THRESHOLDS AND SUPEREXCITABILITY

[Lyle J. Borg-Graham]

>  In a cortical cell, discharge of an action potential will transiently open
>  calcium channels. The potential will return to the resting potential
>  and will be pulled below to more hyperpolarizing levels by the combined
>  action of several voltage and/or calcium-dependent potassium currents,
>  in particular I_C, I_M and I_AHP. They effectively implement a relative
>  refractory period.
 
>  In hippocampal cells, action potentials are sometimes followed by ADP, i.e.
>  after-depolarizations, caused by charge coming back from the dendrites.

Probably the reason a distinct "ADP" has been described in hippocampal
pyramidal cells is that there is a ~1msec transient hyperpolarization
(fAHP, or fast after-hyperpolarization) during the spike
repolarization, due to the fast Ca-dependent K current (I_C). Since
this current inactivates (possibly because of voltage) quickly, a
subsequent depolarization results. However, the distributed charge of
the dendrites which underlies this depolarization is most likely in
other cells (the estimation of which is a good candidate for
modelling), and thus the ADP phase is there as well.

>  I wonder whether anybody knows whether the threshold for action potential
>  initiation goes DOWN during these ADPs. 
 
I suppose it depends on what is meant by threshold, i.e. current or
voltage. If the Na voltage 'threshold' doesn't change [which it does]
then the ADP would lower the current threshold since, of course, the
dendritic charge is lending a hand.

> This would be the closest phenomena to "superexcitability" I know
> of.

For what it is worth, it might be better to reserve the term
"excitability" for the state of intrinsic membrane properties. As far
as "superexcitability" goes, perhaps the I_A and I_D K+ currents in
hipppocampus may participate. The inactivation range of these currents
include voltages below spike threshold, with varying time constants
(I_A on the msec scale, I_D on the 10 to 100 msec scale). Thus spike
activity tends to shut them off, which will contribute to
excitability. Now the interesting thing to do is to identify the
physiological pathways that modulate these guys.


Lyle