AMPULLARY ELECTRORECEPTORS 



495 



a i hhlftl/llUllMllltilllllUliiiiiaiiil 



( llflllll^l tA ' , ' t *-* tl ^ n> /llHllrt/tfllUl>nl 



a 2 W\/\/Vv^^^A^^A^ 



H tvm mn^ mftmiimiimm t 



A3 



*^^ 



SALT BRIDGE 

 LIFTED | 



20 jjV 



*4nA 



* 0.1 ,0.2 S 



Figure 8 Spontaneous activity of the short-circuited receptor. (A1-A3) Oscillations re- 

 corded in an ampulla short circuited by a salt bridge. (B) Transepithelial current measured 

 as voltage drop along the canal before and after lifting the salt bridge. There is a steady 

 inward current through the lumenal membranes (displayed downward) with small super- 

 imposed oscillations until the connection provided by the bridge is removed, after which 

 essentially zero current flows along the canal. (C, D) Oscillations recorded in the short- 

 circuited ampulla (C) are abolished when the ampulla is made 4 mV positive (D) 

 (high-gain a. c. -coupled recordings). (From Clusin and Bennett 1978a.) 



(lumen-positive) stimulus of a few millivolts the oscillations cease (Figure 8d). 

 In the short-circuited condition there is a steady inward current through the 

 lumenal faces with small oscillations superimposed. Unshorting the ampulla 

 blocks this current (Figure 8B). 



Oscillations corresponding to the oscillations in the ampulla can be 

 recorded from the afferent nerve. These oscillations are somewhat delayed, 

 which indicates that they are in fact PSPs and not electrical pickup from the 

 receptor cells (Figure 9C, D). As would be expected, the polarities are such 

 that a response of the lumenal membranes of the receptor cells leads to 

 excitation of the nerve. Although the steady level of postsynaptic activity is 

 difficult to measure directly, evidently the tonic activity of the receptor cells 

 causes tonic PSP activity in the nerve. 



Excitatory stimuli of a few microvolts increase excitation of the nerve and 

 inhibitory stimuli decrease it (the stimuli are 5 and 15 juV in Figure 9A, B). 

 Presumably there is a change in the oscillations of the receptor cells over the 

 population as a whole, but it is difficult to be sure from single sweeps 

 because of the irregularity of the background oscillations. Only moderately 

 larger stimuli produce clear changes in the amplitudes of the oscillations 

 (Figure 9C). 



