General Characteristics and Nuclei
Anatomically, CN VII is divided into two functional components: the motor facial nerve proper and the nervus intermedius (nervus intermedius), which carries sensory and parasympathetic fibers.
The entire system relies on three nuclei located in the brainstem:
- Motor nucleus (nucleus nervi facialis): Located in the caudal pons within the reticular formation. From the perspective of the rhomboid fossa, its projection corresponds to the facial colliculus (colliculus facialis).
- Parasympathetic nucleus (nucleus salivatorius superior): The superior salivatory nucleus associated with the nervus intermedius, responsible for secretory functions.
- Sensory nucleus (nucleus tractus solitarii): The nucleus of the solitary tract, also associated with the nervus intermedius, responsible for taste sensation.
Intracranial Course and Brainstem Exit
The fiber pathways within the brainstem feature a complex topography. The rootlets of the motor and intermediate nerves initially course dorsally. They loop around the abducens nucleus, forming a characteristic bend known as the genu of the facial nerve (genu nervi facialis).
After forming the genu, the fibers course anteriorly toward the ventral surface of the brainstem. Their exit point lies at the cerebellopontine angle, at the caudal border of the pons lateral to the olive.
Course in the Petrous Part of the Temporal Bone
After leaving the brainstem, the facial and intermediate nerves travel toward the internal acoustic meatus and enter it. They then course through the bony facial canal (canalis nervi facialis).
Within the petrous temporal bone, the sensory ganglion of the intermediate nerve forms the geniculate ganglion (ganglion geniculi).
While traversing the facial canal, the intermediate part gives rise to three major branches:
- Greater petrosal nerve (nervus petrosus major): Carries parasympathetic fibers supplying the lacrimal gland.
- Nerve to the stapedius (nervus stapedius): Innervates the stapedius muscle in the tympanic cavity.
- Chorda tympani (chorda tympani): A mixed branch that conveys taste sensation from the anterior two-thirds of the tongue and delivers parasympathetic secretomotor fibers to the submandibular and sublingual salivary glands.
Extracranial Branches and Parotid Plexus
The main trunk of the facial nerve exits the cranial cavity via the stylomastoid foramen (foramen stylomastoideum). Immediately after exit, before entering the substance of the parotid gland, it gives off several motor branches:
- Posterior auricular nerve (nervus auricularis posterior)
- Digastric branch (ramus digastricus)
- Stylohyoid branch (ramus stylohyoideus)
Next, the nerve enters the parotid gland, where it divides into multiple branches forming the parotid plexus (plexus parotideus). This plexus gives rise to five main terminal groups that fan out across the face to innervate the muscles of facial expression:
- Temporal branches (rami temporales)
- Zygomatic branches (rami zygomatici)
- Buccal branches (rami buccales)
- Marginal mandibular branch (ramus marginalis mandibulae)
- Cervical branch (ramus colli)