JEAN BOTTS 95 



fiber membrane would be much too slow for the transmission of the activating 

 process to the interior of a fiber within the required time span of a few milli- 

 seconds. The work of Draper and Hodge (25) suggests a heavy concentration of 

 potassium at the Z membrane; however, since this potassium is presumably in 

 the bound form, there is no assurance that electrical conductivity would be 

 facilitated in this structure. If it is supposed that excitation is somehow rapidly 

 transmitted along the Z membrane, there still remains the problem of 'activat- 

 ing' the contractile protein. 



Guba and Biro (38) have reported the existence of a thin myofibrillar mem- 

 brane. Such a membrane might conceivably support propagated electrical 

 changes (within the length of a sarcomere) analogous to those occurring at the 

 fiber surface. It would also ofTer, per fiber, a total surface area about 100 times 

 that of the fiber membrane, with the possibility of an energy augmentation due 

 to secondary ion fluxes across a large surface. However, Hodge, Huxley and 

 Spiro (49) have found no evidence for such a structure and it seems doubtful 

 that activation depends on a fibrillar membrane. 



Within the myofibril is a regular (hexagonal) array of filaments about 100 200 

 A in diameter and 400-500 A apart (40, 48). Detailed descriptions of the 

 myofibrillar structures and the changes taking place during shortening have 

 recently been given by Hanson and H. E. Huxley (40) and by Hodge (48) based 

 on electron microscope studies. Although speculations on the nature of the con- 

 tractile process differ in these two studies, it is assumed in both schemes that 

 the filaments are largely composed of actin and myosin, and that thin actin 

 filaments pass into the Z membrane. 



Actin, myosin A, myosin B, or some combination of actin and myosin have 

 been variously considered to constitute the 'contractile protein' responsible for 

 contraction and tension in active muscle. The properties of these and other 

 muscle proteins have been discussed at length elsewhere (e.g., 71, 77). Extensive 

 studies have been made of the enzymatic activity of myosin A and myosin B in 

 catalyzing the splitting of the terminal phosphate group from adenosine tri- 

 phosphate (ATP) and related substances. 



Recently a series of substances capable of inducing relaxation in muscle has 

 also been studied. Some of these are naturally occurring enzymes — myokinase 

 (5, 67), ATP-creatine-transphosphorylase + creatine phosphate (37, 66) — and 

 others are much simpler molecules such as pyrophosphate (5, 8) and ethylene- 

 diamine tetraacetate (EDTA) (9, 88). The importance of 'relaxing factors' in a 

 discussion of trigger mechanisms for muscle contraction becomes clear when the 

 possibility of identifying 'active state' with a state of 'suspended relaxation' is 

 considered. It has been found that glycerinated muscle fibers, contracted by 

 addition of ATP and Mg, will relax provided relaxing factor is supplied in the 

 presence of ATP and Mg++. Addition of a small amount of Ca++ can then induce 

 contraction. It has been speculated that in the relaxed state the contractile 



