Left/Righ and DECUSSATION

[SJM Guzman]

> 
> 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!