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 UMN and LMN lesions  

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Why is there flaccid paralysis in lower motor neuron lesion but spastic paralysis in upper motor neuron lesion???

thanks in advance.


lmn lesion- loss of inervation to the muscles so there is no resistance whatsoever to any movments- flaccid

umn lesion-lmn innervation is still intact to there is mild resistance at first and then later becomes more flaccid- spastic paralysis

good site for this subject


GAMMA motor neurons of anterior horne of spine are responsible for innervating the INTRAFUSAL fibers (or muscle spindle),and contract these fibers and therefore make them more sensetivi to even small stretches that happen in muscle.BUT;

these GAMMA neurons are normaly inhibited by UMN ,therefore in dmage to UMN the lack of this inhibition make these stretch receptors(intrafusal fibers)more snesetive so DTR increase

ALPHA neurons again innervate intrafusal fibers and contract them in responce to stretch ,contraction of these fibers makes a tention in muscle ,,,so in LMN damage b/c these ALPHA NEURONS DO NOT work so no contraction tension

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