[From Bill Powers (2010.11.16.2030 MDT)]
Hello, all. I’ve addressed this to CSGnet and added CCs to others who may
not be reached that way. There may be some duplications.
I’m writing this because Nature for November 11 and Science
for Nov 5 have just arrived, and both contain multiple references to
glial cells which occupy half the volume of the brain and, I have read,
outnumber neurons 10 to 1. I’ve long suspected some role in
reorganization for these cells, just because they’re everywhere, but now
the information is getting a lot more specific and looking a lot more as
if this is a, if not the, reoganizing system. Glial cells are responsible
for, among many other things, the pruning of synapses during early
growth; they send out extensions that physically wrap around synapses and
they’re known to react to neural events and perform homeostatic functions
on cellular calcium, as well as secreting large amounts of GABA (an
inhibitory neurotransmitter) and other stuff. There’s way too much in
these two publications for me to get into my head, which I have to admit
doesn’t absorb things the way it used to.
I think all PCTers who have any degree of comfort with this sort of
neuroscience should look into these two journals and wherever else they
can get information to see if the reorganizing-system hypothesis can hold
even a drop or two of water. There are PhD theses and glory here in
abundance if this works out.
Here’s one of the articles in the Nature issue:
Eroghu, Cagla and Barres, Ben A; Regulation of synaptic connectivity by
glia. Nature, Vol 468, p. 223-229
Get that title!
Here’s part of a paragraph from page 228 of the review article:
*“Glia also play important roles in synapse elimination in the
mammalian nervous system. One of the classical examples of
activity-dependent synapse elimination occurs at the mammalian
neuromuscular junction. At birth, postsynaptic muscle cells are
innervated by multiple motor axons. By the second week after birth,
activity-dependent competition permanently eliminates immature inputs,
whereas the sole remaining input is maintained and strengthened.
Eliminated connections detach from the neuromuscular junction, and as
they retract, pieces of axon are shed. In the mammalian peripheral
nervous system, similar to the active engulfment and clearance by glia
that is observed in Drosophila metamorphosis, Schwann cells [another type
of glial cell] break up retracting axons and remove the synaptic
debris.”*My God! This sounds like exactly what Demo 8-1, ArmControlReorg in
LCS3, does, except for cleaning up the debris. I forgot to program in a
janitor. Each of the 14 control systems starts by having random
connections to ALL 14 joint angles, which would mean to all the muscles
that affect joint angles if the model had that much detail. During
activities due to fluctuations in reference signals and disturbances, and
because of the reorganizing system, the weights given to irrelevant
connections get reduced closer and closer to zero, leaving only the
increasing weights for the output signal affecting the joint angle (or
muscle force) that is sensed by the control system producing that one
output signal.
The descriptions go on and on, every one sending chills down my spine.
And there are more articles.
Henry Yin note: the first author, Eroghu, is at Duke University’s Cell
Biology department. Please, please show him Demo 8-1. If he doesn’t want
to look at it, get some students to tie him to a chair and prop his
eyelids open. Just start the demo and explain that the matrix of
rectangles on the screen shows changing weights of signals from the
control system outputs to the muscles operating the joint angles, and
then just get him to watch for about 15 minutes. Use a fast laptop. You
shouldn’t have to say any more.
This is the first evidence I have seen that speaks directly to the
concepts of reorganization that I have been imagining through all these
years. Now if somebody would just find that the synaptic strengths are
varying randomly …
Best,
Bill P.