complex 

 protein called actomyosin. Purified actin and myosin mixtures, which 

 can be made to form fibers, were used in later experiments. If ATP 

 molecules are added to the glycerinated muscle preparations or to the 

 synthetic fibers, the result is a readily measured (albeit slow) contrac- 

 tion. The ATP molecules are used up in this reaction, so that part of the 

 recovery heat must be associated with the resynthesis of ATP. Since in 

 living muscle the ATP is present during the resting state of the muscle, 

 it is clear that the mere presence of ATP molecules is not, by itself, the 

 trigger for the contraction. 



It is of interest to point out that a similar role of contractile proteins 

 exists in an organism very different from those we ordinarily think about 

 as having muscle action. Bacterial viruses attach to the cells they infect 

 by means of appendages which have been called tails, though there is no 

 reason to think of these long slim appendages as tails any more than as 

 heads or necks. It has been shown that these viruses inject their nucleic 

 acids into the cells they attack, and that this injection is accompanied 

 by the contraction of the tails. Further, it has been shown that there are 

 about 100 molecules of ATP in the tail, and that the ATP is used up 

 during contraction in the same way as in the muscle contraction we 

 have been discussing. 



2. Muscle structure 



The light and electron microscopes have afforded us an excellent 

 picture of the substructure of muscle which should eventually lead to the 

 elucidation of the entire contractile mechanism. Unfortunately, up to this 

 point in time, the considerable technical problems in preparing materials 

 for electron microscopy have prevented the detailed substructure from 

 being determined. 



The aspects of muscle visible to the eye resulted in classification of 

 muscles into striated and smooth types. Smooth muscles, by dissection 

 and examination in the light microscope, are found to be composed of 

 short (roughly 20 microns) cells of optically isotropic material. Striated 

 muscle, on the other hand, is found to be composed of bands of mate- 

 rials, the bands being distinguishable initially by being optically differ- 

 ent. There is a band of optically isotropic material, called the I-band, 

 and a band of optically anisotropic material, called the A-band. There 

 are two subdivisions in each band. The I-band is divided into two equal 

 parts by a dark-staining thin band called the Z-band (from the German 

 word Zwischen, implying a "between" band). The A-band is likewise 

 divided into equal parts by a slightly thicker clear band, called the H- 

 band (from the German word for clear — hell). These are shown dia- 



