INITIATION OF IMPULSES AT RECEPTORS 



135 



FIG. 8. Summation of receptor potentials with different 

 intervals between stimuli. Upper trace: stimulus signal and time 

 in msec. Lower trace: receptor potentials. [From Gray & Sato 

 (37)-] 



considerable functional importance in determining 

 maximum sensitivities. It is probably also important 

 in determining the thresholds for sensation at different 

 frequencies of vibration (83). 



Depression 



After the production of one receptor potential by a 

 Pacinian corpuscle a subsequent one, occurring within 

 a few milliseconds, is depressed. This is most easily 

 seen with a preparation in which impulse activity has 

 been prevented; it can then be seen that the depres- 

 sion of the test responses increases as the conditioning 

 stimulus is increased and decreases as the interval be- 

 tween the conditioning and test pulses is increased 

 (18, 37). Depression of the receptor potential is also 

 caused by an impulse set up as a result of mechanical 

 stimulation; this depression is much greater than that 

 produced by a threshold receptor potential alone, 

 though it does not appear to be as great as the de- 

 pression caused by really large mechanical stimuli, 

 whether an impulse is present or not. Antidromically 

 conducted impulses also cause depression of a subse- 

 quent receptor potential, though for any given time 

 interval after the impulse the depression is slightly less 



than when the conditioning impulse is excited me- 

 chanically. At the time of writing there are a number 

 of problems which require elucidation and on which 

 the e\idence is conflicting. 



Depression has not been described for other recep- 

 tor potentials, but this is not surprising as the stim- 

 ulating conditions have been very different. It would 

 be interesting to know, however, if any part of the 

 initial decline of other receptor potentials were due 

 to the same cause as this depression; the decline of 

 the Pacinian corpuscle potential appears to be due 

 to other and more rapid processes. 



SITE OF IMPULSE INITIATION 



There is evidence from the rapidly adapting type 

 of stretch receptor of the crayfish that impulses are 

 set up in the cell body (27). The records in these ex- 

 periments were made through an electrode that was 

 inside the cell body and it was found that the change 

 of memijrane potential required to excite an impulse 

 was the same whether this change was brought about 

 by a receptor potential spreading from the periphery 

 or by current spread from an antidromically con- 

 ducted impulse that had been blocked before it 

 invaded the cell body. If the receptor potential set up 

 impulses peripheral to the cell body, the apparent 

 threshold value of the receptor potential, as recorded 

 by this method, would be less than the true value by 

 the amount of decrement occurring between the site 

 of initiation and the cell ijody; this is in fact what 

 occurs in the slowly adapting stretch receptors of the 

 same species. Direct distortion of the cell body and 

 the larger dendrites does not produce any potential 

 changes; receptor potentials are produced only by 

 stretching the muscle fibers in which the finer termi- 

 nals of the neuron ramify. It therefore seems certain 

 that while the impulses are initiated in the cell body 

 of the rapidly adapting receptor, the receptor poten- 

 tials are developed peripheral to this in the finer 

 dendritic terminals. 



A similar state of affairs appears to occur in the 

 Pacinian corpuscle (20). In this receptor a straight 

 nonmyelinated fiber of 2 pi diameter runs down the 

 central core of the corupscle; at the end of this central 

 core the axon becomes myelinated. One node of 

 Ranvier is regularly found inside the corpuscle about 

 half way between the end of the central core and the 

 point at which the axon leaves the capsule, and the 

 second occurs near the latter point (86). The imme- 

 diate surroundings of the nonmyelinated terminal 



