Efficiency of Synaptic Transmission of Single-Photon Events from Rod Photoreceptor to Rod Bipolar Dendrite

A rod transmits absorption of a single photon by what appears to be a small reduction in the small number of quanta of neurotransmitter (Qcount) that it releases within the integration period (∼0.1 s) of a rod bipolar dendrite. Due to the quantal and stochastic nature of release, discrete distributions of Qcount for darkness versus one isomerization of rhodopsin (R*) overlap. We suggested that release must be regular to narrow these distributions, reduce overlap, reduce the rate of false positives, and increase transmission efficiency (the fraction of R* events that are identified as light). Unsurprisingly, higher quantal release rates (Qrates) yield higher efficiencies. Focusing here on the effect of small changes in Qrate, we find that a slightly higher Qrate yields greatly reduced efficiency, due to a necessarily fixed quantal-count threshold. To stabilize efficiency in the face of drift in Qrate, the dendrite needs to regulate the biochemical realization of its quantal-count threshold with respect to its Qcount. These considerations reveal the mathematical role of calcium-based negative feedback and suggest a helpful role for spontaneous R*. In addition, to stabilize efficiency in the face of drift in degree of regularity, efficiency should be ≈50%, similar to measurements.