Cell type · Lobeworks/17

A cell type is a class of neurons that share the same shape, the same position and the same partners, so that they can be treated as copies of one part; in a connectome it is the unit that makes the graph readable and lets one animal be matched to another.

Cell type. A cell type is a class of neurons that share the same shape, the same position and the same partners, so that they can be treated as copies of one part; in a connectome it is the unit that makes the graph readable and lets one animal be matched to another.

Most types in the fly hold a few cells on each side of the brain, some hold hundreds. The FlyWire brain of 139,255 neurons was sorted into 8,453 types, 3,643 of them already proposed from the hemibrain of a different fly and 4,581 new. Typing is done by comparing morphology and connectivity, and checked by finding the same type again on the other side of the brain and in another animal.

It compresses the graph. Eight thousand types can be read and reasoned about where a hundred and forty thousand cells cannot.

It matches animals. A type found in two flies imaged by different methods is the same part in two copies of the machine, which is how stereotypy is measured.

It joins the map to experiments. Fly geneticists keep lines in which one type, and only that type, can be made to glow, switched on with optogenetics or silenced, so the name of a type is the key between the connectome and the laboratory.

A cell type is the part number of a neuron.

The wiring says how the parts are connected; the type says which part it is, so that what one experiment learns about it applies to every copy.

Questions: How is one cell type switched on in a living fly? Through genetic lines that express a light-sensitive channel in that type and no other. Optogenetics then turns the type on with light, and the experimenter watches what the fly does or which other neurons respond. Because the same type is named in the connectome, the result can be laid on the map, which is how predictions of whole-brain models are tested. Why does a connectome need cell types? Because a graph of individual neurons cannot be read, compared or tested on its own. Sorting the 139,255 neurons of the FlyWire brain into 8,453 types made it readable, let it be matched to the hemibrain of another fly, and, above all, tied each part of the map to genetic lines in which that type can be switched on or off in living flies. Without types, nothing learned in a laboratory could be attached to a node of the graph.