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Conditioning?

William Calvin wcalvin at stein1.u.washington.edu
Fri Oct 22 19:10:29 EST 1993


Harry (and others interested), here is the NMDA/LTP synapse section
from our book that will be out next Spring (Calvin & Ojemann,
CONVERSATIONS WITH NEIL'S BRAIN:  Seeking the Narrator of
Consciousness, Addison-Wesley trade book, April 1994):

      Glutamate is one of the amino acids and widely used for building
proteins everywhere in the body; it is also used as a neurotransmitter. 
The excitatory synapse of cerebral cortex where glutamate is the
transmitter is, so far, the best candidate [in mammalian systems] for a
synapse that encodes memory, for a memory mechanism at the cell
membrane level.  There are at least two types of postsynaptic channels
that open up when glutamate binds to their receptor molecules.  One is
pretty ordinary, as excitatory synapses go.  It allows sodium ions into the
dendrite, which raises its voltage temporarily.
      The other type of glutamate channel -- named "NMDA" for
reasons that are arcane and irrelevant -- allows some calcium ions to
enter the dendrite as well.  But what's really extraordinary about the
NMDA channel is that it won't open unless it has two signals at the
same time:  it takes both the right voltage and the right neurotransmitter
to open up.  That's like a locked entrance door that requires a valid
keycard to be stuck in a slot -- but also requires that the power for the
latch's electronics has to be on.
      Until the discovery of the NMDA channel, all channels had either
been operated by voltage alone (as in the sodium and potassium channels
for the impulse) or by neurotransmitter alone.  And certainly not both in
combination, which allows it to detect near-simultaneous arrivals of
inputs to the dendrite.  That's considerably more interesting than merely
the combination of keycard and power.
      In a NMDA pore, there is a plug.  Typically a magnesium ion
diffuses into the channel and gets trapped, unable to go all the way
through.  When neurotransmitter binds to the channel's receptor
molecule, the gate may open but no sodium or calcium flows through
because of the magnesium ion plug.
      That's what's so interesting.  If the dendrite has received an input
elsewhere, its voltage change may prevent the plug from getting stuck in
the NMDA channel.  And so, if the NMDA synapse is now activated by
neurotransmitter, positive ions flow into the dendrite, creating a synaptic
potential to add atop the original one.  This is what is so exciting to
neurophysiologists -- the calcium entry points to a mechanism for short-
term memory spanning many minutes.  In the hippocampus (an old part
of the cortex with a simpler layered structure), long-term potentiation
sometimes lasts for days, and part of the reason for LTP is the NMDA
business.

    William H. Calvin   WCalvin at U.Washington.edu
    University of Washington  NJ-15
    Seattle, Washington 98195 FAX:1-206-720-1989



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