'44 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



that they have quite different properties from main- 

 iiialian synaptic transmitters. Other e\idence has been 

 presented in an earlier section which suggests that at 

 many mechanical receptors acetylcholine does not act 

 as a chemical transmitter. 



The adaptation of receptors is a subject that has 

 stimulated many hypotheses (47, 74). Until recently 

 these have been based mostly on the concept that 

 the adaptation of receptors is closely related to the 

 accommodation of nerve fibers. It would certainly 

 be expected that this factor would play a part if 

 impulses are set up in the nerve fiber as a result of 

 currents generated by receptor activity in the ter- 

 minals. This factor cannot be entirely discounted, but 

 there are now very good reasons for supposing that 

 other factors may be more important. The time 

 courses of all the receptor potentials so far observed 

 are in general agreement with the corresponding 

 time courses of the impulse discharge. Thus the 

 short receptor potential of the Pacinian corpuscle 

 corresponds to the single impulse produced by rela- 

 tively large stimuli, the dynamic and static phases of 

 the receptor potential of the muscle spindle corre- 

 spond to the initial high frequency burst and the 

 maintained discharge of impulses and the receptor 

 potentials found in the two types of stretch receptor 

 investigated in the crayfish correspond to the fast 

 and slow adaptation of the two endings. The adapta- 

 tion of the receptor potential inay simply reflect 

 changes in the mechanical events going on in the 



terminals. This seems to ije the case in the Pacinian 

 corpuscle where only a brief wave of distortion can 

 be found in the central core during a maintained 

 deformation of the outside of the endorgan. In the 

 crustacean stretch receptors, the difference between 

 the slow and fast cells has been attributed to differ- 

 ences in the mechanical attachments between the 

 dendrites of the two types of cell and the muscle 

 fibers in which they ramify (27). The change in the 

 rate of adaptation of receptors in frog skin, when the 

 skin is stretched, is another example of the importance 

 of mechanical factors. It is impossible to say whether 

 or not such factors can account for the whole phe- 

 nomenon of adaptation of the receptor potential. It 

 is possible that there may be some mechanism that 

 reduces the effectiveness of a stimulus as time passes; 

 such a mechanism might conceivably be related to 

 the depression of the receptor potential observed in 

 the Pacinian corpuscle. 



Many of our ideas on the mechanisms of receptors 

 are at the present time speculative. Definite ideas on 

 these problems may develop as work goes deeper 

 into the mechanisms of those few receptors which are 

 particularly well adapted for such investigations. Also 

 when results, that ha\e already been obtained on 

 some receptors, are repeated or contradicted by 

 work on other types, it may be possible to say how- 

 far we may generalize from such results as have been 

 obtained. 



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