Habituation and Conditioning (II)

Ulf Andrick ura at strix.ruessel.sub.org
Fri Nov 19 12:17:37 EST 1993


In <2c80f6$nkv at truffula.fp.trw.com> Harry Erwin (erwin at trwacs.fp.trw.com) writes:
: Further investigation indicates that vesicle release intensity is a 4th
: power function of the Ca++ concentration at the synapse. Hence the 50%
: reduction in intensity seen in short-term habituation corresponds to a
: concentration decrease of about 10-15%. If we assume short-term
: habituation reflects a decrease in the Ca++ concentration, rather than a
: decrease in the number of available vesicles (see below for rationale),
: then the question comes up--suppose the synapse is potentiated. How much
: does that decrease the Ca++ concentration? If the voltage-dependent Ca++
: channels on the postsynaptic side remove as much Ca++ as the channels on
: the presynaptic side, this implies that the short term increase in
: sensitivity on the postsynaptic side has to more than make up for the 75%
: reduction in intensity predicted by the power law for a 30% reduction in
: Ca++ concentration.

I always thought that the depletion of ions due to ionic currents
through the membrane is usually considered as negligeable in
comparison with the number of ions available from the extracellular
liquid. In fact, a current in the order of 1 nA means an ion flux
in the order of only 10^-14 mol/s. In the extracellular liquid,
you have a Ca concentration of some 10^-3 mol/l. So, you have to
postulate a small volume of extracellular liquid to act quasi as
a compartment.  Diffusion acts quickly in small dimensions (less
than 1 ms across the synaptic cleft).  If you want a long-term
effect, it seems that diffusion has to be impaired somehow. And I
haven't considered the activity of Ca pumps, yet. In the time scale
of learning processes, they probably don't work slow. I wonder if
a significant depletion of Ca in the extracellular fluid can be
found experimentally.

Inside the cell, a Ca influx is more meaningful, as the Ca
concentration there is by the order of 6 magnitudes lower than
outside. Further more, the Ca signal could be sensed chemically
and amplified by enzyme cascades as known from many cases where Ca
acts as 2nd messenger inside the cell.

-- 
Ulf Andrick
ura at strix.ruessel.sub.org         deprecated: ura at strix.cluster.sub.org
Mail might bounce due to the address change.



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