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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



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Stimulus velocity (V/miec) 

 _I I 



FIG. J. Receptor potential amplituide in relation to the 

 velocity of the mechanical stimulus with displacement constant 

 in a Pacinian corpuscle. Abscissa/ stimulus velocity in arbitrary 

 units. Ordinate: receptor potential amplitutie as percentage of 

 maximum. [From Gray & Sato (37).] 



function of the relevant form of energy) of the pulse 

 used to excite; thus the rate of rise of the receptor 

 potential of the frog's muscle spindle gets less as the 

 velocity of the stimulus is reduced. In the Pacinian 

 corpuscle there may Ije some change in rate of rise, 

 but often there is no effect attributable specifically to 

 the stimulus velocity; that is to say that, though the 

 rate of rise of the potential change increases as its 

 amplitude increases, the time course of a receptor 

 potential of a given amplitude is often the same 

 whether it is produced by a small displacement 

 having a high velocity or by a larger displacement of 

 lower velocity. In other words there are many me- 

 chanical pulses having different values of amplitude 

 and velocity that are equivalent as 'stimuli'. 



The duration of static receptor potentials, e.g. that 

 of the frog's muscle spindle and the slowly adapting 

 stretch receptor of the crayfish, is directly dependent 

 on the duration of the applied force. The rate of decay 

 of those potentials, which are velocity sensitive, may 

 possibly depend on the duration of the applied force 

 under certain circumstances; however, the rate of 

 decay of the receptor potentials of the Pacinian cor- 

 puscle, the only end organ in which this particular 

 point has been investigated, is normally independent 

 of the duration of the stimulus (37). Off responses 

 have the same time course as on responses. 



Absolute Magnitude oj the Receptor Potential 



Receptor potentials reach a ma.ximum at a certain 

 value of stimulus strength. It is of considerable theo- 



retical importance to know the absolute value of this 

 potential change. Up to the present, it has been pos- 

 sible to make only a rough estimate of its value in the 

 Pacinian corpuscle (20). This has been done by re- 

 cording the external current flowing along the axon 

 between the second and third nodes of Ranvier during 

 activity of each of these nodes and of the receptor 

 potential. By the use of blocking techniques and by 

 taking diff"erences, the.se components were obtained 

 separately and measured. Under suitable conditions 

 these currents will he proportional to the driving 

 potentials. The results given are that the receptor 

 potential amplitude is 59 per cent (n = 6, S.D. = 

 14 per cent) of the amplitude of the impulse at node 2 

 and 38 per cent (n = 5, S.D. = 17 per cent) of the 

 amplitude of the impulse at node 3. The difference 

 between the figures is due to a decline in the impulse 

 amplitude as the terminal is approached. The atten- 

 uation per internode of the receptor potential is likely 

 to be less than the 0.5 for large myelinated fibers of 

 toads C92), so the absolute value of the receptor 

 potential can be considered as of the same order of 

 magnitude as the resting and action potentials. This 

 conclusion is supported by results from the crayfish 

 stretch receptor (27). The amplitude of the recorded 

 receptor potential at threshold ranges from 8 to 25 mv 

 depending on the type of receptor. It has been esti- 

 mated that the loss due to passive conduction along 

 the nerve filler will have reduced the true value of the 

 receptor potential by 20 to 80 per cent; also a maxi- 

 mum receptor potential must be appreciably greater 

 than a threshold one. The ratio for the Pacinian 

 corpuscle is 10 to i (37). 



Summation oj Receptor Potentials 



If, during a maintained receptor potential, the re- 

 ceptor is subjected to increase in the stimulus strength 

 the final value of the receptor potential will correspond 

 to the final value of the stimulus. In this instance both 

 the stimulus and the receptor potential have summed. 

 With short pulse excitation it has been shown that 

 summation of receptor potentials occurs after the 

 stimulus is over (6, 37) as shown in figure 8. This 

 summation appears similar to that found with end- 

 plate potentials and synaptic potentials. A special case 

 of summation occurs when an 'on' response summates 

 with an 'ofli" response (37). Summation of sub- 

 threshold receptor potentials can in this way set up 

 impulses (6) and it seems likely that this process is of 



