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F. Rattay, C. Wenger:
"Which elements of the mammalian central nervous system are excited by low current stimulation with microelectrodes?";
Neuroscience, 170 (2010), 399 - 407.

Low current cortex stimulation produces a sparse
and distributed set of activated cells often with distances of
several hundred micrometers between cell bodies and the
microelectrode. A modeling study based on recently measured
densities of high threshold sodium channels Nav1.2 in
dendrites and soma and low threshold sodium channels
Nav1.6 in the axon shall identify spike initiation sites including
a discussion on dendritic spikes. Varying excitability
along the neural axis has been observed while studying different
electrode positions and configurations. Although the
axon initial segment (AIS) and nodes of Ranvier are most
excitable, many thin axons and dendrites which are likely to
be close to the electrode in the densely packed cortical regions
are also proper candidates for spike initiation sites.
Cathodic threshold ratio for thin axons and dendrites is about
1:3, whereas 0.2 µm diameter axons passing the electrode tip
in 10 µm distance can be activated by 100 µs pulses with 2.6
µA. Direct cathodic excitation of dendrites requires a minimum
electrode-fiber distance, which increases with dendrite
diameter. Therefore thin dendrites can profit from the stronger
electrical field close to the electrode but low current
stimulation cannot activate large diameter dendrites, contrary
to the inverse recruitment order known from peripheral
nerve stimulation. When local depolarization fails to generate
a dendritic spike, stimulation is possible via intracellular current
flow that initiates an action potential, for example 200 µm
distant in the low threshold AIS or in certain cases at the
distal dendrite ending. Beside these exceptions, spike initiation
site for cathodic low current stimulation appears rather
close to the electrode.

http://dx.doi.org/10.1016/j.neuroscience.2010.07.032