PERIODIC FIRING PATTERN IN AFFERENT DISCHARGES FROM ELECTRORECEPTOR ORGANS OF CATFISH

Spontaneous afferent activity was recorded from 26 single ampullary el ectroreceptive organs of freshwater catfish (Ictalurus nebulosus LeS) at various temperatures. Regular grouping of action potentials was app arent in this secondary sensory system at 35 degrees C and occasionall y at 30 degrees C. Impulse groups consisted of up to seven impulses. T he precise timing of impulse generation and the temporal sequence of i mpulses indicated that oscillating processes are involved. Expectation density functions were calculated for records of afferent activity ob tained at various temperatures below 35 degrees C. In the majority of records the function was periodic. Impulse grouping and expectation de nsity functions became more distinct in units exhibiting extremely hig h thresholds (i.e. being insensitive) to electrical stimuli. The resul ts suggest that the oscillations originate from the postsynaptic membr ane. The temporal pattern of impulse generation within impulse groups of ampullary electroreceptor organs and of specific warm and cold rece ptors was compared and found to be similar. Application of cadmium and menthol, which both reduce calcium entry, suppressed spontaneous acti vity in normal and insensitive electroreceptor systems, attenuated the sensitivity of normal receptors and modified the periodic pattern. Th is indicates that calcium is implicated in sensory transduction and in postsynaptic mechanisms. The data suggest that an oscillating process is one component of signal transmission in ampullary electroreceptor organs of teleost fish.