586 



Comparative Animal Physiology 



tebrates. Some muscle fibers are partly striated and partly smooth (Fig. 

 217, D). In general, sharp, closely spaced striations, many fibrils, a small 

 amount of interfibrillar sarcoplasm, and long fibers are correlated with speed, 

 while absence of striations, abundant sarcoplasm, central nuclei, short fibers, 

 and much connective tissue are correlated with slowness. The size of the 

 sarcomere seems to be less important than the sarcoplasm-fibril ratio. 



Time Constants of Contraction and Excitation. The "speed" of a muscle 

 can be measured by several criteria, not all of which are dependent on a 

 common cellular basis. In Table 71 are given representative data on 

 excitation times (e.g., chronaxie), contraction time, relaxation time, con- 

 duction rate, fusion frequency, and refractory period in muscles. The values 

 given are subject to certain qualifications. Contraction and relaxation rates 

 of Table 71 are for simple twitches or brief periods of tetanus. Lever sys- 

 tems are not always fast enough to record accurately, hence true contrac- 

 tion rates may be faster than recorded. Since some muscles can contract at 

 several rates, depending on the stimulus (see pp. 596-600), the values in 

 Table 71 are for fast contractions. Relaxation rates also are often a function 

 of the lever system, yet differences in relaxation rates are greater than in 



Time 120/second 



Myogram— isometric. 

 M. sortoriuS"-R. pipiens 



Myogram --- M. retractor 

 Q Penis --- P elegans' 



Time intervals 01 sec- 



Myogram-- circular muscle 



Q Turtle Intestine 



Time intervals 6.0 sec. — 



mill iiMMiii 



iiiiiiMiiiiiiiiiiiiiiiiiiiiiiiiiii mil III Mill II mil 



Fig. 218. Contraction curves of three muscles of different rates of contraction and 

 relaxation with adjusted time scales as indicated. From Gilson.^" 



contraction rates of different muscles. Some of the chronaxies are taken 

 with preparations where nerve and muscle excitation could not be distin- 

 guished; chronaxies measured with small electrodes are shorter than those 

 measured with large electrodes and tend to approach nerve chronaxies. An 

 attempt is made in Table 71 to select values which are probably muscle 

 chronaxies. Brief contractions of different muscles with adjusted time scales 

 of recording are rem.arkably similar in shape (Fig. 218). 



The contraction rates of some mammalian and bird muscles are faster at 

 38° than the recorded rates of other muscles. An eye muscle, e.g., the in- 

 ternal rectus, of a mammal contracts in less than 10 msec. A mouse breathes 

 at the rate of 200 times per minute and has a heart rate of 200-300. Bird 

 heart rates are often several hundred per minute, and a humming bird 

 "standing still" in air shows a wing beat 55 times a second. The frequency 

 for smooth tetanic fusion is not known for most of these fast muscles, but 

 for cat internal rectus fusion does not occur below a frequency of 350 per 

 second. 



