THRESHOLDS AND SUPEREXCITABILITY

[Peter Cariani]

In reply to Christoff Koch's message:
I had thought I had made myself clear regarding time-dependent threshold
changes. Raymond and others have found a triphasic threshold time course
following an action potential: the threshold initially goes up (refractory
phase) far above its resting level, then it goes DOWN below its resting
level (superexcitable phase), then it goes back up above its resting level
(depression phase) and returns to its resting level. Superexcitability 
typically begins several milliseconds after the action potential and can 
last milliseconds to hundreds of milliseconds. While almost every neural
model includes the refractory phase, I have yet to see one (outside of
Raymond's model for threshold changes) which includes superexcitability.
    Actually, I had read Christoff Koch's and Tomasso Poggio's otherwise 
excellent review paper "Biophysics of computation: neurons, synapses and 
membranes." in Synaptic Function, Edelman, Wall & Cowan eds Wiley, 1989 
when I began to look at neural models about a year and a half ago, 
and there is a section on conduction blocks but no mention of threshold 
changes or of any of the work by Lettvin & company. I know the book "Methods 
in Neuronal Modelling" and yes it has many detailed state-of-the-art models, 
but I explicitly looked there when the book first came out for a model incor-
porating nonmonotonic threshold recovery curves and as I recall none
of the models in that book  even come close. This is not to say they
are bad models (on the contrary, they are the best we have), but that
there are some phenomena of potential import which are being overlooked.

Peter Cariani