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|Nerve: Facial nerve|
|Cranial nerve VII|
|The nerves of the scalp, face, and side of neck.|
|Gray's||subject #202 901|
|CN 0 – Terminal|
|CN I – Olfactory|
|CN II – Optic|
|CN III – Oculomotor|
|CN IV – Trochlear|
|CN V – Trigeminal|
|CN VI – Abducens|
|CN VII – Facial|
|CN VIII – Vestibulocochlear|
|CN IX – Glossopharyngeal|
|CN X – Vagus|
|CN XI – Accessory|
|CN XII – Hypoglossal|
The facial nerve is the seventh cranial nerve, or simply CNVII. It emerges from the brainstem between the pons and the medulla, and controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity. It also supplies preganglionic parasympathetic fibers to several head and neck ganglia.
The motor part and sensory part of the facial nerve enters the petrous temporal bone via the internal auditory meatus (intimately close to the inner ear) then runs a tortuous course (including two tight turns) through the facial canal, emerges from the stylomastoid foramen and passes through the parotid gland, where it divides into five major branches. Though it passes through the parotid gland, it does not innervate the gland (This is the responsibility of cranial nerve IX, the glossopharyngeal nerve).
The facial nerve forms the geniculate ganglion within the facial canal at the genu, the first bend in the canal.
- Greater petrosal nerve - provides parasympathetic innervation to several glands, including the nasal gland, palatine gland, lacrimal gland, and pharyngeal gland. It also provides parasympathetic innervation to the sphenoid sinus, frontal sinus, maxillary sinus, ethmoid sinus and nasal cavity.
- Nerve to stapedius - provides motor innervation for stapedius muscle in middle ear
- Chorda tympani
- Submandibular gland
- Sublingual gland
- Special sensory taste fibers for the anterior 2/3 of the tongue.
Distal to stylomastoid foramen, the following nerves branch off the facial nerve:
- Posterior auricular nerve - controls movements of some of the scalp muscles around the ear
- Branch to Posterior belly of Digastric muscle as well as the Stylohyoid muscle
- Five major facial branches (in parotid gland) - from top to bottom (a helpful mnemonic being To Zanzibar By Motor Car):
The facial nerve is developmentally derived from the hyoid arch (second pharyngeal branchial arch). The motor division of the facial nerve is derived from the basal plate of the embryonic pons, while the sensory division originates from the cranial neural crest.
The main function of the facial nerve is motor control of most of the muscles of facial expression. It also innervates the posterior belly of the digastric muscle, the stylohyoid muscle, and the stapedius muscle of the middle ear. All of these muscles are striated muscles of branchiomeric origin developing from the 2nd pharyngeal arch.
The facial nerve also supplies parasympathetic fibers to the submandibular gland and sublingual glands via chorda tympani. Parasympathetic innervation serves to increase the flow of saliva from these glands. It also supplies parasympathetic innervation to the nasal mucosa and the lacrimal gland via the pterygopalatine ganglion.
The facial nerve also functions as the efferent limb of the corneal reflex.
In addition, the facial nerve receives taste sensations from the anterior two-thirds of the tongue via the chorda tympani; taste sensation is sent to the gustatory portion of the solitary nucleus. General sensation from the anterior two-thirds of tongue are supplied by afferent fibers of the third division of the fifth cranial nerve (V-3). These sensory (V-3) and taste (VII) fibers travel together as the lingual nerve briefly before the chorda tympani leaves the lingual nerve to enter the tympanic cavity (middle ear) via the petrotympanic fissure. It joins the rest of the facial nerve via the canaliculus for chorda tympani. The facial nerve then forms the geniculate ganglion, which contains the cell bodies of the taste fibers of chorda tympani and other taste and sensory pathways. From the geniculate ganglion the taste fibers continue as the intermediate nerve which goes to the upper anterior quadrant of the fundus of the internal acoustic meatus along with the motor root of the facial nerve. The intermediate nerve reaches the posterior cranial fossa via the internal acoustic meatus before synapsing in the solitary nucleus.
The facial nerve also supplies a small amount of afferent innervation to the oropharynx below the palatine tonsil. There is also a small amount of cutaneous sensation carried by the nervus intermedius from the skin in and around the auricle (outer ear).
Location of cell bodies
The cell bodies for the facial nerve are grouped in anatomical areas called nuclei or ganglia. The cell bodies for the afferent nerves are found in the geniculate ganglion for taste sensation. The cell bodies for muscular efferent nerves are found in the facial motor nucleus whereas the cell bodies for the parasympathetic efferent nerves are found in the superior salivatory nucleus.
Upon reaching the temporal bone, the nerve's path can be divided into the internal auditory canal, labyrinthine segment, intratympanic segment, and descending or vertical segment. The labyrinthine segment is the narrowest portion of this pathway and is described to be approximately 0.7mm in diameter. The descending segment is the area where the branches of the chorda tympani and nerve to the stapedius branch from the facial nerve. The facial nerve eventually exits via the stylomastoid foramen to enter into the parotid where it branches into its peripheral segments.
The facial nerve carries axons of type GSA, general somatic afferent, to skin of the posterior ear.
The facial nerve also carries axons of type GVE, general visceral efferent, which innervate the sublingual, submandibular ("spit"), and lacrimal glands ("cry"), also mucosa of nasal cavity ("snot").
The facial nerve also carries axons of type SVE, special branchial-motor efferent, which innervate muscles of facial expression, stapedius, the posterior belly of digastric, and the stylohyoid.
The facial nerve also carries axons of type SVA, special visceral afferent, which provide taste to anterior two-thirds of tongue via chorda tympani
The facial nerve also carries axons of type GVA, general visceral afferent, which provide sensation to the soft palate and parts of the nasal cavity.
People may suffer from acute facial nerve paralysis, which is usually manifested by facial paralysis. Bell's palsy is one type of idiopathic acute facial nerve paralysis, which is more accurately described as a multiple cranial nerve ganglionitis that involves the facial nerve, and most likely results from viral infection and also sometimes as a result of Lyme disease. Iatrogenic Bell's Palsy may also be as a result of an incorrectly placed dental local-anesthetic (Inferior alveolar nerve block). Although giving the appearance of a hemi-plegic stroke, effects dissipate with the drug. When the facial nerve is permanently damaged due to a birth defect, trauma, or other disorder, surgery including a cross facial nerve graft or masseteric facial nerve transfer may be performed to help regain facial movement.
Testing the facial nerve
Voluntary facial movements, such as wrinkling the brow, showing teeth, frowning, closing the eyes tightly (inability to do so is called lagophthalmos) , pursing the lips and puffing out the cheeks, all test the facial nerve. There should be no noticeable asymmetry.
In an UMN lesion, called central seven, only the lower part of the face on the contralateral side will be affected, due to the bilateral control to the upper facial muscles (frontalis and orbicularis oculi).
Taste can be tested on the anterior 2/3 of the tongue. This can be tested with a swab dipped in a flavoured solution, or with electronic stimulation (similar to putting your tongue on a battery).
Corneal reflex. The afferent arc is mediated by the General Sensory afferents of the Trigeminal Nerve. The efferent arc occurs via the Facial Nerve. The reflex involves consensual blinking of both eyes in response to stimulation of one eye. This is due to the Facial Nerve's innervation of the muscles of facial expression, namely Orbicularis oculi, responsible for blinking. Thus, the corneal reflex effectively tests the proper functioning of both Cranial Nerves V and VII.
The course and connections of the facial nerve in the temporal bone.
- Kliniska Färdigheter: Informationsutbytet Mellan Patient Och Läkare, LINDGREN, STEFAN, ISBN 91-44-37271-X
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