Temporal resolving power of blowfly visual system: effects of decamethonium and hyperpolarization on responses of laminar monopolar neurons

Abstract

1. The temporal resolving power of blowfly (Calliphora erythrocephala) photoreceptors and laminar monopolar neurons was investigated by stimulating the eye with sinusoidally modulated light of different intensities. Temporal resolving power was measured as -3 dB values of the maximal amplitude. 2. -3 dB values of photoreceptors increased only slightly with intensity from 25 to 35 Hz within the intensity range studied. -3 dB values of monopolar neurons increased sigmoidally from 30 Hz at low light intensities to 100 Hz at high intensities. 3. With regard to dark membrane potential, responses of monopolar neurons to stimulation with sinusoidally modulated light were formed by depolarizing and hyperpolarizing components. With iontophoretic application of decamethonium and hyperpolarizing current it was possible to reduce the depolarizing and hyperpolarizing components, respectively. 4. Monopolar neurons, whose depolarizing components were reduced by decamethonium, showed a constantly low temporal resolving power (30-40 Hz), similar to that of photoreceptors. The intensity dependence of cells with reduced hyperpolarizing components had nearly the same sigmoidal shape as that of untreated cells. However, the s-3 dB values were shifted 30–40 Hz to lower values. 5. These results suggest that the hyperpolarizing components of monopolar neurons' responses to sinusoidally modulated light are mainly responsible for a basic level of the temporal resolving power, whereas the depolarizing components produce the increase of temporal resolving power with increasing light intensity.