R7 and R8 photoreceptors
systemVisionspeciesD. melanogaster3D view16 of 626 cellssources5
R7 and R8 are the two inner photoreceptors of each ommatidium in the fruit fly’s compound eye. They come in several spectral classes and are the basis of color vision; in the dorsal rim of the eye they detect polarized light.
| Also known as | Inner photoreceptors · R7 · R8 · Long visual fibres |
|---|---|
| Region | Compound eye (retina); axons end in the medulla |
| System | Vision: eye and optic lobe |
| In the display bundle | 16 of the 626 reconstructed neurons in the site’s MaleCNS display bundle, which shows 626 of the model’s 166,700 neurons: R7 7, R8 9 |
What they are
R7 and R8 sit in the centre of the ommatidium, R7 above R8. In most ommatidia the two cells express different opsins, so each pair compares two parts of the spectrum: short-wavelength ultraviolet against blue in “pale” ommatidia, and long-wavelength ultraviolet against green in “yellow” ommatidia (Schnaitmann et al., 2018).
Where they sit
Their axons, the long visual fibres, pass through the lamina and end in the medulla (Meinertzhagen and O'Neil, 1991), where R7 and R8 terminate in different layers.
What they do
Color-opponent processing begins in the photoreceptor terminals themselves: R7 and R8 of the same ommatidium inhibit each other directly through histamine receptors (Schnaitmann et al., 2018).
Downstream, the medulla neurons Tm5 and Tm9 receive direct input from R7 and R8 respectively, and wide-field Dm8 neurons pool input from 13 to 16 ultraviolet-sensing R7 cells. Dm8 neurons are necessary and sufficient for flies to prefer ultraviolet over green light (Gao et al., 2008).
R7 and R8 also contribute to motion vision alongside R1–R6 (Wardill et al., 2012).
Polarized light
In the dorsal rim area of the eye, R7 and R8 have enlarged rhabdomeres and express the same opsin instead of different ones. This makes them detectors of the polarization of skylight, which flies use for orientation (Wernet et al., 2003).