The cranial nerves are nerves that originate in the brain rather than the spinal cord. There are 12 paired nerves. The seventh cranial nerve, the facial nerve, originates in the brainstem between the pons and the medulla. It controls movement of the facial expression muscles. It is involved in the conveyance of taste from the sensors on the tongue to the brain, and it supplies preganglionic parasympathetic fibers to the head and neck region.
The largest portion of the seventh cranial nerve is made up of branchial motor fibers. These fibers originate in the facial nerve nucleus in the pons, a part of the brainstem. The branchial motor fibers run from the pons to the muscles of the face, where they control the movements involved in facial expressions such as smiling and frowning. Blinking is also controlled in part by this nerve. It also enervates the digastric and stylohyoid muscles, which are located beneath the chin.
The visceral motor portion of the seventh cranial nerve enervates the submandibular and sublingual glands, two of the mouth’s salivary glands. The facial nerve controls production of saliva from these glands. It also controls tear production through the lacrimal gland in the eye.
The special sensory branch of the facial nerve conveys taste sensations from the back part of the tongue to the brain. It also innervates the oropharynx, the part of the throat that begins at the base of the tongue and includes the tonsil and soft palate. The general sensory branch, the other part of the seventh cranial nerve involved in sensory stimuli, carries sensory signals from a small area of skin behind the ear.
If a person is able to voluntarily move the face in typical expressions such as a frown, raised eyebrows or smile, the facial nerve is functioning normally. Unintentional asymmetries such as only one side of the mouth raising in a smile can indicate damage to the seventh cranial nerve. Taste generally is tested by swabbing the tongue with a flavored substance, and if the anterior part of the tongue is unable to taste, it also can be a sign of potential facial nerve damage.
Lesions on the seventh cranial nerve, such as those that occur in facial nerve palsy, can cause weakness in the facial muscles. Facial paralysis is another possible disorder involving this nerve. Facial paralysis can be caused by Lyme disease or some types of viruses.