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Dirk Bruere at Neopax dirk at neopax.com
Mon May 16 11:24:14 EST 2005

SJM Guzman wrote:

>> What about sensory information eg tactile, from left and right sides 
>> of the body eg hands? Is the processing of this info done 
>> preferentially by one hemisphere?
> Essentially yes... so easy.... the whole sensory system works 
> essentially the same way, (i.e left processing is preferentially 
> processed by the right hemisphere). This is because of the crossing over 
> of nerve fibers left to the right and vice versa (decussation). 
> Decussation is at the level of brain stem (for cranial nerves) or spinal 
> cord (for the rest of nerves). However, although decussation represents 
> the main input (contralateral) into the cortex, there's some information 
> coming directly to the same part of the body (ipsilateral). The cellular 
> pathway of sensory information requires a three-step neurons system. 
> First, the receptor neuron for the sensory modality (e.g tactile, 
> visual, nociceptive...) goes to the brain stem or medulla (where the 
> information is partially decussed). Second, from there the information 
> goes to the thalamus and (third) finally to the cortex.
> The motor systems work the same way (corticospinal tracts) but in a 
> two-step neuron system. Neurons from the motor cotex goes travels 
> thought the medulla, where 75/80 %  "decussate", and ultimately synapse 
> on alpha motor neurons in the spinal cord. That's the reason why 
> patients cerebral hemisphere stroke, which involves the left side for 
> example, results in right-side paralysis, as well as partially left-side 
> malfunctionality.
> In the cortex the information is represented by the surface involved for 
> sensory or motor execution (somatosensory cortex or motor cortex). Just 
> have a look to the sensitive homunculus of Penfield and the motor 
> homunculus.
> Hope it helps!
> Yours sincerely!
> SJM Guzman
> PS: My apologizes for my english in advance!

I've seen considerably worse from people whose first language is English!
Anyway, one last question - why is there a crossover?


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