Electroreceptor model of weakly electric fish Gnathonemus petersii: II. Cellular origin of inverse waveform tuning

In part I (Shuai et at. 1998. Biophys. J. 75:1712-1726), we presented a cel lular model of the A- and B-electroreceptors of the weakly electric fish Gn athonemus petersii. The model made clear the cellular origin of the differe nces in the response functions of A- and B-receptors, which sensitively cod e the intensity of the fish's own electric organ discharge (EOD) and the va riations in the EOD waveform, respectively. The main purpose of the present paper is to clarify the cellular origin of the inverse waveform tuning of the B-receptors by using the receptor model. Inverse waveform tuning means that B-receptors respond more sensitively to the 180 degrees inverted EOD t han to undistorted or less distorted EODs. We investigated how the A- and B -receptor models respond to EODs with various waveforms, which are the phas e-shifted EODs, whose shift angle is varied from -1 degrees to -180 degrees , and single-period sine wave stimuli of various frequencies. We show that the tuning properties of the B-receptors arise mainly from the combination of two attributes: I) The waveform of the stimuli (B-stim) effectively sens ed by the B-receptor cells. This consists of a first smaller and a second l arger positive peak, even though in the original phase-shifted EOD stimuli, the amplitudes of the two positive peaks are reversed. 2) The effective ti me constant of dynamical response of the receptor cells. It is on the order of the duration of a single EOD pulse, We also calculated the response pro perties of the A- and B-receptor models when stimulated with natural EODs d istorted by various capacitive and resistive objects. Furthermore, we inves tigated the effect of EOD amplitude on the receptor responses to capacitive and resistive objects. The models presented can systematically reproduce t he experimentally observed response properties of natural A- and B-receptor cells. The mechanism producing these properties can be reasonably explaine d by the variation in the stimulus waveforms effectively sensed by the A- a nd B-receptor cells and by time constants.