|MadSci Network: Neuroscience|
The cross connection of many areas of the central and peripheral nervous systems is based on "decussations" -- neurons which cross the midline from their point of origination. The sensory and motor systems cross at different points in the lower brainstem (the sensory and "pyramidal" decussations in the rostral and caudal medulla oblongata respectively. There are also decussations in the brain itself where sensory representations from one side of a sensory field are mapped onto the other brain hemisphere. There is an excellent interactive anatomy guide to the CNS which describes the locations and paths of some of these crossing points at http://www.anatomy.wisc.edu/Bs97/text/BS/ contents.htm But as to the "why" that has been a poorly examined but often asked question. One possibility (not examined but raised during a conference) was that adding the extra length of neuron allowed a more precise timing for action potential arrival for very long axons, or when you may be getting input from neurons of multiple lengths. The other possibility is derived from some neuronal mutants (and I apologize for not having the exact referents available -- if you email me I will try and find them -- I believe they were developed at Stanford) -- these vertebrate mutants developed normally, but lacked decussations. The only effect? When they were picked up, they split in half and fell apart. So perhaps there is a structural aspect and decussations help bind the bicameral central nervous system together.
Try the links in the MadSci Library for more information on Neuroscience.