MAMMALIAN ROD TERMINAL - ARCHITECTURE OF A BINARY SYNAPSE

The mammalian rod synapse transmits a binary signal (one photon or non e) using tonic, rapid exocytosis. We constructed a quantitative, physi cal model of the synapse. Presynaptically, a single, linear active zon e provides docking sites for similar to 130 vesicles, and a ''ribbon'' anchored to the active zone provides a depot for similar to 640 vesic les. Postsynaptically, 4 processes invaginate the terminal: 2 (known t o have low affinity glutamate receptors) lie near the active zone (16 nm), and 2 (known to have high affinity glutamate receptors) lie at a distance (130-640 nm). The presynaptic structure seems designed to min imize fluctuations in tonic rate owing to empty docking sites, whereas the postsynaptic geometry may permit 1 vesicle to evoke an all-ornone response at all 4 postsynaptic processes.