THE CONDUCTION OF THE NERVOUS IMPULSE 841 



Because of the refractory period the continuous activity of nerve must 

 consist of a series of nerve impulses occurring at brief intervals. Nerv- 

 ous discharge consequently has a discontinuous or rhythmic character. 

 The actual rate of this rhythm has not been determined precisely for 

 the voluntary movements of man, but the length of the refractory period, 

 which is shorter in the nerves of warm-blooded animals than in the frog, 

 renders it unlikely that it is greater than 3000 impulses per second, while 

 certain experiments indicate that it is at least 300 impulses per second. 

 The nerve fiber has been found to be unfatigued even by continuous stim- 

 ulation for twenty-four hours, a fact which is explained by the discovery 

 that the nerve will not conduct an impulse until it has recovered from 

 the fatiguing effects of the preceding impulse. The rhythmic character 

 of nervous activity also suggests a way in which' nerve impulses set up 

 by different stimuli may differ from one another and consequently bring 

 about different reactions. Although the all-or-none law of conduction may 

 preclude the possibility of nerve impulses differing in strength or inten- 

 sity, there is no reason why the rate at which impulses follow one another 

 in normal nervous activity may not vary greatly, and this difference may 

 be a determining factor in the spread of the impulses through the spinal 

 cord and brain. 



Conduction between Neurons 



So far conduction has been considered only with regard to the spread 

 of a nerve impulse through a single nerve cell. In passing from one 

 neuron to another the nerve impulse must pass through the synapse 

 which is the structure uniting their respective fibers. The presence of 

 synapses in the path of conduction imposes certain characteristics on 

 the activities of the reflex arc which do not appear in conduction within 

 a single nerve fiber. The key to an understanding of reflex activity and 

 the higher mental processes by which reflex acts are controlled undoubt- 

 edly lies in the physiology of the synapse. 



The Polarity of the Reflex Arc. In nerve fibers it has been seen that 

 impulses may spread in either direction along the axon. In the reflex 

 arc as a whole the passage of the impulse is in one direction only, from 

 the receptor to the effector organ, because a nerve impulse can pass in 

 one direction only through the synapse, which acts as a valve to prevent 

 impulses passing back in the opposite direction. This fact is demon- 

 strated by the classic experiments of Bell and Magendie, who found 

 that no muscular response followed stimulating the central end of the 

 cut ventral root of a spinal nerve. The motor neurons whose axons 

 compose the ventral root can transmit impulses only to the muscles 

 which they innervate, and in this experiment they were separated by 

 the operation from the point of stimulation. That impulses do not 



