Figure 2.
Dynamic Availability of Microvilli Shapes Responses to Light
(A) Schematic illustration of the main principle of bump summation. Left side shows that trains of bumps in individual microvilli represent discrete, stochastically generated samples of activation. Right side shows that bumps like these sum to generate a continuous noisy LIC response.
(B and C) LICs to dim (3,000 photons/s) and bright (3 × 105 photons/s) pulse stimuli, respectively, recorded from dissociated R1–R6 Drosophila photoreceptors during whole-cell patch clamp. Superimposed on these recordings are the simulated LICs to the same dim (blue trace) and bright (red trace) pulse stimuli. LICs are shaped by the number of activated microvilli (shown in D and E) and negative feedback, which reduces the size of the bumps they produce (shown in F and G).
(D and E) Output of 30,000 microvilli modeled, showing the fractions of used (refractory) and activated microvilli and their sums in the model simulations to reproduce (B) and (C). Note that microvilli counts are practically immune to changes in negative feedback strength, which reduces their bump size simultaneously (F and G).
(F and G) Dotted lines show the difference between normalized LICs (B and C), and the number of activated microvilli (D and E) represents the effect of a reduction in bump waveform on LIC as a function of time (whereas the refractory period affects microvilli usage). During dim stimulation, adaptation (bump size reduction) is slow. In bright stimulation, bump size begins to diminish dramatically already after the first bumps, which shape the initial transient response. Time course of bump adaptation was approximated by single exponentials.
(H and I) LICs to a dim pulse stimulus (300 photons/s) of simulated photoreceptors with either 3,000 or 300 identical microvilli. Too few microvilli generate transient responses, because their ongoing photon capture reduces the number of available sampling units for the next round of photons (saturation effect). Therefore, LIC to dim light pulses in photoreceptors with few microvilli (I) looks similar to (but noisier than) the LIC to bright light pulses in photoreceptors with many microvilli (C). In (C) and (I), stochastic (unphased) phototransduction reactions of different microvilli prevent LIC from complete saturation (arrow, no zero DC-signal); even at the start of intense stimulation, there remain microvilli that cannot be light-activated. Refractory microvilli will recover at variable times to the pool of available microvilli (i.e., can then be activated again by light).