CHAPTEB XC 

 PHYSIOLOGICAL PROPERTIES OP THE SDfPLK KKFLKX ARC 



We may now proceed to study the properties of reflex actios occur- 

 ring through the isolated spinal renters of a spinal animal. There are 

 two aspects of tlic qnestion to be considered: 1) the properties of a 

 single reflex arc, and (2) the action or influence of one reflex arc on 

 another. The importance of llie Latter will be evident when it is re- 

 membered thai complicated muscular movements depend for their proper 

 coordination entirely on the interaction between the various reflex arcs 

 which compose the nervous system. This interaction, as already ex- 

 plained, has been called l>y Sherrington the integration of tht nervous 

 system. 



Probably the simplest way to study the physiologic properties of 

 the simple reflex is to compare the mode of conduction of a 

 nerve impulse through it with conduction along a simple nerve trunk. 

 \)y comparing the two modes of conduit ion we shall be better able to 

 appreciate the modifications to -which the impulse is subjected by con- 

 duction through the reflex are. The important points are these: 



1. The Latent Period. — The latent period, or period which intervenes be- 

 tween the moment of application of the stimulus and the response, is 

 very short in the ease of a nerve trunk, and under normal conditions 

 always the same, but is quite variable and sometimes very long in the 

 case of a reflex arc. Thus, in the ease of the conjunctival reflex, which 

 is produced by applying a stimulus to the corneal conjunctiva (causing 

 a closing of the eyelids), the reflex time is very short and invariable, 

 whereas in the case of the scratch reflex it may vary From two and a 

 half to three and a half seconds, according to the strength of the stimu- 

 lus. The seat of delay in the reflex arc is probably in the synapse, but 

 its cause is obscure. 



2. Grading- of Intensity. In a nerve trunk the intensity of the im- 

 pulse is more or less proportional to the strength of the stimulus. This 

 can be judged by observing either the action current in the nerve by 

 means of a galvanometer or the response of the end organ; e. g., mus 

 attached to the nerve. In the case of a reflex arc, on the other hand, 



there is hy mi means BO evident a parallelism between stimulus and 



response. Reflexes, however, vary considerably in this regard The 

 conjunctiva] reflex and the extensor thrust behave according to tl ■ Med 



"all or nothing principle;" i.e., the intensity of the response is more "r 



independent of the strength of the stimulus, in other reflexes, Buch 

 as the flexion reflex and the scratch reflex, the intensity of the response 



90S 



