This paper describes oscillations of membrane potential and extracellular potassium in a bacterial population (shown indirectly with an optical sensor), which show radial synchronization (ie same Vm for cells at the same radius). The proposed mechanism is as follows. A wave of depolarization is initiated by some metabolic factor which makes a K+ channel open, hyperpolarizing the cell. This releases K+ in the extracellular environment. The extracellular increase in K+ reduces the Nernst potential for K+, so all neighboring cells are depolarized. The K+ channel is voltage-dependent (indirectly in the model), it opens when the cell is depolarized. So there is a hyperpolarization that releases K+ extracellularly. With appropriate nonlinearities, the result is a propagating wave of K+ and Vm, which is faster than diffusion. There is a simple Hodgkin-Huxley type model in the supplementary methods. Some of it might be a little questionable (eg K+ reversal potential increases linearly rather than logarithmically with concentration; but that might be ok for small ion fluxes and probably doesn’t change the results qualitatively), but generally sensible. It is a chain of cells coupled through the extracellular environment. It would be interesting to extend the model to a disk and see whether one can account for radial synchrony.

This is interesting for at least two reasons. One is that there is electrical communication based on ionic channels not just in neurons but also in bacteria; so probably in all living cells. Another is the mode of communication is neither gap junctions (direct electrical coupling) nor synapses (through neurotransmitters), but through changes in ionic composition of the extracellular environment. These changes should occur also in the nervous system, so could it be that neurons also communicate in this way?