|MT > 8. Local Synchronization via Ephaptic Coupling|
Ephaptic coupling refers to the coupling of adjacent cells mediated by local electric fields (E). During an action potential, Na+ ions flow inward while K+ ions move outward (Figure 1). The ion movement across the membrane of a spiking neuron will alter the electric field at the extracellular space, which in turn may affect the excitability of nearby neurons. Since electric fields act instantly, the ephaptic coupling was thought to play an important role in neuronal synchronization. This notion has been supported by several lines of evidence (Dudek et al., 1998; Anastassiou et al., 2011).
Alteration in E at the extracellular space can be converted into the extracellular membrane potential (Ve). The membrane voltage, Vm, is defined as
Vm = Vi - Ve
where Vi is the intracellular membrane potential. Depending on locations, a spiking neuron can generate depolarizing or hyperpolarizing Vm. The action potential that occurs at the axon initial segment (AIS) usually generates depolarizing Vm. By contrast, spikes at apical dendrites initially produces hyperpolarizing Vm (Vigmond et al., 1997; Stacey et al., 2015). Since action potentials initiate at AIS, a group of parallel neurons, such as pyramidal neurons, can have excitatory ephaptic coupling. However, if the apical dendrites of a neuron is near the AIS of the other, their ephaptic coupling would be inhibitory.
Ephaptic coupling depends on the distance between two neurons. In the hippocampus, the pyramidal neurons and dentate granule cells are tightly packed. This may account for their vulnerability to seizures that arise from hypersynchronization (McBain et al., 1990; Dudek et al., 1998). Theoretical calculations show that a single spiking neuron may change Ve up to 1 mV (Vigmond et al., 1997; Stacey et al., 2015). Typically, initiation of an action potential requires ~15 mV depolarization. Hence, ephaptic coupling alone is unlikely to generate spikes from the resting state. However, it may influence spike timing (Anastassiou et al., 2011) and mediate local synchronization (Dudek et al., 1998; Stacey et al., 2015).
Author: Frank Lee