502 



ELECTRICAL SENSES 



due to external fields are developed almost entirely across the skin. 

 Presumably this represents an adaptation to the high resistance of fresh 

 water in that the fish is better impedance matched to its environment. 



In spite of the superficial similarity to freshwater teleost receptors, 

 Potamotrygon receptors should still be classed as ampullae of Lorenzini, for 

 their receptor cells have apical cilia like those of other elasmobranch re- 

 ceptors and they exhibit the same polarity of sensitivity (that is, they are 

 excited by externally negative stimuli and inhibited by externally positive 

 stimuli (Figure 10)). The receptors have not been tested for the opposite 

 effects of very strong stimuli that directly polarize the basal faces. Although 



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STIMULUS- mV 



Figure 10 Responses of an afferent fiber from an ampulla of Lorenzini of the freshwater 

 stingray Potamotrygon. Upper trace was recorded from a single afferent fiber, lower 

 trace represents potential produced on the skin (in air) outside the receptor by a square 

 pulse of current. High resistivity is indicated by the gradual rise and fall of potential. Out- 

 side negative stimuli excite (A-D) and increase the impulse frequency over that of the 

 tonic resting discharge (H). Outside positive stimuli inhibit (E-G). The graph shows the 

 mean discharge frequency during 100-ms pulses of varying amplitudes. A sigmoidal rela- 

 tion, linear for small stimuli of either sign, is evident. Compared to that of marine (skate) 

 ampullae, the sensitivity is very low. 



