It results from damage of descending motor tracts (pyramidal tract alone or together with extra-pyramidal tract). The most common site is the internal capsule where the fibers condense together. The most common causes are hemorrhage or thrombosis of cerebral blood vessels.
Effects of UMNL:
Extent: It is usually wide spread because pyramidal tract fibers condense together. So, there is mono-plegia (paralysis of one limb), hemi-plegia (paralysis of UL & LL in one side or paraplegia (paralysis of both LL.).
Side: Usually in the opposite side because pyramidal tract fibers cross to the opposite side in its pathway from CC to AHC.
Type of paralysis: Spastic paralysis because it is accompanied with ↑ MT.
Recovery: No recovery in UMNL due to absence of neurilemma in its fibers (no cellular sheath in the CNS).
NB: In hemi-plegia some muscles recover (e.g. trunk muscles) because they receive innervation also from the same side.
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