AMPULLARY ELECTRORECEPTORS 



493 



Responsiveness of the Basal Faces 



One would expect that the basal faces had some excitable Ca channels 

 because transmitter release is produced by depolarization of this face, 

 requires Ca, and is blocked by high Mg solutions (Steinbach 1964). 

 Excitability of the basal faces can be demonstrated in ways similar to those 

 used with other gradedly responsive systems, such as the presynaptic fiber of 

 the squid giant synapse (Katz and Miledi 1969). If 2-mM tetraethyl- 

 ammonium (TEA) and high Ca are applied to the basal membranes or if Ba is 

 substituted for Ca, these membranes become able to generate a prolonged 

 action potential (Figure 7). This response is recorded as a long-lasting, 

 lumen-positive potential. It can be evoked by lumen-negative stimuli that 

 excite the lumenal membranes, which then excite the basal membranes; it 

 then follows the briefer lumen-negative action potential of the lumenal 

 membrane (Figure 7C). Alternatively, it can be evoked by lumen-positive 

 stimuli that directly depolarize the basal membranes (Figure 7D). The 

 long-lasting responses of the basal faces are associated with large and 

 long-lasting PSPs in the afferent nerve. The excitability of the basal faces in 

 normal solutions has little effect on the voltage— current relations of the 

 epithelium described in the previous section, but it is important in 

 electroreception, as discussed in the next section. 



E lect rorecep tion 



When the ampulla and canal preparation is held short circuited to zero 

 transepithelial potential (by a salt bridge connecting the two pools of Figure 

 3), the receptor epithelium is in its negative slope region. Because of the low 



Table 1. Membrane resistance values 



Whole ampulla, One receptor 1 cm of membrane, 

 r/n cell, r T X area 



kft 



m£2 



flcm 2 



' LUM 

 r BAS 

 r Ca 

 r LATE 



1/(G 



Ca 



+ G 



LATE 



) 



>3,800 

 112 + 6 

 48 ±6 

 6.1 ± 3.0 

 5.4 ± 2.5 



>38,000 

 1,120 ±60 

 480 ± 60 

 61 ± 30 



54 ± 25 



>3,000 



5,500 ± 300 

 38 ± 5 

 4.9 ± 2.4 

 4.3 ± 2.0 



The values of r Ca and r BAS are based on the assumption that £late ~ ~ ^ m ^ an ^ ^sh 

 = 352 k£7. The errors represent maximum deviations of the listed value from values cal- 

 culated with the extreme assumptions ^late = ar *d -12 mV, and i?sH = 322 and 352 

 k!2. The calculated value of r LU M does not depend on #late- Values for a single cell 

 (second column) assume 10 4 cells. Values in the third column assume the receptor cells 

 to have a lumenal membrane area of 8 jKm 2 and a basal membrane area of 500 |Um 2 . From 

 Clusin and Bennett (19776). 



