8 : 5/ Muscles 153 



Finally, it is cut into sections a few hundred angstroms thick. When 

 these sections are examined in the electron microscope, most are cut at 

 such angles to the myofibrils that they are useless for analysis, but a few 

 will be either at right angles to the myofibrils or along the myofibril. 

 (A great deal of judgment is necessary to discard most of the sections as 

 useless.) 



These studies have been interpreted to show that the / bands consist 

 of thin filaments joined by a membrane at their centers (the Z disc). 

 The H zone consists only of thick fibers and the A band is a region of 

 overlap between the thick and thin filaments. These are arranged in a 

 regular array with a definite number of thin filaments surrounding a 

 thick one, varying from two in the flight muscles of insects to six in some 

 vertebrate muscles. Between the thick and thin filaments, there appears 

 to be a series of bridges spaced about 50 or 60 A apart. 



The length of the A band, with the H zone in its center, is then the 

 length of the thick filament as shown in Figure 8. When a muscle (or a 

 myofibril) contracts, the length of the A band remains constant. This 

 implies that the thick filaments do not change in length. Extraction 

 studies have shown that the thick filaments consist entirely of myosin 

 and that they probably contain all the myosin. Chemical studies com- 

 bined with electron microscopy have shown that the thin filaments 

 contain actin and another protein, presumably tropomyosin. 



When the muscle fiber contracts, both the / band and the H zone are 

 shortened. The decrease in length of both these regions is comparable. 

 Therefore, as is shown in Figure 8, the length of the thin filaments also 

 must remain unchanged on contraction. The interpretation of the 

 electron micrographs, then, is that the thin filaments somehow slide in 

 between the thick ones as the muscle contracts. 



Just how the thick filaments slide along the thin filaments is a matter 

 of speculation. One might imagine that it takes an ATP molecule to 

 open each bridge between thick and thin filaments and that these then 

 moved in some sort of ratchet fashion in finite steps. The rate of splitting 

 of ATP by myosin and the number of ATP molecules used per twitch 

 both make this finite jump-type motion possible. Again, one might 

 suppose that small kinks appear along the thin filaments and that these 

 move along one bridge at a time. No doubt the reader can construct 

 a few other speculative models himself. 



Even if one accepts completely the interpretation of the electron 

 micrographs presented above, there still remain several questions at the 

 molecular level, concerning the mechanism of muscular contraction. 

 It is not known how the muscle action potential triggers the contraction 

 process, although it is known that the action potential always precedes 

 contraction. It is not clear how the numerous filaments all move in a 



