Characterization of the intrinsic dynamics of isolated retinal bipolar cells by a whole-cell patch-clamp technique combined with estimation of effective impulse responses across a range of mean injected currents reveals strikingly adaptive behavior. At resting potential, bipolar cells' effective impulse response is slow and lowpass. Depolarization decreases gain, speeds up response, and, in most cells, induces bandpass behavior.
This adaptive behavior involves two potassium currents, the delayed-rectifier, which accounts for the observed gain reduction, speed increase, and bandpass behavior, and the A-channel, which further shortens the impulse responses but suppresses bandpass features. Computer simulations of model neurons with varying A-channel conductance show that impulse responses largely reflect the flux of electrical charge through the potassium channels and that A-channel broadens frequency response and preempts the action of the delayed-rectifier, thereby reducing the associated bandpass features. Thus, admixtures of the two potassium channels produce the distinctive dynamics of retinal bipolar cells.