Muscle and Electric Organs 



593 



maximal than with maximal stimulation, because both factors— increased 

 number of fibers and incomplete relaxation— are effective. 



Isolated pieces of single frog fibers show summation of the local re- 

 sponse,-^ ^ and isolated entire frog muscle fibers which give all-or-none 

 twitches have tetanus-twitch ratios varying from 1.3 to 6.7.^'*^ Tetanus- 

 twitch ratios for entire frog muscle with maximal shocks are sometimes 

 greater than 2 (Fig. 224), but in mammalian muscle higher ratios are found, 

 e.g., cat gastrocnemius 3.3 and internal rectus 10.7. ''■''• ^-■' The frequency 

 for fusion is higher in white striated muscle than in red (Table 71). At 

 frequencies 1.5 to 3 times the fusion frequency, the response diminishes. 



Not all non-iterative muscles increase in tension with increasing fre- 

 quency of stimulation. Portions of the squid mantle contract as single motor 

 units.-^-^ Mechanical fusion of the response to stimulation through the 

 nerve occurs at 25 per second, but there is no increased tension; as the fre- 

 quency of stimulation increases the response remains constant (Fig. 

 225).!^^ 



Fig. 224. Isometric contractions of frog sartorius at frequencies given. 



Fig. 225. Contraction of mantle of squid to stimulation of giant fiber of mantle nerve 

 at frequencies stated. Prosser and Young.^"^ 



Vertebrate cardiac muscle increases in response hardly at all with in- 

 creasing frequency, and has such a long refractory period that contractions 

 do not fuse (Ch. 15, p. 565). After its refractory period, however, cardiac 

 muscle is hyperexcitable; i.e., its threshold is lower than normal. 



Some muscles have properties intermediate between non-iterative and 

 iterative. Many smooth muscles of molluscs, for example, respond to a 

 single stimulus but relax very slowly, so that contractions build up and ten- 

 sion is held at low frequencies. In the byssus retractor of Mytilus, for ex- 

 ample, the tension doubles when the frequency of direct current stimuH 

 increases from one to four per second. 



In iterative muscles a single maximal stimulus does not elicit an all-or- 

 none response of the entire muscle; there may be local responses. In addi- 

 tion to the mechanical factor mentioned above for increased responsiveness, 

 two other mechanisms are important, neurornuscular facilitation, and spread 

 of excitation from cell to cell in a sheet of muscle. 



