48 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



lated directly by the electrical current and its contractions are recorded by 

 the ergograph, it will give a curve similar to that figured above for the volun- 

 tary contractions, except that the contractions are not so extensive. Under 

 these conditions the muscle', when completely fatigued to electrical stimula- 

 tion, will respond to voluntary stimulation from the nerve centers. It 

 seems likely, as suggested by Hough, that this result is due mainly to the 

 fact that the electrical current cannot be applied to a muscle in its normal 

 position so as to excite uniformly all the constituent muscle fibers, although 

 it is also possible that what we call the normal or voluntary stimulus is more 

 effective or, to use a physiological term, more adequate to the muscle fibers 

 than the electrical shock. On the other hand, after fatigue from a series 

 of voluntary contractions it has been observed that the muscle will still 

 give contractions if stimulated directly by electricity. This fact has been 

 interpreted to mean that, in the neuromuscular complex involved in a mus- 

 cular contraction namely, motor nerve cell, motor nerve fiber, and muscle 

 fiber the first named fatigues most easily, and that the ordinary fatigue 

 curve obtained from the ergograph does not represent pure 'muscle fatigue, 

 but fatigue of the neuromuscular apparatus as a whole, the point of complete 

 fatigue being reached in the neural component of the mechanism before 

 the muscle itself loses its power of contraction. This interpretation, however, 

 is not entirely certain. Wedenski has called attention to the fact that in 

 the neuromuscular apparatus the motor end-plate is a sensitive link in the 

 chain, and that, when the nerve is stimulated strongly with artificial stimuli, 

 at least, this structure falls into a condition in which it fails to conduct the 

 nerve impulse to the muscle. It may be, therefore, that in sustained volun- 

 tary contractions the end-plate fails first, and thus is directly responsible 

 for the failure of the apparatus to perform further work. That the fatigue in 

 ordinary voluntary contractions affects the muscles before the motor nerve 

 centers is indicated by the experiments of Storey.* Making use of a weight 

 ergograph and experimenting upon the abductor indicis he found that after 

 fatiguing this muscle to voluntary contractions with a certain weight, re- 

 moval of the weight enabled the individual to make contractions as high and 

 as rapid as before the fatigue. On the other hand, if after removing the 

 weight the muscle was stimulated electrically the contractions were lower and 

 slower than before the fatigue. So far as our knowledge goes, therefore, 

 fatigue as it appears in sustained voluntary contractions is due probably 

 primarily to a loss of irritability in the muscle and in the motor end-plates. 

 The motor nerve fibers do not fatigue, and as regards the motor nerve centers 

 it is not possible as yet to say what may be their relative susceptibility to 

 fatigue. 



Sense of Fatigue. It should be noted in passing that in con- 

 tinued voluntary contractions we are conscious of a sense of fatigue, 

 which eventually leads us, if possible, to discontinue our efforts. 

 This sensation must arise from a stimulus of sensory nerve fibers 

 within the muscle or its tendons, and it may be regarded as an 

 important regulation whereby we are prevented from pushing our 

 muscular exertions to the point of " straining." 



Muscle Tonus, In addition to the conditions of contraction 

 and of relaxation the living muscle exhibits the phenomenon of 

 "tone." By muscle tone we mean a state of continuous shortening 

 or contraction which under normal conditions is slight in extent 

 and varies from time to time. This condition is dependent upon 

 the connection of the muscle with the nerve centers, and we may 

 assume that under normal circumstances the motor centers are 

 continually discharging subminimal nerve impulses into the muscles 

 * Story, 'American Journal of Physiology," 1903, viii., 355. 



