142 



Muscles /8 : 3 



If one stimulates an excised muscle by means of an electrical shock (or 

 a mechanical impulse, or heat, cold, and so on), a twitch occurs. If 

 the stimuli are spaced a long time apart, the muscle relaxes to its original 



€ r " J/ -A 



Is, 



§ 



5 



5% 



Length of Twitch Varies 



with Particular Muscle, 



Temperature, and pti 



I _L_L_LJ_i_+ 



1 



25 |xsec 



f/\ r\ /v^ ! 



Time 

 (a) 



Time 

 (b) 



AL/L 



Tetany 



Fatigue 



sec 



' * ■ 



Time 

 (c) 



Figure 3. Curves of contraction, (a) Occasional stimulation 

 shows twitches. Arrows indicate stimuli, (b) Frequent 

 stimulation leads to summation, (c) Prolonged tetany leads 

 to fatigue. Note the difference in the time scale as compared 

 to (a) and (b). After S. Cooper and J. C. Eccles, J. Physiol. 

 69:377 (1930). 



length between twitches, and a contraction curve is obtained of the 

 shape shown in Figure 3a, for isotonic contractions. If the stimulus is 

 repeated before relaxation occurs, summation is observed as shown in 

 Figure 3b. With still more rapidly repeated stimulation, a smooth 

 contraction curve results such as shown in Figure 3c. The steady con- 

 traction is called tetany. All muscles will eventually fatigue and fail to 

 contract, even though stimulated. This type of fatigue probably never 

 occurs in the healthy intact animal, as the nervous system undergoes 

 fatigue before the muscles do. 



Curves illustrating the strength of isometric and isotonic contractions 

 are shown in Figures 4a and 4b, in terms of effect of length on tension 

 developed in isometric contraction and of load on shortening produced 

 during isotonic contraction. Only in isotonic contraction is work done. 

 It is easy to show that the maximum work is done at half the maximum 

 load for muscles for which the straight line relationship of Figure 4a is 

 valid. The straight line can be described by 



