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Ring-shaped neurons in the fly connectome

Which VFB neuron types have a ring-shaped morphology, and what VFB holds on them

Asked at the VFB table, NeuroFly 2026 (Cologne), 7 September 2026.

The question

“Can you search for circle shape neurons in fly connectome data?”

A visitor wanted to know whether VFB could find neurons by the shape of their arbour rather than by name — specifically, ones that form a ring or circle.

How we answered it

VFB has no shape-based search (that’s what NBLAST similarity search is for, starting from a seed neuron — see the Similarity session ), so the first step was to check whether “ring-shaped” already exists as a named biological category. It does: the ellipsoid body ring neurons, a well-described class in the fly central complex. The MCP tools used, in order:

  1. search_terms for “ring neuron” and “circular” — confirms the class exists and rules out unrelated hits (mostly visceral muscle, which also happens to be called “circular muscle”).
  2. get_hierarchy (subclass_of, descendants) on the top class — pulls out the full subtype tree.
  3. get_term_info on the two branch classes, to read the literature description and confirm the anatomical claim (“forms ring-shaped arborizations… circling the ellipsoid body canal”) rather than assume it from the name.
  4. run_query (ListAllAvailableImages) on each subtype — counts how many registered images VFB actually holds for each, across which connectomes.

What we found

The adult ellipsoid body ring neuron (FBbt:00003649, synonym TL neuron) splits into two branches: six intrinsic R-neuron subtypes (ER1–ER6, GABAergic, ~150 per hemisphere per the literature) and eight extrinsic ring neuron subtypes (ExR1–ExR8, mixed neurotransmitters). VFB holds 1,217 registered images across the class — mostly from the male CNS v0.9 connectome (Berg et al., 2025), plus FlyWire, BANC and hemibrain.

The figure below shows one representative neuron for each subtype, each a genuine VFB-registered image on the JRC2018U template — not an illustration. That is 13 panels rather than 14: VFB currently holds no registered image for ExR2 (FBbt:00110327).

Ellipsoid body ring neurons — one representative per subtype
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Worth being honest about: this answers “which named class is ring-shaped”, not “find me anything ring-shaped by silhouette” — VFB has no morphology-only search of that kind. The two serotonergic/dopaminergic outliers (ExR3, ExR5) also extend well beyond the ellipsoid body, so “ring-shaped” only describes part of their arbour.

Sources