Band-pass filtering by voltage-dependent membrane in an insect photoreceptor.

The membrane properties of short type blowfly photoreceptors (R1-6) were investigated in dark and light adaptation with single electrode current and voltage clamp techniques. The impedance of the cells was defined in frequency domain by using discontinuous current clamp and white-noise-modulated current injection. We found that the slow activation and relaxation of the voltage-dependent K+ conductance transform the photoreceptor membrane effectively into a band-pass filter. This behaviour could be observed under current clamp as voltage-dependent outward and inward rectification of the membrane. The voltage-dependent band-pass filtering is likely to be present in all neurons with graded potentials and voltage-dependent membrane conductances.