A clockwork hypothesis: synaptic release by rod photoreceptors must be regular.
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*.
Schein
Stan
S
Department of Psychology, and Brain Research Institute, University of California, Los Angeles, Los Angeles, California 90095-1563, USA. schein@ucla.edu
Ahmad
Kareem M
KM
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
D015220:Calcium Channels; D018377:Neurotransmitter Agents