Abstract
An endogenous electrical field effect, i.e., ephaptic transmission, occurs when an electric field associated with activity occurring in one neuron polarizes the membrane of another neuron. It is well established that field effects occur during pathological conditions, such as epilepsy, but less clear if they play a functional role in the healthy brain. Here, we describe the principles of field effect interactions, discuss identified field effects in diverse brain structures from the teleost Mauthner cell to the mammalian cortex, and speculate on the function of these interactions. Recent evidence supports that relatively weak endogenous and exogenous field effects in laminar structures reach significance because they are amplified by network interactions. Such interactions may be important in rhythmogenesis for the cortical slow wave and hippocampal sharp wave–ripple, and also during transcranial stimulation.
Highlights
The electrical field produced by neural activity is commonly viewed as a measure of that activity rather than as a mechanism for influencing it
It is difficult to imagine how an electrical field effect might be useful in neural computations involving neurons interconnected with discrete and plastic synapses
Fields generated by neural activity and the principles of field effect interactions Electrical field effects are generated when extracellular currents associated with electrical activity of the dendrites, soma or axon of one or more neurons are sufficiently large that they are channeled across the membranes of adjacent inactive neurons
Summary
The electrical field produced by neural activity is commonly viewed as a measure of that activity rather than as a mechanism for influencing it. The transmembrane potential, Vm, differs from the intracellular potential (Vi) and instead equals the difference Vi − Ve (Furukawa and Furshpan, 1963; Faber and Korn, 1989) (Figure 1) This notion is straightforward and is based upon first principles, and so called field effects or ephaptic interactions, are well known to be theoretically feasible (Arvanitaki, 1942; Katz and Schmitt, 1942). Fields generated by neural activity and the principles of field effect interactions Electrical field effects are generated when extracellular currents associated with electrical activity of the dendrites, soma or axon of one or more neurons are sufficiently large that they are channeled across the membranes of adjacent inactive neurons. Geometry plays a role, as polarization along the longitudinal axis of a pyramidal neuron results in a dipole and an open field with currents and associated field effects detectable far from the generator (Holt and Koch, 1999; Gold et al, 2006)
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