Left/Righ and DECUSSATION
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
> 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!
> 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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