THRESHOLDS AND SUPEREXCITABILITY

[Christof Koch]

I agree with Peter C. that none of the current models contain
a superexcitable phase. However, do we know to what extent this
phenomena occurs under physiological circumstances in say, cortical cells.
As far as I remember Raymond and Lettvin's J. Physiol. paper on the timecourse
of the spiking threshold, they worked on alpha motoneurons and made the cell
fire hundreds of time.
 
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.
I wonder whether anybody knows whether the threshold for action potential
initiation goes DOWN during these ADPs. This would be the closest phenomena
to "superexcitability" I know of.
 
Christof