CHAPTER C 

 THE INTEGRATION OF ACTION WITHIN THE REFLEX ARC 



In considering the anatomical arrangements which give rise to sensa- 

 tion and motor activity, we have taken the risk of creating a false im- 

 pression that the action of the nervous system is carried out in a rigid 

 and immutable manner, and that nerve impulses travel over the conduct- 

 ing paths with the invariability with which an electric current flows over 

 a fixed system of wires. While the constancy of the anatomical paths 

 by means of which certain nervous functions are carried out is of ines- 

 timable value in clinical neurology it must be emphasized that in the 

 nervous system continuous adjustment is made to the end that the activ- 

 ity of one part may adapt itself to the activities of other parts, and con- 

 sequently the results of a given stimulation are not always strictly pre- 

 dictable. In other words the activity of the various reflex paths is closely 

 coordinated. Before we can consider how this coordination between 

 reflexes is accomplished, we must first examine into how the activity of 

 the various parts of a reflex arc are related so that the reflex may bring 

 about an act of functional significance; i. e., a response in which a local- 

 ized group of muscles act in a coordinated way so as to accomplish some 

 purpose which is related to the exciting stimulus. 



The Receptors. Reflex acts are initiated by the stimulation of recep- 

 tors. Because each receptor is specialized to respond more readily to 

 one quality of stimulus than to any other (page 856), each reflex arc is 

 brought into action only by stimuli of an appropriate sort. The selective 

 excitability of the receptors of the different reflex arcs consequently 

 enables the organism to respond in different ways to stimuli of different 

 kinds when they are applied to the same receptive skin area. As an 

 example we may consider the reactions which may result from stimulat- 

 ing the foot of a spinal or decerebrate cat. By pressing against the under 

 surface of the paw a reflex may be elicited known as the extensor thrust, 

 consisting of a vigorous extension of hip, knee, and ankle of the corre- 

 sponding leg. This response cannot be called out by any other form of 

 stimulus. In life it would be expected to occur when in running the 

 cat's foot comes in contact with the ground, thus throwing the animal's 

 Weight on the leg. If a harmful stimulus is applied to the same part of 

 the foot the response is of a totally different character, consisting of a 

 flexion of the joints of the limb. The foot is pulled awav ^om the stim- 



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