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Now, let’s quickly review the anatomy and physiology of facial innervation. The 12 pairs of cranial nerves are peripheral nerves, also called lower motor neurons, that emerge from the brain or brainstem. The seventh cranial nerve is the facial nerve, which emerges from the brainstem,
and then enters the temporal bone where it travels through the facial canal. The facial nerve then exits the skull through a tiny hole called the stylomastoid foramen.
From there, it branches off to innervate different facial muscles that control facial expression ipsilaterally, so on the same side. The facial nerve also innervates the lacrimal glands, as well as the sublingual gland and submandibular salivary glands, and the mucous membranes of the nose, mouth, and nasopharynx. In addition, the facial nerve also carries sensory information about taste from the anterior ⅔ of the tongue. Finally, the facial nerve innervates the stapedius muscle in the ear, which helps dampen loud noises.
If we look back, the facial nerve receives information from a region of the brain called the motor cortex, which has upper motor neurons that send information to the brainstem nuclei of the facial nerve, and then the facial nerve gives rise to two lower motor neurons, one to innervate the lower side of the face, and one for the upper side. Now, the lower motor neuron that innervates the upper side of the face receives information from both sides of the motor cortex, so both from an upper motor neuron coming from the ipsilateral side, as well as an upper motor neuron coming from the contralateral or opposite side that crosses in the midline. On the other hand, the lower motor neuron that innervates the lower side of the face only receives information from the contralateral motor cortex, so from an upper motor neuron that crosses in the midline.
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