72 



PHYSICAL ASPECTS OF MUSCLES 



Fig. 32. A schematic representation of a muscle showing the A, I, Z, and H 

 bands. The A-band is optically anisotropic. The I-band is optically isotropic. 



grammatically in Fig. 32. More recent work has shown that the H- 

 band contains a thin central M-band, and that the Z-band is similarly 

 constructed. These refinements do not concern us here. 



When a muscle is stimulated, there is a contraction, followed by a 

 relaxation and return to the original state; the entire process is called 

 a twitch. Observation with the light microscope reveals that during the 

 contractile phase of a twitch, the I-band and the H-band shorten by 

 about the same amount; the A-band width does not change. Since the 

 dark part of the A-band increases in size by the amount that the I-band 

 decreases in size, it is plausible that the material of the I-band is sliding 

 into the A-band, and that this is the mechanism of contraction. Of 

 course, the equality of change in the H-band and I-band could be mere 

 coincidence or have an entirely different interpretation. A once popular 

 theory ascribed the changes to alterations in configuration of the long 

 protein molecules making up the fibers; if the physico-chemical environ- 

 ment changed, the proteins would change in about the same amount. 

 This theory has foundered on the rock of measurement, for x-ray studies 

 have demonstrated that there is no such configuration change in the 

 proteins during muscle contraction. Indeed, this last result teaches us 

 that the muscle shortens while its constituent filaments do not; this is a 

 result which may be restated as showing that there are no contractile 

 proteins in muscle but that there are contractable configurations which 

 leave the filamentous proteins unaltered in length. It is further worth 

 noting that when muscles are stretched, the H- and I-bands both in- 

 crease by about the same amount, thereby supporting the notion that the 

 two changes are coupled, since the contractions and extensions occur 

 together. 



This deduction about material from the I-band sliding into the A-band 

 cannot be the whole story. For if the I-band were to disappear entirely, 

 by sliding into the A-band, the shortening would have a maximum value 

 of about 40%, since the I-band is roughly 40% of the total length of the 

 A- plus I-bands. Yet measurements of intact muscles reveal a shortening 



