Spike initiation & time-dependent threshold changes

[Peter Cariani]

    Regarding models for spike initiation, as far as I can see there are 
few (if any) models which take into account the intrinsic time course of
threshold following an action potential. Thresholds for many different 
kinds of axons have a superexcitability period in which the threshold
(that amount of injected charge needed to re-excite the membrane) descends
to a half or third or less of its resting value. (See Chung, Raymond, &
Lettvin, 1970; Raymond & Lettvin, 1978; Raymond, 1979). Furthermore,
these thresholds are altered by activity, so the phenomenon of conduction
block would serve to dynamically modify the effective connectivity between
neurons (in the frequency/interspike interval domain). I don't understand
why this phenomenon has been so overlooked by the neuroscience community
at large, but I would think it would be extremely important for any
models where "intrinsic oscillations" play an important role. I am 
currently working on an extremely rudimentary model of this sort,
trying to understand its functional implications. 
    It's unclear what causes this superexcitable phase--one set of ideas
involves electrostatic interaction between calcium and ion channels on
the outside of membranes (and is therefore related to Hill's model
for accomodation)--but I think it means that neurons have more complex
temporal characteristics than has been generally assumed and that
oscillatory behavior may not necessarily require recurrent loops with 
time delays. Obviously this affects the plausibility of temporally-based
pulse codes, since what required a reverberatory loop may now only need
a single neuron. This all remains to be seen, but it's worth keeping in
mind.

SH Chung, SA Raymond, & JY Lettvin (1970) Multiple meaning in single visual
    neurons. Brain Behav Evol 3: 72-101
SA Raymond & JY Lettvin (1978) Aftereffects of activity in peripheral axons
    as a clue to nervous coding. Physiology and Pathophysiology of Axons
    S Waxman, ed. Raven Press, NY
SA Raymond (1979) Effects of nerve impulses on threshold of frog sciatic
    nerve fibres. J Physiol 290:273-303.

--Peter Cariani