THE NEUROMUSCULAR SYSTEM: FATIGUE 105 



member, and this means that comparatively untrained 

 muscles are called upon. Endurance is thus inevitably 

 lessened. 



We have now recognized two probable elements in the 

 familiar state of the tired man. These are end-plate fatigue 

 and inhibitory impulses mounting to the centers. Addi- 

 tional factors are undoubtedly concerned. Among these 

 we may include some measure of true muscular fatigue, 

 dependent on the local chemical changes in the contractile 

 material and perhaps on changes in the composition of the 

 blood and lymph. We must bear in mind that fatigue 

 substances of this class may be distributed far and wide 

 by the circulation and depress the functional activity of 

 muscles which have not themselves been much used. An 

 unfavorable effect spreading to glands and to the nerve- 

 centers is easily possible. Our pedestrian may be feeling 

 the results of such a diffuse poisoning as he plods along. 



Deterioration of the motor perikarya in the cord and 

 perhaps in the brain may be one source of the observed 

 waning of power. Reference has been made some time 

 since to the demonstration of visible changes in these 

 bodies which Hodge and others have succeeded in making 

 conclusive. A new picture of the central element in general 

 fatigue we owe to the school of Sherrington, and particu- 

 larly to one recently associated with him, Alexander 

 Forbes. 1 This new emphasis is placed upon the synapses 

 through which stimulation of the motor perikarya must 

 be accomplished. Each typic perikaryon has a number 

 of these possible approaches. When it takes part in a 

 simple reflex it is not excited through the same synapse as 

 though the stimulation were conveyed to it from the 

 brain. 



Now, there is reason to believe that synapses, like end- 

 plates, are links of limited endurance" in the neuromuscular 

 chain. If, however, a synapse becomes fatigued and in- 

 capable of efficient conduction, there is the possibility, 

 not present in the case of the end-plate, of substituting a 

 1 American Journal of Physiology, 1912, xxxi, 102. 



