Invariant synaptic density across species

Abstract

The nervous system scales with animal size while preserving function, yet the principles underlying this stability remain unclear. Here, we analyse the ultra-structure and connectivity of thousands of neuronal cells across species, including fly, zebrafish, mouse, and human, and found a conserved feature that stabilises neuronal responses across scales: an average of one synapse per micrometre of dendritic cable. We show that the appropriate synaptic density is shaped by correct axon-dendrite positioning and synaptic transmission during development. We find that this specific synaptic density is linked to wiring optimisation in neurons, where dendrites minimise cable length and conduction delays. Finally, simulations indicate invariant synaptic density as a neuronal design principle, conserved for its ability to synergise with other cell-intrinsic properties to stabilise voltage responses across cell types and species.