Passive membrane impedance vs voltage-gated conductance

[plasticity]

At subthreshold levels of injected current I am guessing that the passive membrane conductance is much greater than the conductance of voltage-gated channels. Is this true? If so then at what transmembrane potential do the voltage-gated conductances start to dominate. Bit of a newbie question!

[ted]

At subthreshold levels of injected current I am guessing that the passive membrane conductance is much greater than the conductance of voltage-gated channels. Is this true?

Consider the reversal potentials of the ionic species to which the membrane is permeable. The resting potential of a cell will be closest to the reversal potential of the ionic species to which the membrane is most permeable. Proof of this true statement is left to the reader as an exercise.

[plasticity]

Hi Ted

I am not sure that answers my question.

[ted]

Never mind--it's true only if the ionic equilibrium potentials can be regarded as fixed. It is not true if there is a permeant species whose concentrations are determined by the transmembrane potential, e.g. chloride.

at what transmembrane potential do the voltage-gated conductances start to dominate

Depends entirely on the cell. In some cases, an active ionic conductance (potassium conductance) dominates even at rest.

[plasticity]

Thanks Ted!

What about for pyramidal cells? Can my question be answered definitively for them?

Cheers!

[ted]

No. You'd have to determine the gating properties and spatial variation of channel density of every kind of ion channel in the membrane of one cell. That cannot be done.