THE CONDUCTION OF THE NERVOUS IMPULSE 843 



of a single nerve impulse, it is found that a second impulse can pass 

 through it, provided it is produced immediately after the termina- 

 tion of the refractory period. For a short interval after the passage 

 of one impulse into a region in which conduction is difficult the nar- 

 cotized nerve becomes better able to conduct a second impulse. The 

 effects of two impulses added together is thus able to produce a re- 

 sponse which a single impulse alone cannot accomplish. This phenom- 

 enon is known as summation in conduction. Such summation is a 

 characteristic of reflex conduction, as we have seen in the experiment 

 in which stimulation with an induction shock fails to bring about a 

 reflex act unless it is repeated several times. In some cases as many 

 as 40 or 50 stimuli must be applied before reflexes are established. 



Two characteristic phenomena of reflex conduction closely related 

 to summation which may be explained on the assumption that one im- 

 pulse can alter the ease with which a second impulse can overcome 

 the resistance at the synapse are Induction and Facilitation. 



Induction is the production of a reflex response by the application to 

 different afferent nerves of two stimuli each of which alone is incapable 

 of setting up impulses which can break through the resistance of the 

 reflex arc. Induction may occur when the stimuli are applied at the 

 same time or when one stimulus is commenced after the other has been 

 discontinued. Facilitation resembles induction except that the stimuli 

 are each capable of producing the response when applied independently. 

 Their combined effect is to produce a greater response than either can 

 elicit when acting alone. 



The underlying assumption in these cases is that the afferent paths 

 over which the impulses travel in to the nervous system impinge upon 

 a common motor path in the synapses of which summation in conduction 

 occurs. 



Inhibition. In studying the heart we have seen that nerve impulses 

 traveling over the vagi may depress or inhibit the action of the cardiac 

 muscle. Inhibition is also an important process in the action of the 

 central nervous system, and is of great importance because when cer- 

 tain groups of muscles are made to contract the activity of opposing 

 groups must be depressed in order that movement may be made without 

 opposition. Coordination in the nervous system depends on the in- 

 hibition of certain reflex activities in order that other reactions may 

 be carried out without confusion. Certain forms of inhibition can be 

 explained by considerations quite similar to those employed in the ex- 

 planation of summation. We have seen that immediately after the pas- 

 sage of a nerve impulse along a nerve a period occurs during which 

 a second impulse cannot be conducted by the nerve fiber. This is the 

 refractory period of conduction. Later still there is a period during 



