488 



ELECTRICAL SENSES 



A 



PSP 



B 



50 



35 



1 20 



> 



i 

 > 



10 



5 - 



D 



V 



P$P -uwjuauUJ> 



r * A ****iUulJLUU» 



100 

 TIME (ms) 



200 



Figure 4 Effects of constant current on the ampullary epithe- 

 lium. Applied current is shown in the top trace, lumen-negative 

 stimuli upward. The transepithelial voltage appears below with 

 the lumen-negative responses shown upward. In A, hyperpolar- 

 izing and subthreshold depolarizing stimuli cause the epithelium 

 to exponentially approach a voltage that is proportional to ap- 

 plied current. There is a 30-mV lumen-positive resting potential, 

 the threshold for the action potential being about mV. In B, a 

 semilog plot of voltage minus final voltage vs time after stimulus 

 onset confirms the exponential time course of the passive re- 

 sponse. The time constant of the inactive epithelium is 126 ms 

 (average value from plots 1 and 2 in B). The inactive or leakage 

 resistance of the epithelium is 372 k£2 and the capacity is there- 

 fore 0.34 [JiF. An ampullary action potential and corresponding 

 postsynaptic potential (PSP) are shown in C. The trace passing 

 through the base of the action potential parallel to the base line 

 indicates mV. The extracellular recording of the PSP (bottom 

 trace) shows superimposed action potentials arising in the nerve 

 fibers. There is no change in postsynaptic activity from the 

 spontaneous level until the threshold is reached. In D, transmitter 

 is released by a strong lumen-positive stimulus which directly 

 depolarizes the basal, secretory membranes of the receptor cells. 

 The current calibration (vertical bar) is 0.4 [lA in A, 1.0 /xAinC, 

 and 2.0 /J.A in D. The transepithelial voltage calibration (vertical 

 bar) is 70 mV in A, 40 mV in C, and 200 mV in D. The post- 

 synaptic voltage calibration (vertical bar) is 2.0 mV in C and 

 0.8 mV in D. The time calibration (horizontal bar) is 0.4 s in A 

 and C and 0.2 in D. From Clusin and Bennett 1977a. 



