Effect of calcium on the differentiating operation of the ampullary electroreceptor inIctalurus nebulosus

Summary

• 1. Nerve impulses from the afferent fibre of the ampullary electroreceptor inIctalurus were recorded and changes of the response characteristics induced by calcium ions in the medium surrounding the fish were examined.

• 2. Under normal calcium concentration (2 mM/1) set and break of long lasting electrical stimuli lead to frequency changes, which are equal in size (and have opposite direction) for anodal and cathodal currents, respectively. With calcium deficiency only anodal currents evoke a response (Fig. 1).

• 3. Also in the case of low calcium, responses to intensity steps depend on direct current flow, but are largely unaffected by the d.c.-level under normal and high calcium (Fig. 3).

• 4. The decline of the frequency response does not alter the height of a succeeding response, and the off-response is fullsized already at the beginning of frequency decline. The response to the second current-step is only influenced by a preceding current onset, if the interval is shorter than 50–90 ms. For longer delays the frequency response is the same as a reaction unaffected by a prestimulus (Figs. 4–6). This time dependent decrease of the prestimulus effect is not seen with low calcium: there is a permanent (sensitivity enhancing) effect of direct current flow (Fig. 7).

• 5. From these results it has become evident that the differentiating operation mode of the ampullary electroreceptor is based on two different processes: (1) a calcium dependent mechanism, which makes the receptor independent of direct current and (2) the decline of the frequency response during continuous current flow which is not affected by calcium ions.