NATURE OF THE NERVE IMPULSE. 



117 



for experimental purposes, the arrangement being as represented 

 in Fig. 54. 



The Refractory Period. In the case of the heart, the nerve 

 cell, and the muscle it has been shown that for a short period after 

 the tissue enters into a condition of functional activity it is non- 

 irritable toward a second stimulus. This condition of loss of 

 excitability following upon or accompanying functional activity 

 is designated as the refractory period. It is interesting to find that 

 a tissue so irritable as a nerve fiber exhibits the same phenomenon. 

 For a very brief period (0.002 to 0.006 of a second), after it enters 



into action, as indicated by the electrical response, a second 

 stimulus throw in will be found to be ineffective. As the elec- 

 trical change passes off, that is, as the state of activity subsides, 

 the nerve regains its normal irritability. The refractory period 

 of the nerve fiber may be much prolonged by conditions which slow 

 the processes underlying activity, for example, by low tempera- 

 tures, or by the action of certain drugs, such as yohimbine (Tait). 

 The Question of Fatigue of Nerve Fibers. An important 

 question in connection with the nature of the nerve impulse has 

 been that of the suscep- 

 tibility of the nerve fibers 

 to fatigue. The obvious 

 fatigue of muscles and 

 of nerve centers has been 

 referred to the accumula- 

 tion of the products of 

 metabolism of their tis- 

 sues or to the actual 

 consumption of the en- 

 ergy-yielding material in 

 them. Functional activ- 

 ity in these tissues im- 

 plies the breaking down 



of complex organic material (catabolism) and the setting free 

 of the so-called chemical energy. The internal energy of the 

 compound is liberated as kinetic energy of heat, etc. 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 



Fig. 54. Schema to show the method of block- 

 ing the nerve impulse by means of a polarizing cur- 

 rent: a, The stimulating electrodes; 6, the battery, 

 the current of which is led into the nerve. The de- 

 pressed irritability at both anode, +, and cathode, , 

 prevents the nerve impulse started at a from reaching 

 the muscle. 



