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Journal of Neuroscience, Vol 7, 3512-3524, Copyright © 1987 by Society for Neuroscience
The interaction of ionic currents mediating single spike activity in retinal amacrine cells of the tiger salamander
S Eliasof, S Barnes and F Werblin
Graduate Group in Neurobiology, University of California, Berkeley 94720.
We investigated the ionic interactions responsible for the characteristic
nonrepetitive spike activity of amacrine cells. First we measured 4
pharmacologically separable ionic components: a voltage- gated, transient
inward sodium current, a voltage-gated, sustained inward calcium current, a
calcium-gated, sustained outward potassium current, and a voltage-gated,
transient outward potassium current. The measurements provided the time
course and magnitudes of the underlying conductances as functions of
voltage. Each current was simulated following conventional Hodgkin-Huxley
theory. A composite of the simulated currents was analytically reassembled
to generate an approximation of the voltage response to a current step. By
artificially varying the magnitude and kinetics of the different
conductances in the simulation, we determined the range of values that
supported the nonrepetitive spike-like response. Amacrine cells tend to
remain refractory following an initial spike because (1) the entire
activation range for potassium is located at positive potentials with
respect to sodium inactivation, so sodium inactivation is never fully
extinguished, and (2) the fully activated sodium conductance is of
insufficient magnitude to subsequently reach threshold, given this residual
inactivation. Shifting the sodium inactivation range by 10 mV, or
increasing sodium conductance by 5 times, leads to a more repetitive form
of activity. Changes in the magnitude, time course, or activation range of
the potassium conductance cannot alter these conditions.
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