title: A clockwork hypothesis: synaptic release by rod photoreceptors must be regular.
abstract: We can see at light intensities much lower than an average of one photon per rod photoreceptor, demonstrating that rods must be able to transmit a signal after absorption of a single photon. However, activation of one rhodopsin molecule (Rh*) hyperpolarizes a mammalian rod by just 1 mV. Based on the properties of the voltage-dependent Ca2+ channel and data on [Ca2+] in the rod synaptic terminal, the 1 mV hyperpolarization should reduce the rate of release of quanta of neurotransmitter by only 20%. If quantal release were Poisson, the distributions of quantal count in the dark and in response to one Rh* would overlap greatly. Depending on the threshold quantal count, the overlap would generate too frequent false positives in the dark, too few true positives in response to one Rh*, or both. Therefore, quantal release must be regular, giving narrower distributions of quantal count that overlap less. We model regular release as an Erlang process, essentially a mechanism that counts many Poisson events before release of a quantum of neurotransmitter. The combination of appropriately narrow distributions of quantal count and a suitable threshold can give few false positives and appropriate (e.g., 35%) efficiency for one Rh*.
authors.affiliation: Department of Psychology, and Brain Research Institute, University of California, Los Angeles, Los Angeles, California 90095-1563, USA. schein@ucla.edu
authors.forename: Stan
authors.initials: S
authors.lastname: Schein
authors.forename: Kareem M
authors.initials: KM
authors.lastname: Ahmad
mesh_terms: D015220:Calcium Channels / Q000502:physiology* / Q000528:radiation effects; D003198:Computer Simulation; D008027:Light; D008564:Membrane Potentials / Q000502:physiology* / Q000528:radiation effects; D008956:Models, Chemical; D008959:Models, Neurological; D015233:Models, Statistical; D018377:Neurotransmitter Agents / Q000378:metabolism*; D017948:Retinal Rod Photoreceptor Cells / Q000502:physiology* / Q000528:radiation effects; D013569:Synapses / Q000502:physiology* / Q000528:radiation effects; D009435:Synaptic Transmission / Q000502:physiology* / Q000528:radiation effects
chemical_list: D015220:Calcium Channels; D018377:Neurotransmitter Agents