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Figure 7 Evidence for excitability of the basal faces of the receptor cells recorded as in 

 Figure 9, but at lower gains. (A) Normal monophasic action potential recorded across the 

 current-clamped ampulla in physiological saline. (B) Strong lumen-positive stimuli can 

 cause nerve responses by direct depolarization of the basal faces. Inexcitability of the 

 presynaptic membrane in physiological saline is indicated by the absence of a positive- 

 going regenerative response and by the immediate cessation of transmitter release at the 

 end of the stimulus. The transepithelial potential declines slightly during the response. 

 This presumably results from membrane breakdown associated with the large stimuli 

 applied, although slight responsiveness of the basal faces cannot be excluded. There is 

 some direct pickup in the nerve recording of this very large stimulus. (C, D) The basal 

 faces become excitable when perfused with 20-mM calcium and 2-mM TEA: (C) Stimula- 

 tion of the lumenal membranes by a lumen-negative stimulus produces a diphasic receptor 

 action potential, the prolonged lumen-positive phase arising in the basal faces. This re- 

 sponse is associated with a large PSP and intense postsynaptic impulse activity. (D) Direct 

 stimulation of the basal faces by a lumen-positive stimulus produces a lumen-positive 

 regenerative response (onset at arrow), which starts during the stimulus and ends after its 

 termination. This response is also accompanied by augmented transmitter release and 

 postsynaptic impulse activity. 



resistance of the canal, the epithelium is rather well clamped at zero 

 potential. However, as stated in the previous section, the receptor cells 

 cannot be clamped at this potential because of the resistance of the basal 

 membrane. And high gain recordings of the epithelial potential show small 

 oscillations that wax and wane (Figure 8A, C). Evidently the individual 

 receptor cells (of which there are several thousand) are not clamped 

 but are oscillating under these conditions. If one applies an inhibitory 



