Polyadic synapse · Lobeworks/17

A polyadic synapse is a chemical synapse in which one presynaptic release site faces several postsynaptic cells at once, so that a single packet of transmitter reaches all of them; it is the usual form of synapse in the insect brain.

Polyadic synapse. A polyadic synapse is a chemical synapse in which one presynaptic release site faces several postsynaptic cells at once, so that a single packet of transmitter reaches all of them; it is the usual form of synapse in the insect brain.

In the fly the release site is marked by a dense body shaped like a T in cross-section, the T-bar, with vesicles clustered around it, and across the cleft sit the processes of several receiving cells. The hemibrain counted about 9.5 million T-bars and 64 million postsynaptic densities, close to seven partners per release site. Vertebrates have the arrangement in places too, at the ribbon synapses of the retina, where one release site feeds a pair of cells.

Connectomes count it by pairs. A dataset that reports a synapse count between two neurons is counting partner pairs, so one release site can add to the count of several connections.

Models split it into wires. A graph of pairwise weights treats a broadcast to several cells as independent connections, which ignores that they share one packet of transmitter and one moment of release.

Questions: How can one fly synapse reach several cells at once? At a polyadic synapse one release site, marked by a T-shaped density, faces several postsynaptic cells across the cleft, so the transmitter of one vesicle reaches all of them. The hemibrain counted about 9.5 million such release sites and 64 million postsynaptic densities, close to seven partners each. A model built from pairwise weights treats this broadcast as several separate connections. What does a synapse count between two fly neurons actually count? Partner pairs. Because one release site can face several cells, a dataset counts each presynaptic site once for every postsynaptic partner it contacts, and the count between two neurons is the number of such pairs. It is a good proxy for how strongly one cell can drive another, and it says nothing about receptors or release probability.