Re: Why does the nervous system of many metazoans 'cross over'?

Hi Chris --

Great question!  For the optic chiasm, there actually is a pretty good 
explanation for crossing over -- it allows for binocular vision without any 
sort of duplication of visual cortex.  In binocular animals, a portion of 
retinal ganglion cells cross over; another population remains ipsilateral.  
This allows the right side of the visual cortex to process the left visual 
field, and the left side of the visual cortex to process the right visual 
field. This link has a good representation of crossed and uncrossed neurons:  
http://lshome.utsa.edu/Courses/
introneurolab/Assign02Anat/SheepBrain/html/optic_chiasm.htm 


If you were to try building a binocular visual system without crossing at 
the chiasm, you would need to create two primary visual cortex areas, one 
for the left eye and one for the right.  Then, you would need to create 
intracortical connections between these two areas to create binocular 
coordination.  You would probably face more complicated axon guidance 
problems using that strategy, rather than allowing crossing-over to occur 
earlier in the visual pathway.

Explaining the optic chiasm is the easy part; it's much harder to come up 
with a good explanation for the large-scale crossing that is seen in other 
pathways.  Commissural neurons in the spinal cord cross over to coordinate 
local motor neuron activity.  Without those local connections, coordinated 
motion between the two sides of the body (like walking) becomes difficult.  
In the brain, large commisures facilitate communications between the two 
halves and hold the brain together.  The brains of transgenic mice 
with commissural defects actually fall apart from the lack of these 
important connections.  Similarly, flies and nematodes with midline crossing 
defects have severe and often lethal nervous system defects.

The vast majority of corticospinal fibers cross over at the decussation of 
the pyramids, at the junction of the medulla and the spinal cord.  Crossed 
and uncrossed fibers innervate different pools of motor neurons and 
interneurons at the appropriate level of the spinal cord.  Why does the 
corticospinal tract decussate?  My colleagues and I had opinions, but no 
data to support them.  It's an open question, and perhaps some studies in 
the expanding field of axon guidance will provide us with answers down the 
line.

For more details on crossed and uncrossed neuroanatomy, see the new edition 
of Kandel's textbook, Principles of Neural Science 4th ed..

Hope this helps,
Amanda Kahn
amandak@phy.ucsf.edu

Thanks to my colleagues in the Tessier-Lavigne and Bargmann labs at UC San 
Francisco for discussions about axon guidance!