A Neuronal Jam in a Primitive Brain

The neuronal packing in the brain does not conform to the norm of jamming, which typically includes a single type of particle. This is because the brain comprises a mix of different types of neurons, and same-type neurons avoid contacts between one another. This unique packing and its property remain to be explored. We simulate neuronal packing with particles that are colored differently but otherwise identical, with concolor particles repelling each other. We show that two levels of jamming can be reached simultaneously: one for all particles (“global jamming”) and another for each set of concolor particles (“sub-jamming”). Reaching this double-jamming requires a minimum number of colors, and that number increases with the range of repulsion. Experimentally, we characterize the packing of peptidergic neurons in the planarian, one of the most basal animals known to have a brain. We found that homotypic neurons pack independently of other neuronal types around the jamming point. Consistent with the simulation, this packing is driven by short-range, homotypic repulsion, mediated by conserved cell adhesion molecules. This study opens a new line of inquiry: how the two levels of jamming are related, and how the configurational space of global jamming projects onto sub-jammed states.