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heyy very nice explanation.

i have a confusion.. how does UMN lesion result in lower limb weakness and LMN lesion result in upper limb weakness? eg. ALS.. kaplan says both upper and lower motor affected.. so FLACCID paralysis of upper limbs (so means LML upper limbs) and similarly SPASTIC paralysis of lower limbs ( so UML lower limbs).. the same thing applies to ASA occlusion also.

i thought UMN in lower limb supply the flexors more than extensors.. but cant really understand the application.. very confused.
I think it depends on the level of lession. Lession at the spinal cord level of brachial plexus origin (C5-T1) will cause weakness in upper limb, while at the level of lumbosacral plexus origin (L2-S3) will cause weakness in lower limb. Whether it is spastic or flaccid paralysis will depend on type of lession (UMN or LMN).

In case of ALS, it usually affects the cervical spinal cord that's why you'll see LMN type of lession of upper limb and UMN type of lession in lower limb.
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