844 CENTRAL NERVOUS SYSTEM 



which the conductivity of the fiber is supranormal, and although the 

 first impulse has failed to pass through a region of resistance such as 

 a narcotized area or a synapse, the second impulse falling in this period 

 of supranormal conductivity may be powerful enough to pass through 

 and excite the muscle. This is the phenomenon of summation. Between 

 the refractory period and the period of supranormal conduction the 

 nerve fiber is recovering its ability to conduct. Impulses set up at 

 this period will be of subnormal strength and will be less able to pene- 

 trate regions in which conduction is difficult. Consequently if the rhythm 

 of nervous discharge is such that each impulse falls in the period of 

 subnormal conductivity which follows the passage of the preceding im- 

 pulse, the discharge will be of impulses of subnormal strength. Such 

 a series of impulses may be unable to pass through the resistance of the 

 synapse and no activity can result. Whether a series of impulses will 

 produce summation or inhibition depends on the relation between their 

 frequency and the time required for the conducting tissue to recover 

 from the effects of each impulse, that is, on whether each impulse falls 

 in the period of supranormal or of subnormal conduction set up by its 

 predecessor. 



If the synapses connecting a single afferent path with two motor 

 neurons have different rates of recovery, the impulses might fall in the 

 period of supranormal conductivity of one synapse and be summated 

 and cause a contraction of the corresponding muscle, while they might 

 fall in the period of subnormal conductivity of the other synapse and 

 be inhibited, the corresponding muscle remaining inactive. In this way 

 we obtain a picture of how the reciprocal inhibition of antagonistic 

 groups of muscles may be accomplished. 



It must be remembered that physiologists have just made a beginning 

 in analyzing nervous activity from this point of view, and that our present 

 ideas are no doubt crude and subject to revision. Also the facts on which 

 our conception of the nature of nervous conduction is based have been made 

 out chiefly by a study of the motor nerves. It is conceivable that the im- 

 pulses conducted by sensory nerves are of a different nature from those 

 motor impulses, but so far as they can be studied they appear to be the 

 same. It is also possible that the impulses set up by electrical stimulation 

 of motor nerve trunks are different from those arising through voli- 

 tional or reflex activity. 



Canalization. The frequent use of a path through the nervous system 

 appears to lower permanently the resistance of the synapses along its 

 course. This process is known as canalization, and results in a greater 

 facility in bringing about movements of the muscles to which the path 

 leads. The bearing of this property of the synapse on the development 

 of skill in mechanical manipulation and in habit formation is obvious. 



