438 - Multicellular Animals, Especially Man 



Now, however, it is apparent that the myo- 

 fibril is composed of subsidiary, exceedingly 

 delicate threads, called filaments (Figs. 24-7 

 and 24-8). The filaments are of two kinds: 

 thick filaments, of about 100 Angstrom di- 

 ameter, which appear to be composed of 

 myosin, and thin filaments, about 50 Ang- 

 stroms in diameter, apparently composed of 

 actin. Moreover, the filaments are not very 

 long. The thick filaments terminate at the 

 edges of the A-band (Fig. 24-8), so that their 

 total length is about 1.5 microns. The thin 

 filaments are slightly longer (about 2 mi- 

 crons). They extend from the margin of one 

 H-band, through the Z-band. to the margin 

 of the next successive H-band (Fig. 24-8). 

 Thus the banded appearance of a striated 

 myofibril results mainly from a regular pat- 

 tern of overlapping in the filaments. This 



produces bands of greater and lesser protein 

 density at regular intervals along the length 

 of the fibril. Thin filaments are lacking in 

 the H-band; both thick and thin filaments 

 are present in the darker parts of the A-band; 

 and thick filaments are lacking in the Fband 

 (Fig. 24-8). The precise nature of the Z-band 

 is still unknown, although it appears to be a 

 membrane that supports each of the thin fila- 

 ments at the center. 



Decision as to the composition of the fila- 

 ments is based mainly upon extraction ex- 

 periments, although density measurements 

 made by interference microscopy have yielded 

 confirmatory results. Muscle fibers, subjected 

 to extraction with solutions that dissolve 

 myosin, yield electron microscope prepara- 

 tions that are devoid of thick filaments; 

 whereas fibers treated with actin-dissolving 



STRETCHED MUSCLE 



THICK (MYOSIN) FILAMEN T 

 THIN fACTINl FILA MENT ' 



RELAXED MUSCLE 



; 1 CROSS; BRIDGES 



-H— ; 

 ■A — 



CONTRACTED MUSCLE 



Fig. 24-8. Fine structure of a skeletal muscle fiber, after H. E. Huxley. The diagram is based on analyses of 

 electronmicrographs, such as the one shown in Figure 24-7. Note that the striated appearance of muscle results 

 from the regimented overlapping arrangement of the thick and thin filaments (see text, p. 437). 



