76 The Physiology of Sense Organs 



its plausibility, Katz dismissed his proposal on quantitative 

 grounds. The hypothesis put forward by Bennett has so far held 

 up under the scrutiny of various experimental procedures. 



Tonic, Ampulla ry Receptor 



Phasic, Tuberous Receptor 



\// ^A — ^v^-T— w^- -, 



r. -L 



receptor external, internal, and 

 cell skin resistances 



Fig. 34. Diagrams of the anatomy of two types of electroreceptors found 

 in Mormyrids and their electrical analogs. AmpuUary, or tonic, receptors 

 act as series resistances to current flowing as the result of an impressed 

 voltage. Outward current across the inner face of the receptor cells 

 presumably liberates a chemical transmitter and excites second-order 

 sensory nerve fibers. The tuberous, or phasic, type of receptors have a 

 large membrane capacitance associated with their outer face; thus, even 

 small changes in voltage of the external medium result in large current 

 fluxes across the membrane of the inner face of the cell. When this 

 current is directed outward, it triggers an action potential across the 

 membrane and a transmitter substance is liberated. (Courtesy of 

 M. V. L. Bennett.) 



