CHAPTER XC 

 PHYSIOLOGICAL PROPERTIES OF THE SIMPLE REFLEX ARC 



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

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

 two aspects of the question 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 the latter will be evident when it is re- 

 membered that 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 by Sherrington the integration of the 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. 

 By comparing the two modes of conduction we shall be better able' to 

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

 duction through the reflex arc. 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 case 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 case 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., muscle, 

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

 there is by no means so evident a parallelism between stimulus and 

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

 conjunctival reflex and the extensor thrust behave according to the so-called 

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

 less independent of the strength of the stimulus. In other reflexes, such 

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



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