Functioning of catfish electroreceptors: Statistical distribution of sensitivity and fluctuations of spontaneous activity

Abstract

1. Properties of electroreceptors (small pits) in the dorsal skin of the head of catfish,Ictalurus nebulosus, were determined by recording the impulse activity from the lumen of individual receptors in intact fish. 2. The sensitivity of a small pit to sinusoidal current was defined as the difference of the maximum and minimum average impulse frequency during the stimulus period, divided by the stimulus current. 3. The statistical distribution of the sensitivity was found to be unimodal, with a modus at 0.5 Hz/nA. The sensitivity of 96% of the small pits is between 0.04 and 1.6 Hz/nA (Fig. 1). 4. The input/output relation of the small pits (impulse frequency versus stimulus current) is sigmoidal, but has a nearly linear segment at small stimulus amplitudes (Fig. 3). 5. The statistical distribution of the mean spontaneous impulse frequency has a modus at 40 Hz. The spontaneous activity of 95% of the small pits is between 15 and 70 Hz (Fig. 2). 6. The frequency of the spontaneous activity of individual small pits fluctuates considerably. The standard deviation (RMS value) of this “frequency noise” is between 6 and 35 Hz, and varies little with sensitivity (Fig. 5). 7. Both the sensitivity and the frequency noise are reduced by an outward current of 0.5 to 6 μA, and by exposition to a calcium free medium. 8. The origin of these phenomena in the receptor structures is discussed.