Journal of Neurophysiology, Vol 59, Issue 3 706-716, Copyright © 1988 by APS
Effect of subthreshold voltage-dependent conductances on the transfer function of branched excitable cells and the conduction of synaptic potentials
K. Yoshii, L. E. Moore and B. N. Christensen
Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston 77550.
1. Impulse response functions were determined from complex point impedance
and transfer functions from cultured NG-108 cells to simulate the
propagation of a synaptic potential in response to the release of
transmitter. In general, the flow of synaptic current has a much shorter
duration than the normal membrane time constant, thereby making the use of
impulse response functions useful approximations to synaptic events. 2. The
resonance observed during the activation of the potassium conductance was
reflected in the impulse response function as a pronounced damped
oscillation. A comparison of the impulse response functions calculated from
point impedance and transfer functions showed similar results for current
injections in the growth cone. 3. In addition to the resonance effects of
the voltage-dependent conductances on transfer and impulse response
functions due principally to the activation of conductances for outward
currents, transfer functions were measured during the activation of a
steady-state negative conductance. Under these conditions the phase
function approaches 180 degrees, indicating that the voltage response is
out of phase with the current. 4. In the steady state, the effect of a
negative conductance is to algebraically add to the positive conductances
and generally decrease the absolute conductance unless there is a net
negative current. The decreased conductance enhances the impulse response
and the DC space constant, thus leading to a better propagation of slow
potentials. This effect can be seen as a decrease in the electrotonic
length, L, with intermediate depolarizations. At large depolarizations the
steady-state activation of the K conductance generally dominates and leads
to a greatly increased electrotonic length. 5. Both the net conductances
and the associated kinetics play a role in shaping the potential changes
during a synaptic current. This is especially critical if there is a net
negative steady-state conductance. Under these conditions there is a
surprising reduction in the impulse response function. 6. Thus, during a
subthreshold activation of the voltage-dependent negative conductances, the
observable synaptic potentials would be either large potential responses
due to an apparent increase in the impedance (algebraic summation of
positive and negative conductances with a net positive conductance) or a
minimal response because of the phasic cancellation due to a net negative
conductance. The latter condition could exist near the synaptic reversal
potential due to a large synaptic drive and would appear experimentally as
a form of inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)
