A stroke patient has a weak arm that is stiff, with brisk reflexes and an upgoing toe. This is an upper motor neuron lesion — the corticospinal tract is damaged above the spinal cord. UMN signs: spasticity (stiffness), hyperreflexia, Babinski positive (toes fan upward). Contrast polio (LMN): flaccid weakness, fasciculations, absent reflexes. The UMN/LMN split localizes every weakness — brain/cord vs nerve/muscle — the most examined distinction in neurology.