HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



Stances with different final effects is suggestive of the 

 behavior of an enzyme. It is highly relevant to this 

 that substances which are able to displace acetyl- 

 choline from the enzyme cholinesterase are also able 

 to displace it from the receptor. The receptor appears 

 almost certainly to be a protein constituent of the 

 muscle fiber membrane with its reactive sites exposed 

 on the outer surface. As a result of the combination 

 of these sites with acetylcholine, the physical proper- 

 ties of the membrane alter and a new path appears 

 for the diffusion of ions of various species through it. 

 In electrical terms transmitter action may be ap- 

 proximately described as the placing of an addi- 

 tional conductance across the membrane which short- 

 circuits any previously existing potential difference. 

 In that the experimental findings are in agreement 

 with this interpretation, they exclude the possibility 

 of electrical transmission by which the junctional 

 response is considered to be produced by an externally 



generated current impressed upon the muscle fiber. 

 At the same time they eliminate the possibility that 

 the response may be of the nature of a local response, 

 a specific increase in membrane permeability to 

 sodium ions boosting an initially small potential 

 change, such as may occur when the membrane is 

 depolarized to near the threshold for setting up an 

 action potential. It appears that the junctional 

 respon.se cannot be brought about by any means of 

 electrical stimulation of the postjunctional structure 

 but only by a specific chemical reaction of the re- 

 ceptor. The presence at the junction of a region capa- 

 ble of responding in this way does not appear to 

 affect the action potential developed there, except 

 by an addition of the independent actions of the two 

 types of activity. The probable significance of this is 

 that the area occupied by the receptor is small and 

 does not detract appreciably from the area engaged 

 in producing the action potential. 



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