Coding in tuberous and ampullary organs of a gymnotid electric fish

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Abstract
The coding mechanisms of the tuberous and ampullary organs of Gymnotus carapo were studied. The tuberous fibers had no or irregular spontaneous discharges and showed phasic on and/or off responses to 50 msec electric pulses, while the ampullary fibers had regular spontaneous discharges and showed tonic responses to the pulses. The threshold measured with the electric pulses was 1.5 mV/cm on the average for the tuberous fibers and 0.26 mV/cm for the ampullary fibers. For the tuberous fibers, a 0.5–1.0 msec electric pulse was sufficiently long to evoke the maximum response when the intensity of the pulse was 3–10 times higher than the threshold of a given fiber. However, responses of the ampullary fibers were more dependent on duration.To repetitive stimuli of 0.5 msec electric pulses, the tuberous fibers discharged several impulses per stimulus; while the ampullary fibers responded by changingpontaneous discharges from an unlocked to a locked state only in a certain range of intensity and repetition rate of stimulus pulses. The number of impulses per stimulus in the locked state was not modified by change in stimulus intensity. More than half the tuberous fibers and all amullary fibers were mechanically excitable. However, these organs are not considered to act as mechanoreceptors in natural conditions, but to act as electroreceptors.