X-Message-Number: 2
From: Kevin Q. Brown
Subject: memory mechanisms
Date: 25 Jul 1988
Since cryonicists want to preserve their memories, information on how memory
actually works is of great interest. I have appended below an article
concerning the biochemical mechanisms of memory, which I have reproduced
(with permission, provided the credit below is included) from the July 1988
issue of Claustrophobia.
- Kevin Q. Brown
...{ihnp4|att|clyde|cuae2}!ho4cad!kqb
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A Glimpse of the Chemical Mechanism of Memory
by Thomas Donaldson
Claustrophobia, July 1988, Vol. 12, No. 7, Issue No. 139
1402 SW Upland Dr., Portland, OR 97221
(503) 227-6848
One fundamental fact about memory, already known 10 years ago, consists of the
distinction between short-term memory and long term (ie. permanent) memory.
Even 10 years ago, of course, no one claimed that this dichotomy was complete.
That is, perhaps there were still other forms of memory yet to be found.
But the fundamental distinction remained.
This distinction remains today. However more recent studies on animal
preparations suggest that indeed there are more than 2 stages in progress of a
memory from short term to long term. Studies of Aplysia in particular, for
instance, have shown a need for at least one more form of memory, intermediate
term memory. This acts as a bridge between short term and long term.
If stimulated by electric shock, the mollusc Aplysia will withdraw its siphon.
Much more important, a series of such shocks will make the animal much more
ready to withdraw its siphon. The effect is called short term sensitization.
Short term memory only lasts for a few minutes, while short term sensitization
persists for an hour. It acts as a bridge between short and long term memory.
For long term memory, the nerve cells must make new protein. But they need no
new protein for this short term sensitization. Nevertheless, both long term
memory and short term sensitization act on the same synapses, with the same
ultimate chemical effects. How can this happen?
Recently in NATURE (329 (1987) 62-65) Steven M. Greenberg, Vincent Castelluci,
and others at Columbia University in New York presented their work on a likely
mechanism by which short term sensitization can occur.
The first chemical event in memory consists of phosphorylation (adding a
phosphate group to) a protein at the synapse. This change affects how easily a
synapse will receive an impulse. Special enzymes (called kinases) promote this
phosphorylation. Because it is a beautiful example of how nanomachinery works,
I'll describe the mechanism which Greenberg, Castelluci, and their coworkers
found, in detail.
There are really two different nanomachines directly involved. One of them is
the kinase. The second is another machine (the regulator) which attaches to
the first whenever the level of one cell chemical, cAMP, is low. When it
attaches to the kinase, the regulator prevents this kinase from acting. But the
regulator itself has another spot to which cAMP can attach. If there is lots
of cAMP, the regulator is turned off. It releases the kinase, which can then
go off attaching phosphate to proteins in the synapse.
And what role does cAMP play in this process? cAMP is a chemical which floats
freely about in the cell. In other cell types, it acts as a messenger
substance: attachment of a hormone to the outer cell wall causes cAMP levels
to increase. This increased cAMP level then causes a change in the cell
nucleus, to make more protein. Much the same thing happens in memory for nerve
cells. Nerve impulses at the synapse cause cAMP levels in the whole cell to
go up.
This is exactly the interesting point. Because cAMP levels have increased, the
kinases are released to do their work. They proceed to change the synapses, so
that later impulses act even more easily. That means that cAMP levels inside
the neuron will stay high much longer than otherwise. A single nerve impulse
creates a condition in which the nerve will respond much more readily to later
impulses. Once touched, the cell rings like a bell for an hour or more.
This process, of course, will only preserve memory for hours, not for days or
weeks. We still need to know about the earlier stages (that is, exactly how
the synapses are altered) and the later stages (how proteins are synthesized
for a long term memory). What is important about this work is that it uncovers
quite fully one entire stage in the process of memory. The other stages cannot
be too far behind.
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