Erroneous Persistent Currents Errors in Persistent Inward Currents
Generated by Space-Clamp Errors: A Modeling Study.
J.A. White, N.S. Sekar, and A.R. Kay.
Department of Biological Sciences and Neurosciences Program, University of
Iowa, Iowa City, Iowa 52242-1324.
APStracts 2:0014N, 1995.
SUMMARY AND CONCLUSIONS
1. The effects of imperfect space clamp on inactivating inward currents were
examined with the use of a "ball-and-stick" neuronal model with uniform active
and passive membrane properties. With poor space clamp, both transient and
steady-state (persistent) components were distorted. The ratio of steady-state
to peak current (is/p), measured at the soma, was sometimes smaller but
usually larger than would be the case with uniform space clamp. For a fast Na+
current, the anomalous persistent component was largest for large electrotonic
lengths, low-conductance densities, and voltage-clamp potentials near the
threshold of the current. Under some conditions, steady-state current could
take one of two values, depending on the holding potential. 2. Membrane
potential as a function of distance was examined, revealing a steady-state
voltage gradient in which distal portions of the neuron were more positive
than in the passive case, and often more positive than the command potential
itself. These reversed voltage gradients, caused by the uncontrolled "window"
Na+ current at remote electrotonic distances, produced steady-state axial
current flow into the soma, thereby increasing the persistent current measured
somatically. 3. The time at which the current peaked (tp) was sensitive to
imperfections in the space clamp. This phenomenon made somatic membrane
current and axial current at t sensitive to the fidelity of space clamp as
well. The ratio of steady-state axial current to that at t = tp was a good
predictor of the degree of distortion of is/p. 4. In a model with slower
voltage-dependent kinetics, the same principles applied, but tp was relatively
independent of space-clamp quality, making changes in the steady-state axial
current a more dominant factor in determining is/p. For this reason, is/p in
the slow model grew with increasing conductance density, instead of shrinking
as in the fast model.
Received 11 July 1994; accepted in final form 23 January 1995.
APS Manuscript Number J428-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 3 April 1995.