A Phenomenological Model of Synaptic Activity in the Blowfly Lamina

Estimates of the effective Poisson rate obtained from voltage recordings of Large Monopolar Cells (LMCs) in the blowfly lamina, measured in response to repeated light-contrast stimuli, are on the order of 5x105 events per second. These values imply that if vesicle exocytosis at photoreceptor-LMC synapse consisted of statistically independent quantal release events, the mean release rate would need to be at least this large to account for the observed signal and noise characteristics. Such a scenario is highly implausible, as it would correspond to an exocytosis rate of approximately 400 vesicles per second per active zone, or equivalently to the turnover of a membrane area comparable to that of all six photoreceptor axon terminals within a lamina cartridge more than twice per second. Relaxing the assumption of statistical independence, however, allows for substantially larger effective Poisson rates. In earlier work, we demonstrated that temporal regularization of release events- particularly through a reduction in low-frequency noise power spectral density- can increase the effective Poisson rate without altering the underlying mean release rate. Additionally, the contrast-to-current impulse response function of the extracellular space has been shown to be approximately equal in magnitude and opposite in sign to that of the LMCs, suggesting that both signals arise from the same chloride current. This observation provides a potential physiological mechanism for vesicle regularization, although it was previously described only in qualitative and simplified terms. In the present work, we develop a phenomenological model that incorporates these findings together with additional physiological parameters from literature to account for the regularization of vesicle release.