Field effects (also called ‘ephaptic interaction’) refer to neuronal interactions mediated by electrical current flowing through the extracellular space. Simply put: When current is generated across a cell membrane, for example as during an action potential, a current will also be generated in the extracellular space around that cell; this extracellular current may then effect a neighboring cell by passing through its membrane. Dr. Faber previously demonstrated functional field effects between small groups of CNS neurons including goldfish Mauthner/inter-neurons and rat cerebellar cortex (Faber and Korn 1973; Faber and Korn 1983). Despite advances in our understanding of the basic mechanisms of field effects, their functional role in modulating spike timing in large central neuronal networks remains largely unknown and strongly debated (Faber and Korn 1989; Bullock 1997; Dudek 1998).
A significant amount of our work on applied weak electrical stimulation [link Writing-Brain] is in fact directly relevant to this topic, in particular our consideration of how apparently “weak” electric fields, whether endogenous of applied, are functionally “amplified” by the nervous system as the single cell and network levels. Therefore concepts we have developed in characterizing “applied” fields are also referenced within the following endogenous section.
It is well established, in humans and animal models, that application of exogenous fields (by passing current between two stimulating electrodes) can profoundly affect nervous system function. These findings have lead to speculation that endogenous fields (fields generated by the nervous system) would also modulate CNS function. Though the effects of uniform fields have been characterized extensively in brain slice (link to uniform), the effects of physiologically-relevant non-uniform extracellular fields on neuronal membranes have so far not been systematically quantified. Dr. Bikson and Dr. Parra have received funding from NIH precisely to address this fundamental question for endogenous fields. Knowledge of field-effect coupling-strength and time-constant (i.e. how much neuronal polarization is induced for a given extracellular potential profile (link to coupling) is a prerequisite to understanding how field-effect induced polarization will affect the behavior of a neuronal network. We predict that non-uniform fields will be more effective than uniform field in polarizing cell membranes (and specifically cell somatic compartments) and that quantifying this issue is a necessary first step toward a rational understanding of the role of endogenous fields.