Facial Nerve Revision Flashcards
What are the modalities of the facial nerve?
- Motor
- Sensory
- Special sensory
- Parasympathetic
What is the motor function of the facial nerve?
- Stapdeius
- Posterior belly of digastric
- Stylohyoid
- Muscles of facial expression (via 5 terminal motor branches)
What is the somatic sensory function of the facial nerve?
A small area around the concha of the external ear
What is the special sensory function of the facial nerve?
Taste - anterior 2/3 of tongue via the chorda tympani
What is the parasympathetic function of the facial nerve?
Glands of head and neck EXCEPT parotid:
- Submandibular and sublingual salivary glands
- Lacrimal glands
- Nasal, palatine and pharyngeal mucous glands.
Anatomically, the course of the facial nerve can be divided into two parts:
- Intracranial: the course of the nerve through the cranial cavity, and the cranium itself.
- Extracranial: the course of the nerve outside the cranium, through the face and neck.
Where does CN VII emerge from?
Cerebellopontine angle (ventral aspect of pons)
How does the facial nerve enter the cranium?
Through the internal acoustic meatus in the petrous part of the temporal bone –> closely related to inner ear here
Where is the internal acoustic meatus?
Petrous part of temporal bone
Still within the temporal bone, the roots leave the internal acoustic meatus, and enter what?
The facial canal –> this is ‘Z’ shaped
Within the facial canal in the temporal bone, what ganglion is formed by the facial nerve?
The geniculate ganglion
What is the geniculate ganglion?
- A collection of pseudounipolar sensory neurons of the facial nerve located in the facial canal of the temporal bone.
- Function:
- Sensory ganglion of the facial nerve
- Contains cell bodies of the fibres responsible for conducting taste sensation from the anterior 2/3 of tongue
Within the facial canal in the temporal bone, what 3 branches does the facial nerve give rise to?
- Greater petrosal nerve
- Nerve to stapedius
- Chorda tympani
What is the function of the greater petrosal nerve? What type of nerve is it?
- Type:
- Mixed nerve - carrying parasympathetic, taste, and sensory fibers
- Sends parasympathetic fibres to the mucous glands and lacrimal gland.
What is the function of the nerve to stapedius? What is the function of stapedius?
- Nerve: Sends motor fibres to stapedius muscles of the middle ear
- Function: Stapedius dampens the vibrations of the stapes helping to control the amplitude of sound waves
What can damage to the nerve to stapedius result in?
Resulting paralysis of stapedius leads to hypersensitivity to loud noises (hyperacusis).
What is the function of chorda tympani?
Special sensory: innervates taste to the anterior 2/3 of the tongue (carries taste back to brain)
Parasympathetic: Fibres to the submandibular and sublingual glands (via lingual nerve)
After arising in the facial canal, which nerve does the chorda tympani ‘hitch hike’ with? Why?
- ‘Hitchhikes’ with the lingual nerve.
- The parasympathetic fibres of the chorda tympani stay with the lingual nerve, but the main body of the nerve leaves to innervate the anterior 2/3 of the tongue (taste)
After giving off these branches in the facial canal, where does the facial nerve exit the cranium?
Via the stylomastoid foramen
Where is the stylomastoid foramen?
located just posterior to the styloid process of the temporal bone.
After exiting the skull, and before dividing into its terminal motor branches, what does the facial nerve supply?
- Posterior auricular nerve –> motor innervation to the some of the muscles around the ear
- Motor innervation to the posterior belly of digastric and to the stylohyoid muscle
After innervating the posterior belly of the digastric muscle and the stylohyoid muscle, where does the facial nerve then pass?
continues anteriorly and inferiorly into the parotid gland –> the facial nerve does not contribute towards the innervation of the parotid gland, which is innervated by the glossopharyngeal nerve
Within the parotid gland, how does the facial nerve terminate?
By splitting into 5 motor branches which supply the muscles of facial expression:
- Temporal
- Zygomatic
- Buccal
- Marginal meningeal
- Cervical
Intracranial lesions occur during the intracranial course of the facial nerve (proximal to the stylomastoid foramen). What would be the effect of this on:
- Muscles of facial expression?
- Tongue?
- Salivation?
- Hearing?
- Lacrimal fluid production?
- Muscles of facial expression will be paralysed or severely weakened - 5 terminal branches
- Loss of taste on the ipsilateral 2/3 of tongue - chorda tympani
- Reduced salivation - chorda tympani via lingual nerve
- Ipsilateral hyperacusis (hypersensitive to sound) - nerve to stapedius
- Ipsilateral reduced lacrimal fluid production - greater petrosal nerve
Define ipsilateral
belonging to or occurring on the same side of the body.
What is the most common cause of an intracranial lesion of the facial nerve?
Infection related to the external or middle ear
What is Bell’s palsy?
Bell’s palsy is temporary weakness or lack of movement affecting 1 side of the face
Extracranial lesions occur during the extracranial course of the facial nerve (distal to the stylomastoid foramen). What is the only function of the facial nerve that is affected?
The motor function (as other branches given off intracranially)
What do extracranial lesions of the facial nerve result in?
paralysis or severe weakness of the muscles of facial expression.
What are 4 causes of extracranial lesions of the facial nerve?
- Parotid gland pathology – e.g a tumour, parotitis, surgery.
- Infection of the nerve – particularly by the herpes virus.
- Compression during forceps delivery – the neonatal mastoid process is not fully developed and does not provide complete protection of the nerve.
- Idiopathic – If no definitive cause can be found then the disease is termed Bell’s palsy.
