My colleagues Greg Stacey – ResearchGate Profile, Tom Quail – ResearchGate Profile, and myself have published a new article in the Journal of Neurophysiology, “Computational Study of Synchrony in Fields and Microclusters of Ephaptically Coupled Neurons“. Intrigued by a small set of previous works hinting at electric field-medited coupling between pairs of extremely close neurons, so-called “ephaptic coupling” (in contrast to biochemically mediated synaptic coupling), we set out to estimate if this could lead to synchronization phenomena.
Using a combination of physically realistic simulations in the NEURON network and an abstracted, but much more performative, mathematical model based on coupled oscillators, we found things we expected, but also things we did not expect at all:
- Yes, given that two neurons are close enough, an action potential in one neuron can precipitate or delay the firing of a second neuron – without the need of a direct, synaptic connection, and under several limiting conditions. (Expected, but still good to know.)
- In groups of similar neurons, which are close enough and ordered in a laminar structure, this can indeed lead to spontaneous synchrony. This requires that neurons are sufficiently close – in the way of a critical proximity that is required for synchronization. This fits quite well with animal experiment and clinical data, where a reduction of the space between neurons put these neurons more at risk for dynamics associated with epileptic seizures and their onset. (This was a main goal, and that it fit so well with experiments was indeed unexpected.)
- The surprise came with further reduction of neuron spacing. Locally synchronized micro clusters emerged, which have an own local frequency, and are uncoupled from other clusters. These micro clusters have, equally, been observed in recent brain slice experiments – where synaptic connections between neurons were suppressed, and therefore other mechanisms for the micro clustering are required. In our study, we could explain the experimental findings in a lot of detail, and therefore believe that ephaptic coupling is a promising mechanistic explanation of these observations. (Now, this was quite unexpected when we started this project.)