486 ELECTRICAL SENSES 



lumen-positive stimulus is inhibitory in terms of nerve discharge, and a 

 lumen-negative stimulus is excitatory. This polarity inversion results from 

 excitability of the lumenal faces of the receptor cells. A lumen-negative 

 stimulus depolarizes and excites the lumenal faces. Their regenerative 

 depolarizing response depolarizes the basal faces so that they secrete 

 transmitter. There is activity of the receptor cells even in the absence of 

 applied stimuli. A lumen-positive stimulus decreases this activity and leads to 

 relative hyperpolarization of the basal faces and reduced release of 

 transmitter. The detailed evidence for these mechanisms is described in the 

 next sections. 



Excitability of the Lumenal Membranes of the Receptor Cells 



One of the beauties of the ampulla of Lorenzini as an experimental 

 preparation is that it can be removed from the animal with sections of canal 

 and nerve attached. The canal can then be rinsed with nonelectrolyte 

 (sucrose-urea) solutions to electrically isolate the epithelium. The responses 

 to controlled currents or voltages applied across the epithelium in such a 

 preparation can be recorded from the distal end of the canal or by electrodes 

 penetrating it near the ampulla (Figure 3). 



The nerve response also can be recorded. In the presence of tetrodotoxin 

 (TTX), nerve impulses are blocked and postsynaptic activity ("postsynaptic 

 potentials," or PSPs) that initiate impulses can be measured by external 

 electrodes with relatively little distortion. A further important advantage is 

 that the canal and basal surface of the epithelium can be independently 

 perfused with experimental solutions. 



When the ampulla is electrically isolated it develops a lumen-positive 

 resting potential of about 10-30 mV (Clusin and Bennett 1977a). Moderate 

 lumen-positive rectangular current pulses and small lumen-negative ones 

 produce exponentially rising and falling voltages (Figure 4A, B) and give a 

 linear voltage current relation (Figure 5). The receptor epithelium behaves 

 like a simple resistance and capacity in parallel. Somewhat larger lumen- 

 negative stimuli, at a threshold of about mV, elicit a regenerative response, 

 an all-or-none action potential up to 100 mV in amplitude and about 100 ms 

 in duration (Figure 4A, C). This action potential is lumen negative and is an 

 active response because it can outlast and is much larger than a near- 

 threshold stimulus (Figure 4C). Its polarity is consistent with its being a 

 depolarizing response of the lumenal faces, and we have generally displayed 

 it lumen-negative-upward so that it appears like an ordinary action potential 

 recorded intracellularly from a single cell. Actually, the ampulla behaves in 

 many respects like a single, inside-out cell. 



The origin of the response as a calcium spike in the lumenal membranes is 

 established by perfusion experiments (Clusin and Bennett 1977a). Drastic 

 changes of ions bathing the basal face have little or no effect on the 

 response. Perfusion of the lumen with Co, an ion that blocks Ca channels in 

 many tissues, blocks the response as does zero Ca, EGTA solution. TTX, 



