NATURE OF THE NERVE IMPULSE. 117 



there is an absolutely refractory condition in which no stimulus, 

 however strong, can excite the nerve. This is followed by a period 

 lasting to about 0.013 second after the first stimulus in the frog's 

 nerve, and designated as the relatively refractory period, in which 

 the excitability is returning rapidly to normal. The excitability, 

 in fact, may go beyond normal, giving a supernormal phase, during 

 which the nerve is more excitable than in the resting stage.* The 

 refractory period of the nerve fiber may be much prolonged by con- 

 ditions which slow the processes underlying activity, for example, 

 by low temperatures, or by the action of certain drugs, such as 

 yohimbin (Tait). 



The Question of Fatigue of Nerve Fibers.— An important ques- 

 tion in connection with the nature of the nerve impulse has been 



Fig. 54. — Schema to show the method of blocking the nerve impulse by means of a polar- 

 izing current: a, The stimulating electrodes; b, the battery, the current of which is led into the 

 nerve. The depressed irritability at both anode, +, and cathode, — , prevents the nerve im- 

 pulse started at a from reaching the muscle. 



that of the susceptibility of the nerve fibers to fatigue. The obvious 

 fatigue of muscles and of nerve centers has been referred to the ac- 

 cumulation of the products of metabolism of their tissues. It has 

 been accepted, therefore, that if the nerve fiber could be dem- 

 onstrated to show fatigue as a result of functional activity, 

 this fact would be probable proof that the conduction of the im- 

 pulse is associated with a chemical change of a catabolic nature in 

 the substance of the fiber. Experimental work, however, has 

 shown that under normal conditions the nerve fiber shows no 

 fatigue. The experiments made upon this point have been nu- 

 merous and varied. The general idea underlying all of them has 

 been to stimulate the nerve continuously, but to interpose a block 

 somewhere along the course of the nerve so that the impulses should 

 not reach the end-organ. This precaution is necessary because 

 the end-organ — muscle, gland, etc. — is subject to fatigue, and 

 must therefore be protected from constant activity. From time 

 to time or at the end of a long period of stimulation the block is 

 removed and it is noted whether or not the end-organ — for in- 

 stance, the muscle — gives signs of a stimulation. The removable 

 * Keith-Lucas, "The Conduction of the Nerve Impulse." 1917. 



