Ephaptic

Ephaptic refers to a non-synaptic form of neuronal interaction in which the electric field generated by activity in one neuron influences the membrane potential of a neighboring neuron through the surrounding extracellular space. The term often appears in discussions of ephaptic coupling or ephapse, describing field effects rather than direct chemical or electrical synapses.

Mechanism and context: Ephaptic interactions arise when extracellular potentials change sufficiently in magnitude or orientation to depolarize or hyperpolarize nearby membranes. Such effects are more likely in tightly packed, anisotropic tissue with narrow extracellular clefts, such as certain white matter regions or layered cortical areas. The geometry and relative orientation of neurons, along with the ionic environment, determine the strength and direction of the influence. While typically weaker than synaptic transmission, ephaptic coupling can modulate excitability and timing, particularly during synchronized activity or high-frequency firing.

Significance and examples: Ephaptic coupling has been proposed to contribute to synchronization and rhythmic activity in neural networks and to certain epileptiform phenomena under specific conditions. It is also studied as a mechanism that can shape network dynamics without forming direct synapses. The concept is contrasted with chemical synapses and with electrical synapses (gap junctions), the latter providing direct cytoplasmic continuity between cells.

Relation and research: The idea extends to other excitable tissues, including the heart, where extracellular field effects can influence neighboring cells. Research methods include computational modeling of extracellular fields and experimental measurements using extracellular recordings and stimulation paradigms to assess field-mediated interactions.