Facial paralysis: A critical review of accepted explanation
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Historically, paralysis of facial muscles has been divided into "upper motor neuron injury" and "lower motor neuron injury". Patients who experience a stroke in the cortex or internal capsule have UMN injury and cannot purse their lips or smile on command. They are, however, able to wrinkle their forehead, raise their eyebrows, and completely close their eyes. Patients with LMN injury, in addition to the aforementioned impairments cannot raise their eyebrows. The classical explanations for these clinical findings are that the upper facial muscles receive bilateral innervation from the cerebral cortex and the lower facial muscles receive only unilateral innervation from the contralateral cerebral cortex. However, a review of the basic science literature indicates that commonly accepted explanations and the pattern of cortical projections are not consistent with anatomical studies. Studies in monkeys demonstrate that both the upper facial nucleus and the lower facial nucleus receive bilateral cortical projections. As well, there is no direct anatomical evidence in human beings that the facial nucleus (upper or lower) receives any innervation from the cortex.
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