Electrical synapses continuously transfer signals bi-directionally from one cell to another, directly or indirectly via intermediate
cells. Electrical synapses are common in many brain structures such as the inferior olive, the subcoeruleus nucleus and the
neocortex, between neurons and between glial cells. In the cortex, interneurons have been shown to be electrically coupled
and proposed to participate in large, continuous cortical syncytia, as opposed to smaller spatial domains of electrically
coupled cells. However, to explore the significance of these findings it is imperative to map the electrical synaptic microcircuits,
in analogy with
in vitro studies on monosynaptic and disynaptic chemical coupling. Since “walking” from cell to cell over large distances with a glass
pipette is challenging, microinjection of (fluorescent) dyes diffusing through gap-junctions remains so far the only method
available to decipher such microcircuits even though technical limitations exist. Based on circuit theory, we derive analytical
descriptions of the AC electrical coupling in networks of isopotential cells. We then suggest an operative electrophysiological
protocol to distinguish between direct electrical connections and connections involving one or more intermediate cells. This
method allows inferring the number of intermediate cells, generalizing the conventional coupling coefficient, which provides
limited information. We validate our method through computer simulations, theoretical and numerical methods and electrophysiological
paired recordings.
Keywords Gap-junctions - Electrical coupling - Networks - ZAP current - Impedance - Interneurons - Cortex - Layer VI
Action Editor: Xiao-Jing Wang
Corrado Calì and Thomas K. Berger are equally contributing authors.