Sensitivity of catfish electroreceptors: Dependence on freshwater ions and skin potential

Summary

• 1. The sensitivity of the electroreceptors in the skin ofKryptopterus depends on the ionic concentration of the freshwater surrounding the fish (Figs. 1, 3 upper half). The mechanism of this dependence was investigated.

• 2. Each freshwater solution polarizes the fish skin (‘skin potential’, cf. Fig. 2); the sensitivity-enhancing solutions (CsCl, NaCl, KCl) in the anodal direction (fish outside positive vs inside), the sensitivity-decreasing solutions (CaCl₂, CoCl₂, LaCl₃) in the cathodal direction (Figs. 2, 3 lower half).

• 3. Each solution produces not only a specific skin potential, but also a characteristic dependence of the receptor sensitivity on the skin potential with a specific sensitivity maximum (Fig. 5). With the sensitivity-enhancing solutions, both the skin potential and the potential of the sensitivity maximum lie near zero mV or are anodal, and are nearly equal; with the sensitivity-suppressing solutions, both the skin potential and the potential of maximal sensitivity are cathodal and have widely different values. There are indications of an additional more direct reaction of La^{+ + +} and Co^{+ +} ions with the receptor, probably at sites accessible from within the sensory cells.