94 PHYSIOLOGICAL TRIGGERS 



relaxation indicates that it is sensitive to environmental changes. For example, 

 the abnormal anions discussed above intensify the depth of the latency relaxa- 

 tion (57). At about the time that the latency relaxation is beginning, changes in 

 the optical properties in muscle are also observed (46, 47). D. K. Hill has sug- 

 gested that these changes may be related to the outflow of potassium ions during 

 the declining phase of the action potential. Abbott and Lowy (i) have found no 

 latency relaxation in mantle muscle of squid, but report an increased trans- 

 parency during the declining portion of the action potential. These observations 

 suggest the possibility of a sudden change of state in some muscle protein 

 deUcately poised near the critical point between its soluble and insoluble states. 

 Such a 'discontinuous' change — in response to, say, a small shift in ion concen- 

 trations — is a favorite example of a triggered process and one which may well 

 be operative in muscle. 



Following the lead of A. V. Hill in studying the initiation and duration of the 

 active state, Ritchie (80) has recently measured the time at which tension first 

 begins to decline following cessation of multiple-electrode, tetanizing stimula- 

 tion. He has found a correlation between this time interval and the latent period 

 for contraction, the ratio of these two times being independent of temperature. 

 Although no unique interpretation can be assigned to these results at present, 

 this approach offers one more tool with which to probe the active state. 



INITIATION or CONTRACTION 



The largest sub-units within a muscle fiber are the long, more or less cylindri- 

 cal myofibrils which are parallel to the fiber axis and about i micron in diameter. 

 These fibrils display the characteristic A and I bands of striated muscle and, 

 depending on the animal source, may be firmly interconnected at the Z 

 membrane (49). Extensions of the Z membrane substance also appear to be 

 continuous with the fiber sarcolemma (26). Recent reports have again raised the 

 question of whether the Z membrane is a distinct 'disc' or whether it is part of 

 a continuous spiral down the length of the fiber. A. Engelhardt (28), for ex- 

 ample, reports a spiral arrangement in the fibers of human eye muscles as well 

 as in frog muscle fibers. 



A. F. Huxley and Taylor (53, 54), applying a micropipette to the fiber surface 

 of a frog semitendinosus muscle, observe a localized contraction when a 

 depolarizing current is applied opposite an I band, but no response opposite an 

 A band. (A hyperpolarizing current causes no contraction when applied to 

 either A or I bands.) These investigators support the suggestion of earlier 

 workers that changes initiated by depolarization at the fiber surface are nor- 

 mally conducted to the fiber interior along the structurally continuous Z 

 membrane located in the center of the T band. Exactly how the Z membrane 

 may serve in this capacity is not known. A. \'. Hill (43) has calculated that 

 diffusion of some hypothetical activating substance released just inside the 



