R1–R6 photoreceptors
systemVisionspeciesD. melanogaster3D view16 of 626 cellssources6
R1–R6 are the six outer photoreceptor cells in each ommatidium, or unit eye, of the fruit fly’s compound eye. They all belong to one spectral class and are the main input to the fly’s motion vision.
| Also known as | Outer photoreceptors · R1-6 |
|---|---|
| Region | Compound eye (retina); axons end in the lamina |
| 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: R1-6 16 |
What they are
Each ommatidium contains eight photoreceptors, R1 to R8 (Meinertzhagen and O'Neil, 1991). R1–R6 are arranged around the outside of the ommatidium and R7 and R8 in its centre. Whereas R7 and R8 come in several spectral classes, R1–R6 all belong to a single one (Wardill et al., 2012).
Light is absorbed by rhodopsin in the rhabdomere, a stack of membrane folds, and converted into an electrical signal by a cascade that is the fastest G-protein signalling pathway known (Hardie and Raghu, 2001). Like other fly photoreceptors, R1–R6 respond with graded changes in membrane voltage rather than spikes.
Where they sit
Their cell bodies lie in the retina and their axons end in the lamina, the first neuropil of the optic lobe. There R1–R6 terminals provide the photoreceptor input to the lamina’s repeating modules, the cartridges, and synapse onto the lamina monopolar cells (Meinertzhagen and O'Neil, 1991). R1–R6 cells in six neighbouring ommatidia that look at the same point in space send their axons to the same cartridge, an arrangement called neural superposition.
What they do
Genetic dissection of the lamina places R1–R6 at the start of the achromatic motion channel: their signal passes mainly through lamina neurons L1 and L2, which together are necessary and largely sufficient for motion-dependent behavior (Rister et al., 2007). Downstream, the pathway splits into ON and OFF channels that end in the T4 and T5 motion detectors (Joesch et al., 2010; Maisak et al., 2013).
R1–R6 are not the only route: R7 and R8 also feed the motion pathway, which broadens its spectral tuning and improves motion discrimination (Wardill et al., 2012).
Behavior
Turning responses to moving patterns depend on motion signals that begin in R1–R6 and pass through L1 and L2 (Rister et al., 2007; Maisak et al., 2013).