842 CENTRAL NERVOUS SYSTEM 



spread backward from the motor neurons to the afferent neurons can be 

 demonstrated by attaching a galvanometer to the afferent nerve, when 

 it will be seen that no action current is set up in the afferent neuron 

 if the stump of the ventral root is stimulated. Impulses set up in 

 these fibers can not pass to other neurons within the spinal cord, be- 

 cause the synapses will not allow them to spread in that direction. 



Thus it is seen that the polarity which is characteristic of the central 

 nervous system is imposed upon it by the synapse. In the primitive 

 nerve net, in which synapses can not be demonstrated, polarity does 

 not exist. 



Resistance to Conduction due to Synapse. The synapse is also a re- 

 gion which offers some resistance to the passage of the nerve impulse 

 and may, prevent its passage altogether. It is usually impossible to 

 cause reflex response by applying a single induction shock to a sensory 

 nerve, although the stimulus may be quite strong enough to excite a nerve 

 muscle preparation. Not until the stimulus is repeated several times, 

 setting up a series of nerve impulses, will the resistance of the synapses 

 in the reflex arc be overcome so that an impulse can pass through to the 

 muscle. 



The synapses in which the collaterals of a single neuron terminate 

 differ considerably in the resistance which they offer to the passage of 

 the nerve impulse. If a series of weak stimuli are applied to the foot 

 of a frog from which the brain has been removed a flexion of the leg 

 may be induced. If the stimulus is increased in strength movements 

 of the opposite leg will occur, while still stronger stimuli will cause the 

 excitation to spread to the muscles of the trunk and forelimbs. This 

 observation indicates that the impulses set up by a weak stimulus can pass 

 only through those synapses which connect the afferent neurons with 

 the motor neurons of the same limb, while stronger stimuli are required 

 to set up impulses which can pass to the synapses leading to the motor 

 paths -to more remote muscles. This graded synaptic resistance is con- 

 sequently an important mechanism in determining what paths an im- 

 pulse shall follow in its course through the greatly branching systems 

 of nerve fibers which occur in the nervous system. 



Summation in Reflex Conduction. The resistance presented to the 

 passage of the impulse by the synapse suggested to Lucas that con- 

 duction in a synapse is comparable to 'that in a narcotized area of nerve. 

 A second point of resemblance is that the impulse travels slower through 

 both synapse and through a length of nerve treated with alcohol. lie 

 consequently studied with care the conditions of conduction through 

 narcotized nerve and discovered several facts which are of value in 

 understanding the peculiarities which the synapses give to reflex conduc- 

 tion. If an area of narcosis is just deep enough to check the passage 



