The sensitivities of dragonfly photoreceptors and the voltage gain of transduction

Abstract

1. The spectral, polarisation and absolute sensitivities of darkadapted retinula cells of the ventral retina of the dragonflyHemicorduliatau are measured by making intracellular recordings of receptor potential. 2. On the basis of their spectral sensitivities the retinula cells fall into two distinct classes. The “single pigment” cells have narrow spectral sensitivity functions corresponding to the absorption spectrum of either a UV (360 nm) or a blue (440 nm) or a green (510 nm) rhodopsin photopigment (Fig. 1). The “linked pigment” cells have broadened spectral sensitivity functions which suggest that at least three rhodopsins contribute to their response (Table 1, Fig. 2). 3. The “single pigment” UV cell has a high PS of 7.1 whereas the “linked pigment” cells are insensitive to polarised light (Figs. 4, 6). The PS(λ) functions of “linked pigment” cells (plots of PS against stimulus wavelength) show that the UV cell acts as a dichroic filter placed in front of the “linked pigment” cells (Figs. 5, 6, 7) and that self-screening plays no role in downgrading “linked pigment” cell PS (Fig. 6). 4. The absolute sensitivity of all cell types is precisely calibrated using monochromatic parallel rays of light of the most effective (peak) wavelength directed along the optical axis of the ommatidium. The PAQ50 (PeakAxial cornealQuantal irradiance required to give a transient response of 50% maximum) is measured and its reciprocal defines the APS50 (AxialPeakSensitivity as determined at the 50% level). 5. When one knows the spectral and angular sensitivities of units APS50 measurements are comparable from cell to cell and organism to organism. In dragonfly ventral retina the “linked pigment” cells and “single pigment” green cells have almost identical absolute sensitivities (APS50 = 1.5× 10−12 S.D. = 1.2× 10−12 whereas the “single pigment”UV cell is 12 times more sensitive (APS50 = 1.8× 10−11, S.D. = 2.2× 10−11 (Fig. 8). 6. The UV cell has a peak-to-peak voltage noise level 7 times greater than that of the “linked pigment”cells, (Figs. 9, 10). The analysis of noise in terms of equivalent intensity (Appendix 1) shows that this voltage noise is generated by the random absorption of photons (photon shot noise) and/or intrinsic noise that is statistically identical (Figs. 11, 12). 7. The high voltage noise levels of the UV cell result from its transducer having a voltage gain greater than that of the other cells. Thus higher gain gives the UV cell a greater absolute sensitivity which compensates for the relative scarcity of UV photons and enables the UV cell to operate in sunlight with a voltage output similar to that of “linked pigment” cells. 8. It is concluded that the retinula cells of the ventral retina show a division of labour into colour, PS and contrast-coding types but absolute sensitivities are carefully matched so that all the cell types described can operate simultaneously with almost identical dynamic response ranges.