Answer :
Final answer:
Supranuclear lesions primarily cause lower facial weakness only due to the bilateral control of upper facial muscles and more unilateral control of lower facial muscles by the cerebral cortex. These lesions often result from conditions like strokes, while Bell's Palsy is an example of a lower motor neuron disorder.
Explanation:
Supranuclear lesions affecting the face primarily cause lower facial weakness only. Supranuclear pathways are responsible for transmitting voluntary movement signals from the brain's cortex to the lower motor neurons in the brainstem that control the muscles of facial expression. Upper facial muscles are controlled bilaterally by these pathways from both hemispheres of the brain, whereas lower facial muscles are more heavily innervated unilaterally. Thus, a supranuclear lesion, commonly due to a stroke, on one side of the brain may lead to contralateral paralysis or weakness of the lower half of the face, while the upper half often retains its function because it still receives innervation from the unaffected side.
Bell's Palsy, on the other hand, is a lower motor neuron disorder characterized by muscle weakness that causes one half of the face to droop, indicating a lesion of the facial nerve itself or its branches, rather than a supranuclear lesion affecting the upper motor neuron pathways.
