Facial nerve anatomy, course, branches & innervation is complex. This details the facial nerve from brainstem to termination at muscles of facial expression.
Origin of Facial Nerve
The facial nerve arises from the medullopontine angle of the brain stem.
There are two distinct roots which join as they leave the brain stem to form the facial nerve:
- Facial Nerve Proper: the motor fibers, which loop dorsally around the abducens nerve nucleus and exits the cerebellopontine angle.
- Nervus Intermedius: parasympathetic and sensory fibers.
The facial nerve enters the petrous temporal bone at the internal auditory meatus alongside cranial nerve VIII.
Intracranial Facial Nerve
Course of Intracranial Facial Nerve
From this origin, the facial nerve can be divided into 5 intra-cranial segments:
- Cisternal segment in the cerebellopontine angle
- Intracanalicular segment
- Labyrinthine segment
- Tympanic segment – separated by the anterior genu, where the geniculate ganglion is located and the origin of 3 branches.
- Mastoid segment – separated by the posterior genu and the location of 3 branches.
Branches of Intracranial Facial Nerve
There are 6 intra-cranial branches of the facial nerve. These branches occur in two segments of its course: the geniculate ganglion in the tympanic segment and the mastoid segment.
Geniculate Ganglion (Tympanic Segment)
The geniculate ganglion is the location of the first 3 branches and mediates parasympathetic branches:
- Greater petrosal nerve – secretomotor to the lacrimal gland, taste from soft palate and parasympathetic fibres.
- External petrosal nerve
- Lesser petrosal nerve to the parotid gland
The next 3 branches of the facial nerve occur in the mastoid segment:
- Nerve to Stapedius muscle
- Sensory auricular branch
- Chorda Tympani – supplies taste sensation to the anterior 2/3 tongue via the lingual nerve and parasympathetic innervation to the submandibular and sublingual glands via the submandibular ganglion.
After this, the facial nerve becomes extra-cranial as it exists the stylomastoid foramen.
By understanding this anatomy, proximal facial nerve injuries involving the intracranial segments will cause loss of taste (ageusia) to the anterior 2/3 tongue and hyperacusia (nerve to stapedius). This does not occur in extra-cranial injuries.
Extra-Cranial Facial Nerve
The facial nerve becomes extra-cranial when it exits the stylomastoid foramen.
Course of the Extra-Cranial Facial Nerve
After exiting the stylomastoid foramen, the extra-cranial facial nerve passes anterior to the posterior belly of the digastric muscle and lateral to the styloid process of the temporal bone.
The nerve trunk lies approximately 1 cm inferomedial to the tip of the tragal pointer, and runs anterior to the mastoid apex and C1 transverse process, within 9 mm and 14 mm, respectively1
After providing multiple branches to muscle bellies, it enters the parotid gland.
The nerve next enters the parotid gland high up on its posteromedial surface and passes forwards and downwards behind the mandibular ramus
Patterns of Branching
Six distinctive anastomotic patterns were originally classified by Davis et al in 19564. Numerous micro-dissection studies have demonstrated that branching patterns and anastomoses between branches, both within the parotid and on the face, exhibit considerable individual variation5 6.
These anastomoses and branches patterns explain why a division of a small branch often fails to result in the expected facial nerve weakness. There is significant overlap and cross-innervation.
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