AMPULLARY ELECTRORECEPTORS 



487 



Figure 3 Experimental setup for passing constant current across 

 the epithelium and recording its responses. The isolated ampulla 

 lies in one pool, and its canal crosses an air gap to a second pool. 

 Current applied between pools enters the canal and exits mainly 

 through the ampullary epithelium. A microelectrode in the lumen 

 records resulting transepithelial voltages, and current is measured 

 by an operational amplifier. The nerve is led into an oil-filled pipette, 

 and its responses are recorded by fine wires. (From Clusin and 

 Bennett 1977a.) 



which blocks Na channels in many cells, has no effect when applied to either 

 face. 



Voltage clamp experiments allow further characterization of the response 

 of the lumenal membranes. For lumen-positive clamping pulses, the 

 epithelium behaves linearly. For lumen-negative stimuli, there is an inward- 

 outward current sequence like that observed in excitable cells in general 

 (Figure 5A, C). The inward current leads to the regenerative rising phase of 

 the action potential, and the outward current terminates the response. There 

 are, however, several important differences between these records and those 

 from, for example, the squid axon. If one plots the peak inward current vs 

 voltage, the relation is linear from about 25-mV lumen-negative to very large 

 lumen-negative stimuli. When the late current is plotted, this relation is also 

 linear over an appreciable range, but for large lumen-negative stimuli the 

 onset of the outward current is progressively delayed (Figure 5A, 71-119 

 mV) and finally blocked altogether (148 mV). 



