left-right reversal of CNS: Why?

Jerry Larson jlarson at hsc.usc.edu
Sun Sep 15 17:33:51 EST 1996

In article <323BAF6A.A9B at postoffice.worldnet.att.net>, kenneth paul
collins <KPCollins at postoffice.worldnet.att.net> wrote:

> Jerry Larson wrote:

> > The brain doesn't know its own topological layout. 
> The nervous system =is= so self-aware... such selfe-awareness is built right 
> into, and derives from, the TD E/I nerual Geometry that I described in
> posts... it follows from the way that "neighborhood" relationships are 
> preserved, globally, within the neural topology... it's this one global 
> ordering principle that allows "self" to be distinguished from not-self
> neural-activation states... 

Well, YOUR brain may know its topological layout, since you studied it.
;-)  A thing working a certain way doesn't mean it's aware of the

> > If you could take and
> > twist it around through space, it would still work; in fact just that has
> > happened.
> ...not if neighborhood relationships are not preserved within the global 
> Geometry... frogs whose optical tecta are surgically relocated, for instance, 
> strike with their tongues, at insects, in the wrong direction... missing the 
> insects... (technical point... beyond normal joint movement, the nervous 
> system doesn't respond well to being subjected to "twisting" manipulation)...

I said IF, didn't I?  Again there are two levels that have to be
distinguished. When I said it had happened, I meant, of course, in the
course of evolution.   If you could take a circuit and twist it all around
through space without distorting the connectivity, or creating problems
with heat, blood flow, or whatever, it would still work.  I pointed this
out to show that whether the "image" on the cortex was right side up or
not doesn't matter.  If I understand the above paragraph, you're saying,
yes it does, if you could twist something around it would get all confused
and work backwards.  Unproven.

I know about the shufflebrain salamanders and stuff.  That's not the
hypothetical case I was talking about, since the connective  relationships
are NOT preserved. If it affects the case, it goes to show that the
cross-connectedness is at lower, more primitive levels.  There are
experiments with human beings who learned to function just fine with
inverting glasses.  A frog doesn't have the plasticity to learn something
like that, is all.  There is some plasticity in human somatomotor cortex;
a lot of functions can expand or contract in the homunculus (after loss of
a body part, or through intensive use of certain skills such as playing an
instrument), though they can't switch places, of course, and if the hand
area gets injured, that function can't come back.

Suppose you took a mammal of some sort, say a chimp, and disconnected  the
face area of cortex from the hand area, with some nice small subpial
cuts.  I suggest it wouldn't have a lot of effect.  Certainly the chimp
isn't going to start reaching backward for bananas in front of him!  The
arm and hand cortex is still connected to the contra arm, and still in
communication with the visual system.  Now do the experiment under local
anesthesia, and retract the isolated gyri away from each other, changing
the spatial relationship of the two motor control areas.  As long as those
two areas of cortex are connected to what they're supposed to connect to,
and have blood supply and so on, things will continue to work properly.

In fact, as I pointed out, the "homunculi" in the cerebellum do NOT
maintain the topological relationships, and they get rather distorted in
thalamus too.

> ...the topology that was described =permeates= the nervous system... it's 
> functionality has only become augmented as evolutionary development 
> occurred...

I'm inclined to agree with that.  The point is, the original questioner
asked  why is there crossed control?  I suggest it isn't so the contra
somatomotor cortex can be a few centimeters closer to the ipsi
cerebellum--even if that's true, it's a later development--but because of
the way nervous systems evolved in critters with bilateral symmetry.  Your
ideas about the cerebellum are interesting, but remain speculative, imo.
It's quite clear that you believe very strongly in them, and that I'm not
convinced, but let's not keep arguing back and forth, let's try to bring
in new ideas or deepen the understanding.

> >
> > Does anybody know offhand, what is the lowest thing on the evolutionary
> > ladder that demonstrates contralateral representation?  Do insects have
> > it?  I would guess they do, because they don't have much of a brain, so
> > there have to be "subcortical" control circuits that coordinate the two
> > sides of the body.
> > 
> > Do all vertebrates have it, even critters like snakes without legs or
> > fins? (I would guess they do, because they USED to have feet or fins in
> > their evolutionary derivation). How about planaria?  (Planaria, i.e.
> > flatworms, have a pair of nerve trunks, right and left, running lengthwise
> > down the body.  I believe they do have some sort of rudimentary brain, but
> > how do the two sides work together-- are they controlled by the brain,if
> > any,  or do they talk directly to each other?)
> ...as has been discussed, the antecedants of mammalian decussation can be 
> traced all the way back to organisms having radial symmetry... ken collins

Well, we did talk about that before, or you did, but I don't think you
proved your case at all.  You said something to the effect that nervous
impulses diffuse around the nervous system of a radial organism in a way
that's slow and not well organized, and that proves the NEED for
decussation. Looks to me like they need a central nervous system all
right, but lacking sides, the question of fibers crossing to the other
side doesn't arise.

I was asking about where do you actually SEE EVIDENCE of crossed control? 
Do snakes have crossed motor control?  Beetles?  Planaria?  Are there any
animals that have bilateral symmetry and a rudimentary nervous system, but
no crossed fibers? (Ken, I know you're going to say no.  I wonder if you,
or anyone else, can offer some interesting examples, though).

Anyway, I think this is an interesting point about radial vs. bilateral
symmetry.  Radial organisms do look as if they could stand to have some
more organization, i.e. a central nervous system, and I can imagine a
radial critter, longer than it is wide, with a central nervous system,
where?  At the center, where else, with a front and a back, putting mouth
and eyes (5 of them, say, facing in 5 directions) at the front, grasping
tentacles (5) at the front, swim fins (5, why not?) at the back.  But it
didn't happen, not on this planet at any rate; invention of the front-back
distinction, the left-right distinction, and the central nervous system,
all came pretty much together in a package deal.  And I bet crossed
control came in the same box.


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