500 ELECTRICAL SENSES 



1977b, Huse et al. 1977). The duration may be of the order of a hundred of 

 the oscillations of the short-circuited epithelium, and comparable to the time 

 for accommodation to weak stimuli. Careful analysis of the time course of 

 the facilitation should illuminate the time course of removal of cytoplasmic 

 Ca and perhaps provide insights into mechanisms that are of some wider 

 application. 



M ultimo dality of Responsiveness 



Given the many factors involved in the electrical sensitivity of the receptors 

 and the high degree of sensitivity, it is not surprising that the ampulla is 

 responsive to many modes of stimulation (Murray 1974). The receptors are 

 as sensitive to temperature as mammalian skin receptors, but no analysis is 

 available of the site of action. The accommodation patterns indicate that this 

 is not a simple thermoelectric effect, but temperature would be expected to 

 act on many of the receptor mechanisms. Again, one notes that the receptors 

 are most unfavorably situated for sensing temperature, and behavioral 

 experiments demonstrate an electrical perception mediated by the ampullae 

 (Kalmijn 1974, 1978). Mechanical sensitivity, although present, is unremark- 

 able, particularly compared to the ordinary lateral-line receptors, and the 

 evidence indicates little activation of ampullae by the usual mechanical 

 stimuli in the animals' environment. 



Sensitivity to magnetic fields is present through induced electric fields 

 when the animal is moving, and Kalmijn gives evidence that this sensitivity is 

 used in navigation. 



Sensitivity to salinity changes is transient, if quite marked (Murray 

 19655). Whether such responses are present as the animal swims through 

 large gradients, or whether they require local application to the canal 

 opening, is unknown. 



Accessory Structures and Sensitivity of Electroreception 



In the marine elasmobranchs the ampullary canals radiate from the several 

 capsules in which the ampullae are located (Murray 1967, Bennett 19715). 

 The resistance of the canal walls is extremely high, so that their space 

 constants are extremely long (Waltman 1966). The resistance of the 

 ampullae that terminate the canals is also high compared to the axial 

 resistance of the canals. Thus, there is little voltage drop along the canals. 

 The resistance of the skin, in the skate at least, is quite low, and the body 

 interior is of somewhat greater resistance than seawater. Thus, a voltage 

 gradient in the water is not greatly distorted by the fish itself (Murray 1967). 

 Given these conditions, the stimulus for the ampulla is the difference 

 between the voltage at the opening of the canal and thatjn the body interior 

 just outside the basal faces of the receptors cells. The different receptors, 

 with different lengths and orientations, would thus have different absolute 

 sensitivities to uniform gradients of various orientations. Each receptor 

 would be most sensitive to gradients parallel to the axis of the canal. The 



