Facial Nerve (CN 7)
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Facial Nerve Fiber Courses
Although each fiber track has its individual course through the brainstem, basal cisterns, and skull, its easiest to learn the major motor component, first, and then simply focus on how the other fibers differ from it.*
The major motor component
- Exits the brainstem at the pontomedullary junction as the motor root, passes through the cerebellopontine angle cistern, enters the petrous bone through the internal acoustic meatus.
- Then gives off the stapedius nerve branch at the beginning of the mastoid segment before it drops straight down through the stylomastoid foramen where it divides into several nerve branches, which innervate the muscles of facial expression.
- As a clinical pearl, indicate that hyperacusis, an abnormal sensitivity to loud sounds, can occur in Bell's palsy.
- The stapedius muscle contracts the neck of the stapes, ostensibly to prevent the transmission of high-energy sounds through the middle ear ossicles, and thus it often fails in CN 7 injury.
The parasympathetic fibers
- Exit the brainstem at the pontomedullary junction as the nervus intermedius, join the major motor component, divide at the geniculate ganglion (which is the sensory ganglion of CN 7) into an upper division.
- The upper division exits the petrous bone as the greater petrosal nerve to innervate the pterygopalatine ganglion, which provides parasympathetic postganglionic innervation to the lacrimal, nasal, and palatine glands.
- & The lower division, which continues with the major motor component until it joins the chorda tympani to innervate the submandibular ganglion, which innervates the submandibular and sublingual glands.
- Thus, a proximal Bell's palsy will affect the upper and lower divisions of parasympathetic output and cause upper facial and oral secretions, whereas a distal lesion may only affect oral secretions.
- This explains why there can be intense drying of the eye, so much so that the eye must be well lubricated to avoid corneal abrasion.
The taste sensation fibers
- Follow the exact same course as the lower division parasympathetic fibers, except they innervate taste sensation of the anterior 2/3rds of the tongue.
- Thus, both proximal and distal Bell's palsy can affect taste.
The clinically insignificant sensory fibers to the external ear
- Follow the major motor component to its exit at the stylomastoid foramen.
Ramsay Hunt syndrome
- Herpes zoster reactivation in the geniculate ganglion (the sensory ganglion) triggers a Bell's palsy.
- The zoster vesicles may or may not be visible on the palate, tongue, or external acoustic meatus.
- Importantly, in Ramsay hunt, there is CN 8 involvement, as well, so patients manifest with hearing loss and vertigo in association with their CN 7 injury, because CN 8 runs through the internal acoustic meatus along with CN 7; thus herpes zoster activation in the geniculate ganglion affects both CNs.
- The chorda tympani is the nerve bundle of the lower parasympathetic fibers and the taste fibers.
- It merges with the lingual nerve, a branch of the mandibular division of the trigeminal nerve, which carries sensory afferent information from the floor of the mouth.
- This helps explain why Bell's palsy commonly causes facial dysthesias (sensory abnormalities) even though CN 7, itself, doesn't supply facial sensation.
The CN 7 Anatomical Segments
- In the meatal segment, CN 7 passes through the internal acoustic meatus.
- In the labyrinthine segment, it enters the facial canal.
- This segment terminates at the geniculate ganglion, which is identified by its abrupt bend, the external genu.
- In the horizontal (aka tympanic) segment, CN 7 runs posteriorly (horizontally).
- In the mastoid segment CN 7 drops straight down and exits the skull through the stylomastoid foramen.