Figure 1. Schematic representation of adaptive quantal light information sampling by a Drosophila R1–R6 photoreceptor.

A, each R1–R6 samples photon influx by ∼30,000 microvilli, which together form its photosensitive light guide, the rhabdomere. Single‐photon responses (quantum bumps) from individual microvilli integrate a macroscopic response. B, top, each microvillus contains full phototransduction reactions, generating one quantum bump (sample) to an absorbed photon at a time; voltage and Ca²⁺‐dependent feedbacks regulate sample size and speed. Bottom, stochastic processes simulate bump generation. Molecular participants in microvillar phototransduction reactions: C, Ca²⁺‐dependent negative feedback to multiple targets; D, DAG; M, metarhodopsin; P, G protein‐PLC complex; T, TRP/TRPL channels (^*, activated form). Red and green dotted arrows indicate negative and positive feedbacks, respectively, as used in the stochastically operating R1–R6 model (Song et al. 2012; Song & Juusola, 2014; Juusola et al. 2015). The gating mechanisms are yet unresolved, but these probably include production of DAG, InsP ₃, proton, and physical microvilli contraction (Hardie & Franze, 2012).