485 



SYNAPSE 



Figure 2 Diagram of cell types of the sensory 

 epithelium. Two receptor cells are adjoined by 

 supporting cells. Zonulae occludentes (ZO) 

 partition the cell membranes into lumenal and 

 basal faces. In the basal region are characteris- 

 tic ribbon synapses with the afferent fibers. 

 (From Clusin and Bennett 1977a.) 



chemically. About five afferent fibers innervate each ampulla. Efferent 

 synapses are absent. 



There is some historical merit and heuristic simplicity in describing first 

 the inferred operation of teleost tonic receptors; the comparison will be 

 made later in this exposition in any case. The morphology of teleost tonic 

 receptors is essentially the same, although the lumenal face is a somewhat 

 greater fraction of the total membrane area. Stimuli applied across the 

 epithelium are developed largely across the basal face because the lumenal 

 face is inexcitable and of relatively low resistance (Bennett 19716, c). 

 External positivity causes outward current through the basal face, tending to 

 depolarize it. There is in the absence of stimulation a resting Ca activation 

 and influx that mediate the tonic release of transmitter that causes the tonic 

 nerve activity. Further depolarization causes increased Ca activation and 

 increased release of transmitter; hyperpolarization decreases Ca activation 

 and reduces transmitter release. In freshwater teleosts these changes in Ca 

 conductance themselves cause little change in membrane potential, and in 

 terms of stimulus strength versus impulse frequency the receptor behaves 

 quite linearly over a moderate range of stimuli; stronger stimuli cause a 

 flattening of the impulse frequency relation or block nerve activity entirely. 



Although Ca activation caused by depolarization leads to inward current 

 and a regenerative response, evidently the evoked currents are too small to 

 cause significant nonlinearities, at least over a moderate range. An analogous 

 situation is observed in the squid giant synapse, in which a Ca action 

 potential can be demonstrated only when K activation is blocked and the 

 driving force for Ca is increased by increasing external Ca (Katz and Miledi 

 1969). In the marine catfish Plotosus a regenerative Ca response can be 

 recorded (Akutsu and Obara 1974, Obara 1974, 1976). 



The mode of operation of the ampulla of Lorenzini is distinctively 

 different from that of teleost receptors (Obara and Bennett 1968, 1972). A 



