02 - Crossed Circuits

Cross references: 


On page 68, the book's "Principle 3" is that "The
Brain's Circuits Are Crossed".  This is confirmed for
the sensory input by the picture on 385 and for the
motor output on 367.  What the discussion on 367
doesn't mention is that the motor nerves that don't
cross in the pyramids connect with interneurons
farther down in the spinal cord which do cross. 

But why do they cross? 

One version of the generally accepted explanation for
why the tracts cross is given by the link to the Washington
Post
article, below.  If it doesn't work right away, try it again. 
Sometimes the server gets bogged down. 

Note:  The graphic which was originally part of the link has been taken down. 

There are two problems with this version of the explanation,
and my excuse for forwarding it is that it is readily available,
provides a basis for discussion, and I couldn't find a better
version online in a reasonable amount of time. 

The first problem is that the motor tracts of fish are crossed,
not uncrossed as is shown in the diagram. 

The second problem is that this explanation leans heavily
on the fact that an optical lens reverses its image. 

The nervous systems of both the amphioxus and the
acorn worm are also crossed even though the light
receptors of the amphioxus are lensless and the
acorn worm has no light receptors at all.

The original version proposed by Cajal did not depend
on the reversal of the visual image. 

In the original version, if an early animal, including an
amphioxus or an acorn worm, were touched on the
left side, the message would need to cross to the
right side so that the muscles on the right side would
contract to move the body away from the stimulus
on the left side. 

The problem with this is that, in an aqueous environment,
if the muscles on the right side contract, the animal will be
propelled to the left, toward the stimulus. 

A long time ago I found a more convincing explanation. 
Unfortunately, I failed to photocopy the article, so the
explanation below is from memory. 

It is widely accepted that the first nervous system
evolved in a primitive coelenterate, among whose
modern descendants are the sea anemone and the
jelly fish, and that all more recently evolved systems
evolved from this early coelenterate configuration. 
Although modern coelenterates have nervous systems
that are more complex, this early nervous system is
hypothesized to have been a network that looked
something like this during the phase of the animal's
 life cycle when it remained pretty much stationary
on the bottom. 




There is a "node" at the base of each tentacle which
sends two nerves diagonally down-across the body of
the animal.  There is an electrochemical synapse
everywhere the nerves cross, and the nerves conduct
equally well in both directions. 

The nodes provide both sensory input to, and
motor output from, the nervous system.

The nerve network allows the input at
one node to influence the output from its neighbors. 

Since there are no nerves in the tentacles, the input
to the nodes is chemical, primarily from the tentacle
with which it is most closely associated.  The input
diffuses down the tentacle from stinging cells,
which discharge on physical contact. 

The output from the node is also chemical, and as it
diffuses back up the tentacle it informs the muscles
when they need to contract because the stinging cells
on the neighboring tentacles have been firing in large
numbers.  This is important information since it indicates
that prey has been captured and is ready to be conveyed
to the mouth. 

The coelenterates have radial symmetry and consist of
many identical units arranged side-by-side in a
circle.  The hypothesis suggests that the genetic
guidance for this growth pattern has two components. 

One genetic component produces the basic structural
element which is repeated, and the second genetic
component controls the repetition. 

The hypothesis suggests that during the evolution of
bilateral body form, the first genetic component was
maintained and the second genetic component was
modified so that there was but a single repetition. 

This results in two identical body structures, as
produced by the first genetic component, lying
side-by-side and interconnected more or less as would
have been the case had the second genetic component
allowed the repetition to continue. 

As you can see, the two nodes would become the two
halves of the brain and each side of the brain would
communicate primarily with the opposite side of the
body. 








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