Crossed nervous systems

[barry at neuron.uchc.edu]

In article <pgrobste-220794173416 at bfac1.brynmawr.edu> pgrobste at brynmawr.edu (Paul Grobstein) writes:
>
>
>> Brian Scott (scott at PSYCH.TORONTO.EDU) wrote:
>> 
>> > I think this would be the place to ask such a question, so here it is.
>> >
>> > What are the current theories as to why the nervous system is mostly crossed?
>> > What are the benefits which are thought to be gained from such a setup?
> FIrst of all it isn't mostly crossed in the brainstem.
> An approach is to try to think of systems for which crossing might be an advantage.
However, I'm not s
ure if any such speculations are testable. My favorite is the 
Mauthner cell or similar reticulospinal neuron mediating escape. The
requirements of such a system are that the interneuron (Mauthner cell) get 
massive sensory (primarily monosynaptic) input from one side but directly 
innervate many motor neurons on the contralateral side 
so as to effect a bend of the fish away from the threatening sensory stimulus. 
Thus the cell body and dedrites are on one side of the body, 
but the axon crosses to innervate contralateral axial motor neurons. 
The mauthner cell is ideally designed to acomplish this task with a minimum of 
synapses. Alternatively many primary eighth nerve afferents would have to cross the 
midline to innervate an interneuron(s) on the contralateral side.  
But what about the case where the interneurons are more numerous and complexly organized,
and (where speed may be less critical) as in the midbrain-forebrain. Here it seems to be 
more of a toss up as to the relative advantage of crossing. All it would take is for one 
system such as the visual to become crossed for some reason; the others systems
would likely follow. Like Paul Grobstein, I haven't been satisfied with the speculations
as to what the "some reason" was.

Mike Barry