CHANNELS, RECEPTORS, AND ELECTRICAL SIGNALING |
A clockwork hypothesis: Synaptic release by rod photoreceptors must be regular
Stan Schein 1* and Kareem M Ahmad 1
1 University of California, Los Angeles
* To whom correspondence should be addressed. E-mail: schein{at}ucla.edu.
Submitted on July 14, 2005
Revised on August 15, 2005
Accepted on 7 September 2005
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Abstract |
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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 following 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 prior to 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*.
Key Words:
Erlang, Poisson, quantal noise, ribbon synapse, rod bipolar cell, single photon
