AMPULLARY ELECTRORECEPTORS 499 



extraordinarily sensitive relation between transmitter release and presynaptic 

 voltage (Bennett 19716, c). However, in the ampulla of Lorenzini the 

 oscillations in individual receptor cells may have such large amplitudes that 

 no unusual sensitivity of the secretory process is required. 



Accommodation 



A highly sensitive low-frequency receptor is analogous to a high-gain d.c. 

 amplifier, and each needs a balance adjustment to remain in its operating 

 range. In the skate the electroreceptor accommodates more or less 

 completely to moderate stimuli but does not lose its incremental sensivity. 

 That is, it is still sensitive to very small voltage changes (Murrary 19656). 

 This property is of obvious value to an electroreceptor that could be 

 subjected to steady voltages from external sources or changes in resting 

 potentials due to slight changes in the compositions of body fluids. 



The tonic activity of the receptor cells may be an important factor in the 

 accommodation. An increase in activity would cause an increased Ca influx 

 through the lumenal membranes; this would increase the Ca-dependent 

 outward current and tend to reduce the activity. If activity were lowered, 

 cytoplasmic Ca concentration would drop, turning off Ca-activated channels 

 and allowing the cell to depolarize. Presumably, with appropriate activation 

 curves and rates of Ca removal these mechanisms could give the requisite 

 uniformity of incremental sensitivity. Of course one could give the same 

 description substituting voltage-sensitive outward current for Ca-activated 

 outward current. However, it may be that the Ca-activated mechanism gives 

 the cell a better "memory" of its recent activity pattern. Accommodation 

 does occur, at least in part, in the receptor cells as demonstrated by the 

 fall-off in transepithelial voltages during small stimuli (Figure 9C and much 

 more extensive observations. ) 



A component of accommodation may be mediated by the afferent nerve, 

 as suggested by Murray (1962). However, his observations of accommoda- 

 tion during polarization applied to the nerve should be reexamined in view 

 of the more recent demonstration that the receptor is sensitive to electric 

 stimuli that could be applied to the ampulla by the nerve itself. The nerve 

 does not appear to accommodate to stimuli lasting for periods of the order 

 of the interval between spontaneous impulses. If one or several extra 

 impulses are evoked antidromically, the next orthodromic impulse arises at 

 about the time it would have if an equal number of impulses had been 

 generated orthodromically (Murray 19656). Over this time scale, the nerve is 

 more of an integrator than the differentiator required for accommodation. 

 For trains of brief stimuli applied to the epithelium (at frequencies greater 

 than about 5/s) the receptor also behaves as an integrator and responds 

 essentially to the d.c. component of the stimulus (Murray 1965a). 



The role of the receptor cells in accommodation can be illuminated by 

 voltage clamp experiments with repetitive pulses. The onset of the late 

 outward current is facilitated; that is, it requires less Ca influx for some 

 seconds after a prior stimulus that causes a Ca influx (Clusin and Bennett 



