Left/Righ and DECUSSATION
jose.guzman at medizin.uni-leipzig.de
Mon May 16 11:04:04 EST 2005
> 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
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
Hope it helps!
PS: My apologizes for my english in advance!
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