The muscles of facial expression are controlled by lower motor neuron axons in the facial nerve. These neurons are in the facial nucleus in the lower brainstem. The upper motor neuron axons for the facial nucleus come from the lateral part of the primary motor cortex, and descend through the deep cerebrum and upper brainstem in the corticobulbar tract. The part of the facial nucleus that controls the muscles of the upper face, including strong eye closure, usually receives upper motor neuron innervation from bilateral corticobulbar tracts. The part of the facial nucleus that controls the muscles of the lower face, however, usually receives upper motor neuron innervation mainly from axons of the contralateral corticobulbar tract. Therefore, unilateral dysfunction of the corticobulbar tract superior to the facial nucleus usually causes contralateral lower facial weakness while sparing the upper face.
When something solid, or a puff of air, contacts the cornea of one eye, both eyes usually close involuntarily, which is called the corneal reflex. The afferent part of this reflex consists of information from mechanoreceptors in the cornea travelling into the brainstem via the ipsilateral trigeminal nerve. The efferent part of this reflex consists of lower motor neurons in bilateral facial nerves causing strong eye closure bilaterally.