152 Muscles /8 :5 



(that is, 20,000 A) long, whereas the thin ones are about 50 A in 

 diameter and 1.5 /x long. These filaments also possess a periodicity or 

 striation, but it is only about 400 A long, a distance that is short compared 

 to the striations on the myofibril. (Indeed, the entire filament is com- 

 parable in length to one "unit" along the myofibril.) The dimensions 

 and periodicities of the filaments have been measured independently 

 by X-ray diffraction and by electron-microscope techniques. The two 

 types of data agree well when changes due to dehydration (necessary 

 for electron microscopy but not X-ray diffraction) are included in the 

 calculated results. 



At one time, X-ray studies of the form of the filaments were inter- 

 preted to show that the general arrangement of amino acids within the 

 proteins changed from a so-called "a form" to a "jS form" during 

 contraction (see Chapter 15). Subsequent studies have shown that this 



Figure 8. Sliding model of myofibrillar structure. The dis- 

 tance from one Z disc (or membrane) to the next is one myo- 

 fibrillar unit. During contraction the thick and thin filaments 

 keep the same length but intermesh more completely. The 

 thick filaments are myosin. The thin one contains actin and 

 presumably also tropomyosin. After H. E. Huxley and J. 

 Hanson, "Structure of cross-striated myofibrils," Biochim. 

 Biophys. Acta 23: 229 (1957). 



interpretation was wrong ; the form of the filaments remains unchanged 

 during contraction. The filaments are made up of helical protein 

 chains but with a nonintegral number of amino acid residues per turn. 

 The entire structure repeats about every 400 A. Theories which assign 

 muscular shortening to a change in the length or form of the protein 

 molecules all have difficulties explaining these data from electron 

 microscopy and X-ray diffraction, which show that the protein mole- 

 cules do not change in shape or form during contraction. 



Modern electron-microscope techniques permit the determination of 

 still more details of the structure of the myofibrils. It is possible to make 

 electron micrographs of the "ultra structure" of the muscle without 

 dispersing or homogenizing it in any way. For these studies, the muscle 

 is first fixed to harden the protein elements. Then it is "stained" with a 

 heavy metal to increase contrast in the electron microscope. Next, it is 

 filled with, and imbedded in, a plastic such as butyl methacrylate. 



