THE CONDUCTION OP THE NERVOUS IMPULSE 



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ditioii of the nerve at that time. This conception is known as the all 

 or none law of conduction, because the tissue acts with all its power or 

 none at all. It is quite comparable to the all or none action of cardiac 

 muscle with which we have already become familiar (page 177). 



The importance of the conclusion that the energy of the nerve im- 

 pulse is derived from the nerve fiber itself is this. No matter in how 

 complex a fashion a neuron may be subdiveded into collaterals or den- 

 drites an impulse set up in it may pass with undimiiiished intensity along 

 each of these subdivisions. Since each collateral may be united with 

 other neurons, the impulse may spread from the one neuron to several 

 others without becoming attenuated by the multiplicity of the paths 



A 



B 



r 



n n 



Fig. 209. Diagram illustrating the effect of areas of narcosis (n) on the strength of the nerve 

 impulse. A indicates the gradual decline in the strength of the impulse as it penetrates an area 

 just long enough to cause its complete extinction. B and C illustrate the two possible results of 

 sending an impulse through two shorter areas, separated by a length of normal nerve. If no re- 

 covery occurs in the normal tissue, the impulse may be extinguished in the second narcotized area, 

 as B indicates. If recovery occurs in this area, the impulse may reach the muscle in undiminished 

 strength. Adrain showed the latter alternative to be true. 



in which it is traveling. Hence the conception of the older physiology 

 of the presence of special reinforcement centers in the central nervous 

 system for the purpose of reinforcing the strength of the impulse as it 

 spreads may be dispensed with, since every part of every nerve fiber 

 contributes the energy necessary to keep the impulse going as it travels 

 along. 



The Refractory Period. Although the nerve impulses induced by ar- 

 tificial stimulation may be momentary in duration the activity* of the 

 reflex arc brought about by normal conditions in life is usually main- 

 tained for some time in order to achieve continuous contraction of the 



