CHAPTER XCII 



INTERACTION AMONG REFLEXES 



A single reflex acting independently of the rest of the central nervous 

 system does not really occur. An afferent impulse on entering the cord 

 spreads so as to involve a large variety of motor neurons, each of which 

 may, however, be excited through other afferent fibers arriving either 

 from other receptors or from higher nerve centers. The motor neuron 

 itself may therefore be a pathway occupied at different times by very 

 different types of nerve impulse. Hence it is appropriately called the 

 final common path, and its activity at any moment must depend on the 

 nature of the various afferent impulses that are transmitted to it through 

 the synapses. In other words, an entering afferent fiber must communi- 

 cate in the cord with internuncial paths which are available in various 

 degrees to other afferent fibers. Since it is through internuncial paths 

 that the impulse is transmitted to the final common path, it is obvious 

 that, if afferent impulses in several of these paths were competing at 

 the same time for the possession of the final common path, confusion of 

 movement would result unless some provision were made whereby only 

 one kind of stimulus could be transmitted at one time. "One kind of 

 stimulus must be inhibited and the other facilitated in its occupancy of 

 the final common path.'* 



To understand the nature of this integration of the central nervous 

 system, it is therefore necessary for us to consider the factors which de- 

 termine which of two competing afferent impulses shall obtain possession 

 of the final common path. Let us take the competition between the flexion 

 reflex and the scratch reflex of the spinal dog. If we elicit the scratch 

 reflex and, while it is in progress, apply some nocuous stimulus to the 

 skin of the hind leg and thus induce the flexion reflex, it will be found 

 that the scratching movement subsides and the flexion movement conies 

 on without any overlapping or confusion. If, however, the stimulus 

 responsible for the scratching movement is a strong one, and that ap- 

 plied to the skin of the hind leg a feeble one, then the displacement may 

 not occur (see Fig. 216). 



In considering this integration of reflexes, as it is called, we must dis- 

 tinguish between those that are allied and those that are antagonistic, 

 and we must further distinguish between reflexes that are simultane- 



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