Organization of the facial nucleus and corticofacial projection in the monkey: a reconsideration of the upper motor neuron facial palsy.

The somatotopic organization of the facial nucleus and the distribution of the corticofacial projection in the monkey were studied by the use of retrograde and anterograde transport of horseradish peroxidase. Facial motor neurons innervating lower facial muscles were primarily found in the lateral part of the nucleus, those supplying upper facial muscles in the dorsal part of the nucleus, and those innervating the platysma and posterior auricular muscles in the medial part of the nucleus. Descending corticofacial fibers innervated the lower facial motor nuclear region bilaterally, although with contralateral predominance. The upper facial motor nuclear regions received scant direct cortical innervation on either side of the brain. Our results indicate that upper facial movement, like that at the shoulder, is relatively preserved in upper motor neuron palsy because these motor neurons receive little direct cortical input. By contrast, the lower facial muscles, like those of the hand, are more severely affected because their motor neurons normally depend upon significant cortical innervation.