We studied how intrinsic membrane properties affect the gain and temporal pattern of response in bipolar cells dissociated from retinae of tiger salamanders. Currents specified by a pseudorandom binary sequence, an m-sequence, superimposed on various means, were injected into the cells. From the resultant membrane voltage response for each mean current, impulse responses were estimated. From each impulse response, transfer function, gain and time constant were calculated.
The bipolar cells acted as quasilinear adaptive filters whose gain and response speed are determined by the mean input current. Near resting potential, gain and time constant were maximum. Dynamics were slow and low-pass, characterized by an approximately exponential impulse response. With depolarization, gains were reduced sharply, responses were much faster, and dynamics became band-pass, as indicated by an undershoot in the impulse response. For any given mean current, the shape of the impulse response did not depend on the amplitude of the m-sequence currents. Thus, bipolar cells behaved in a quasilinear fashion. The adaptive behavior was eliminated by blocking a potassium current, which implicates the role of a voltage-gated potassium conductance.
Computer simulations on a model neuron including a delayed-rectifier reconstructed the observed behavior, and provided insight into other, less readily observable, parameters.
Thus, bipolar cells, even when isolated, possess mechanisms which regulate, with unsuspected elaborateness, the sensitivities and dynamics of their responsiveness. Implications for adaptation and neuronal processing are discussed.