AMPULLARY ELECTRORECEPTORS 



491 



LATE 



Rsh' 



X 



J 



Figure 6 Equivalent circuit of the ampullary epithelium. Active 

 currents arise in the lumenal membranes of the receptor cells. These 

 lumenal membranes are represented by a fixed resistor r LUM in 

 parallel with two variable resistors. The early calcium current 

 flows through r Ca and the late current through r LATE . The 

 basal faces of the receptor cells are represented by fixed resistance 

 r BAS- Most of the leakage current flows through the supporting 

 cells, the marginal zone cells, or the intercellular clefts. These 

 pathways are represented by a single resistor i? SH . The batteries 

 £ Ca- £ lum- £late> ^bas* and ^sh are drawn but not labeled. 

 The ampullary capacity is attributed to cells that are electrically 

 in parallel with the receptor cells, namely the supporting cells and 

 marginal zone cells, whose basal membrane area is large. Some of 

 the ampullary capacitance is in series with a significant fixed 

 resistance. (From Clusin and Bennett 1977a.) 



resistances (for d.c. measurements the resistance in series with one of the 

 capacities is irrelevant). 



The voltage current relation of the epithelium allows measurement of only 

 three slopes. However, further information is available. As an excitatory 

 stimulus is increased, current flowing inward through the lumenal mem- 

 branes and outward through the basal membranes is reduced. Consequently, 

 the basal membranes are less depolarized and less transmitter is released. If 

 shunt resistance Rsh were infinite, there would be no current inward 

 through the receptor cells, when there was no net current across the 

 epithelium. However, transmitter release persists well beyond the point at 

 which net epithelial current changes in sign. Evidently in this range current 

 flows inward through the lumenal membranes and on outward through the 

 basal membranes, while a larger current flows outward through the shunt 

 resistance. 



If one assumes the basal membranes to be sensitive to outward current, as 

 they should be in an electroreceptor, the point at which the PSP fails will be 

 close to the point at which all the current across the epithelium is going 



