The facial nerve provides motor innervation to the muscles of facial expression, lacrimal gland, and submandibular and sublingual salivary glands, as well as taste to the anterior two-thirds of the tongue.
The facial nerve traverses the internal acoustic meatus carrying four modalities (Figure 17-5A).
- Branchial motor neurons. Supply muscles derived from the second branchial arch including the muscles of facial expression as well as the stapedius, posterior belly of the digastricus, and stylohyoid muscles.
- Visceral motor neurons. Provide parasympathetic innervation to almost all glands of the head (e.g., lacrimal, submandibular, sublingual, nasal, and palatal). The only exception is the parotid gland, which receives its visceral motor innervation from CN IX.
- Special sensory neurons. Transmit taste sensation from the anterior two-thirds of the tongue.
- General sensory neurons. Transmit general sensation from a portion of the external acoustic meatus and auricle.
A. The facial nerve (CN VII). B. Branchial motor innervation for CN VII. C. Special sensory distribution of the vestibulocochlear nerve (CN VIII).
Two distinct fascial sheaths package the four modalities carried by CN VII, with branchial motor neurons in one sheath and visceral motor, special sensory, and general sensory neurons in another sheath called the nervus intermedius.
Branchial Motor Nerve Trunk
The branchial motor components constitute the largest portion of CN VII. After entering the temporal bone via the internal acoustic meatus, a small branch of CN VII courses to the stapedius muscle, where branchial motor neurons course through the facial canal to exit the skull via the stylomastoid foramen. In the parotid gland, five terminal branches (i.e., temporal, zygomatic, buccal, mandibular, and cervical) provide voluntary control of the muscles of facial expression, including the buccinator, occipitalis, platysma, posterior digastricus, and stylohyoid muscles.
The nervus intermedius gives rise to the following nerves:
- Greater petrosal nerve. Contains preganglionic parasympathetic neurons that synapse in the pterygopalatine ganglion en route to the lacrimal, nasal, and palatine glands.
- Chorda tympani. Arises in the descending part of the facial canal and crosses the medial aspect of the tympanic membrane, passing between the malleus and incus. The chorda tympani exits the skull through the petrotympanic fissure and joins the lingual nerve from CN V-3 in the infratemporal fossa. The chorda tympani contains preganglionic parasympathetic neurons that synapse in the submandibular ganglion en route to innervate the submandibular and the sublingual salivary glands. The chorda tympani also contains special sensory neurons (taste) from the anterior two-thirds of the tongue, with cell bodies located in the geniculate ganglion.
- Auricular branches. Arise from the external acoustic meatus and auricle and carry general sensory neurons through the geniculate ganglion to the brainstem. The geniculate ganglion is a knee-shaped bend in CN VII, located within the temporal bone and housing sensory cell bodies for the special sensory neurons for taste and general sensory neurons from the ear.
Injury to CN VII,
after it exits the brainstem, results in paralysis of the facial muscles (Bell's palsy
) on the ipsilateral side. Fracture of the temporal bone can result in the abnormalities just described, plus increased sensitivity to noise (hyperacusis
) due to the lack of innervation to the stapedius muscle, dry mouth due to a decrease in salivation, dry corneas due to the lack of lacrimal gland activity, and a loss of taste on the anterior two-thirds of the tongue. It should be noted that a brainstem injury to CN VII results in paralysis of the contralateral facial muscles below the eye.