White matter fiber bundles enable robust conduction and time-critical signaling despite axon caliber variation

Axon caliber determines signal conduction velocity and varies within and across the bundles that form the brain’s wiring. Our knowledge of this organization derives largely from 2D histology, assuming cylindrical axons whose calibers are described by their radius. However, recent 3D histology reveals substantial along-axon variability in radius, calling into question 2D-derived insights into axon radius and conduction velocity.

We show in 450,000 3D axon reconstructions from male rats that—despite this individual variation—axon bundles possess stable radius distributions at the ensemble level. These distributions are faithfully captured by 2D cross-sections and yield robust conduction velocity predictions, as along-axon variability has only a modest effect. Conduction in large axons is particularly stable, supporting time-critical signaling.

With 2D sampling validated, we leverage 46 million human corpus callosum axons from 2D histology to determine sample size requirements across neuroscience applications. Our findings show how axonal organization gives rise to stable ensemble statistics and conduction, explaining and reinforcing decades of 2D histology-based research. We thank Tilo Reinert for insightful discussions.

We used large language models to polish language and to assist with code generation. The authors take full responsibility for the content. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement number 616905.

This work was supported by the German Research Foundation (DFG Priority Program 2041 “Computational Connectomics", [MO 2397/5-1, MO 2397/5-2, MO 2249/3-1, MO 2249/3-2, WE 5046/4-2], by the Emmy Noether Stipend: MO 2397/4-1; MO 2397/4-2). Funded by the European Union. Views and opinions expressed are, however, those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency.

Neither the European Union nor the granting authority can be held responsible for them. This work is supported by ERC grant (Acronym: MRStain, Grant agreement ID: 101089218, DOI: 10.3030/101089218). Open Access funding enabled and organized by Projekt DEAL.