Can Negative Feedback Improve the Efficiency of Information Transmission In The Fly Photoreceptor-LMC Synapses?

Measurements of the contrast power transfer spectrum and the noise power spectral density of large monopolar cells (LMCs) of the blowfly C. vicina predict a sustained minimum rate of >10⁵ synaptic vesicles per second per LMC. The calculation of this rate assumes, as per widespread ideas about how chemical synapses function, that the exocytosis of vesicles occurs as a Poisson process. The typical dimensions of photoreceptors and vesicles makes this a logistically implausible scenario. If instead, we assume that vesicle release is more structured, noise may be suppressed at the low end of the spectrum, substantially bringing down the release rate required for observed experimental results. This could be particularly useful for the transmission of information across the synapse, since the information being transmitted is already low pass filtered. Weckström and Laughlin (2010) show that significant electric potentials are generated in the lamina cartridge extracellular space as a response to visual activity. We propose that regularization of vesicles occurs through the control of the voltage sensors responsible for vesicle release by these feedback potentials. We demonstrate signatures of synaptic control experimentally, and through modeling, the consequences of extracellular responses on the bit rate per vesicle.