490 ELECTRICAL SENSES 



Extrapolation of the resting resistance line and the peak inward current 

 line shows that the point at which the Ca current reverses is the point at 

 which the late current fails. We have termed this point the suppression 

 potential for the late outward current. It is analogous to the suppression 

 potential at the squid giant synapse, which is the potential, inside positive, at 

 which transmitter release is blocked (Katz and Miledi 1967, Kusano et al. 

 1967). It is also the point at which Ca entry fails (Llinas and Nicholson 

 1975), and thus it is the same as the Ca equilibrium potential. If the lumen 

 of the ampulla is perfused with low Ca solution, the potential required to 

 reverse the early current is reduced, but it remains identical to the 

 suppression potential for the outward current (Clusin and Bennett 19776). If 

 Ca entry is blocked by Co or zero Ca, there is no late outward current. The 

 conclusion we have reached is that this outward current is activated by 

 calcium influx. Comparative considerations suggest that it is a K current, 

 although we cannot yet exclude a CI contribution. Calcium-activated K 

 conductances have now been found in a number of excitable as well as 

 inexcitable cells (cf. Meech and Standen 1975). 



When activation of the late current is blocked, the early current persists 

 unabated; it shows little or no inactivation. This is a property commonly 

 found in Ca conductance systems (Katz and Miledi 1969). 



When the epithelium is repolarized after a clamping pulse that activates 

 the late outward current, there is a tail current that lasts about 600 ms 

 (Clusin and Bennett 1977&). When only Ca activation has occurred tail 

 currents are much briefer and are difficult to distinguish from the 

 capacitative transients. Thus Ca activation reverses relatively quickly, while 

 activation mediating the late outward current is much longer lasting. 

 Presumably its duration represents, in part, time for reduction in cyto- 

 plasmic Ca concentration by sequestration or extrusion. 



The responsiveness of the ampulla must arise, at least in part, in the 

 receptor cells, because PSPs and afferent discharges are associated with the 

 action potential generated by the electrically isolated epithelium (Figure 4C). 

 From measurements of the electrical capacity and membrane areas of the 

 ampulla, we have inferred that the supporting cells behave as passive 

 elements during the ampullary response. 



Equivalent Circuit of the Epithelium 



In the preceding discussion the ampulla was seen to behave like a single 

 inside-out cell. In this section we analyze the complexities due to the series 

 resistance of the basal membranes of the receptor cells and the shunt 

 pathways around these cells. An equivalent circuit of the epithelium, in 

 which all the receptor cells are lumped together, is given in Figure 6. There 

 are three resistances in the lumenal membrane of the receptor cells: the 

 resting or leakage resistance, that of the voltage-sensitive Ca system, and that 

 of the Ca-activated outward current system. There are also the resistances of 

 the basal membranes and the shunt pathway. This is a total of five 



