Strength–duration relationship for intra- versus 3 extracellular stimulation with microelectrodes
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Abstract
Abstract—Chronaxie, a historically introduced excitability
time parameter for electrical stimulation, has been assumed
to be closely related to the time constant of the cell membrane.
Therefore, it is perplexing that significantly larger
chronaxies have been found for intracellular than for extracellular
stimulation. Using compartmental model analysis,
this controversy is explained on the basis that extracellular
stimulation also generates hyperpolarized regions of the
cell membrane hindering a steady excitation as seen in the
intracellular case. The largest inside/outside chronaxie ratio
for microelectrode stimulation is found in close vicinity of
the cell. In the case of monophasic cathodic stimulation,
the length of the primarily excited zone which is situated
between the hyperpolarized regions increases with
electrode–cell distance. For distant electrodes this results
in an excitation process comparable to the temporal behavior
of intracellular stimulation. Chronaxie also varies along
the neural axis, being small for electrode positions at the
nodes of Ranvier and axon initial segment and larger at
the soma and dendrites. As spike initiation site can change
for short and long pulses, in some cases strength–duration
curves have a bimodal shape, and thus, they deviate from a
classical monotonic curve as described by the formulas of
Lapicque or Weis