INITIATION OF IMPULSES AT RECEPTORS 



137 



Thirty minutes soaking is about the time needed 

 to abolish the impulse from a Pacinian corpuscle 

 preparation (37). In neither instance is the receptor 

 potential effected. 



The times of action of these solutions are remark- 

 ably similar for the two preparations and in both 

 instances are very long compared with the time such 

 solutions take to act on isolated single nerve fibers. It 

 seems likely that diffusion times play an important 

 part. It is known from direct experiments with 

 labelled sodium, potassium and bromine that diffu- 

 sion through the capsules of the Pacinian corpuscle is 

 slow C38). 



In the Pacinian corpuscle, however, it is possible to 

 perfuse the receptor through the capillary loop that 

 enters the corpuscle with the axon and ramifies in its 

 proximal pole (19). Using a perfused preparation of 

 this kind it is found that procaine in a concentration 

 of 0.02 to 0.05 per cent in the perfusion fluid abolishes 

 the impulse; 0.05 per cent and higher concentrations 

 of procaine cause a reduction of the receptor potential 

 amplitude. The abolition of the impulse occurs within 

 1.5 min. 



If these preparations of the Pacinian corpuscle are 

 perfused with a sodium-free solution the amplitude of 

 the receptor potential falls and after about 20 min. 

 the amplitude is constant and very small. This is 

 illustrated in figure 9. This reduction in amplitude 

 occurs whether the sodium chloride of the physi- 

 ological solution is replaced by choline chloride or 

 by sucrose. The effect, under faNoraisle conditions, 

 is reversible and recovery occurs on changing the 

 perfusion fluid back to a physiological solution. 

 When different concentrations of sodium are per- 

 fused it is found that the amplitude of the receptor 

 potential, measured after a constant le\el has been 

 reached, is related in a graded manner to the con- 

 centration of sodium. When sodium is absent there 

 is a small remnant of the receptor potential; it is 

 probable that this represents a genuine property 

 of the receptor (19). 



The receptor potentials of other types of receptor 

 have also been found to be resistant to local anes- 

 thetics. Cocaine (0.5 per cent) applied externally has 

 little or no effect on the potentials of the olfactory 

 mucous memljrane, though the same application 

 abolishes the responses of the olfactory bulb (78). 

 Procaine in concentrations of 0.05 to o. i per cent in 

 the bathing fluid abolishes the impulses but not the 

 receptor potential of the crayfish stretch receptor. 



The position at the present time seems to be that 

 while receptor potentials are more resistant than 



FIG. 9. Effect of perfusion with a sodium-free solution on 

 receptor potential amplitude. Abscissa: time in min. Ordninle: 

 receptor potential amplitude, arbitrary units. Sodium chloride 

 was replaced with sucrose and changes in recording resistance 

 have been corrected for. Impulses were abolished with pro- 

 caine but were allowed to reappear during the period marked 

 by the dotted line. [From Diamond, J., J. A. B. Gray & D. 

 Inman. Unpublished figure.] 



impulses to procaine, in the Pacinian corpuscle at least 

 quite low concentrations (0.05 per cent) do affect 

 the receptor potential if the diffusion barriers are 

 avoided by perfusion. Perfusion also reveals that the 

 receptor potential is almost completely abolished in 

 the absence of sodium. 



TRANSMISSION OF ENERGY TO THE RECEPTOR ELEMENTS 



It has long been recognized that there are factors 

 in the transmission of the exciting energy to the re- 

 ceptors that are important in the functioning of the 



