Ferreira Castro, A.; Cardona, A.

We measured neuronal cell structure and connectivity from fly and zebrafish to mouse and human and discovered an invariant feature: an average of one synapse for every micrometer of dendritic cable. We provide evidence of how this pattern emerges from precise spatial positioning, electrical activity, and resource conservation in neurons. Our results imply that nervous systems converged onto synaptic density invariance to stabilise neuronal responses, suggesting a simplifying principle for neural circuit function, assembly and evolution.