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 (Q(count)) that it releases within the integration period ( approximately 0.1 s) of a rod bipolar dendrite. Due to the quantal and stochastic nature of release, discrete distributions of Q(count) 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 (Q(rates)) yield higher efficiencies. Focusing here on the effect of small changes in Q(rate), we find that a slightly higher Q(rate) yields greatly reduced efficiency, due to a necessarily fixed quantal-count threshold. To stabilize efficiency in the face of drift in Q(rate), the dendrite needs to regulate the biochemical realization of its quantal-count threshold with respect to its Q(count). 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 approximately 50%, similar to measurements.
Schein
Stan
S
Department of Psychology and Brain Research Institute, University of California, Los Angeles, 90095-1563, USA. schein@ucla.edu
Ahmad
Kareem M
KM
D000818:Animals; D003198:Computer Simulation; D003712:Dendrites / Q000502:physiology* / Q000528:radiation effects; D006801:Humans; D008027:Light; D008959:Models, Neurological; D017785:Photons; D051245:Retinal Bipolar Cells / Q000502:physiology* / Q000528:radiation effects; D017948:Retinal Rod Photoreceptor Cells / Q000502:physiology* / Q000528:radiation effects; D009435:Synaptic Transmission / Q000502:physiology* / Q000528:radiation effects