Facial nerve
The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue. The nerve typically travels from the pons through the facial canal in the temporal bone and exits the skull at the stylomastoid foramen. It arises from the brainstem from an area posterior to the cranial nerve VI and anterior to cranial nerve VIII.
The facial nerve also supplies preganglionic parasympathetic fibers to several head and neck ganglia.
The facial and intermediate nerves can be collectively referred to as the '''nervus intermediofacialis.'''
Structure
The path of the facial nerve can be divided into six segments:- intracranial segment
- meatal segment
- labyrinthine segment
- tympanic segment
- mastoid segment
- extratemporal segment
From the brain stem, the motor and sensory parts of the facial nerve join and traverse the posterior cranial fossa before entering the petrous temporal bone via the internal auditory meatus. Upon exiting the internal auditory meatus, the nerve then runs a tortuous course through the facial canal, which is divided into the labyrinthine, tympanic, and mastoid segments.
The labyrinthine segment is the shortest and narrowest segment of the facial nerve and ends where the facial nerve forms a bend known as the geniculum of the facial nerve, which contains the geniculate ganglion for sensory nerve bodies. The first branch of the facial nerve, the greater petrosal nerve, arises here from the geniculate ganglion. The greater petrosal nerve runs through the pterygoid canal and synapses at the pterygopalatine ganglion. Postsynaptic fibers of the greater petrosal nerve innervate the lacrimal gland.
In the tympanic segment, the facial nerve runs through the tympanic cavity, medial to the incus.
The pyramidal eminence is the second bend in the facial nerve, where the nerve runs downward as the mastoid segment, the longest segment of the facial nerve. In the temporal part of the facial canal, the nerve gives branch to the stapedius muscle and chorda tympani. The chorda tympani supplies taste fibers to the anterior two thirds of the tongue, and also synapses with the submandibular ganglion. Postsynaptic fibers from the submandibular ganglion supply the sublingual and submandibular glands.
Upon emerging from the stylomastoid foramen, the facial nerve gives rise to the posterior auricular branch. It then gives rise to the branch to the posterior belly of the digastric, and then the branch to the stylohyoid. The facial nerve then passes through the parotid gland, which it does not innervate, to form the parotid plexus. The nerve then bifurcates at the pes anserinus to become the upper and lower divisions of the facial nerve. It then splits into five branches, innervating the muscles of facial expression.
Intracranial branches
The greater petrosal nerve arises at the superior salivatory nucleus of the pons and provides parasympathetic innervation to several glands, including the nasal glands, the palatine glands, the lacrimal gland, and the pharyngeal gland. It also provides parasympathetic innervation to the sphenoid sinus, frontal sinus, maxillary sinus, ethmoid sinus, and nasal cavity. This nerve also includes taste fibers for the palate via the lesser palatine nerve and greater palatine nerve.The communicating branch to the otic ganglion arises at the geniculate ganglion and joins the lesser petrosal nerve to reach the otic ganglion.
The nerve to stapedius provides motor innervation for the stapedius muscle in middle ear
The chorda tympani provides parasympathetic innervation to the sublingual and submandibular glands, as well as special sensory taste fibers for the anterior two thirds of the tongue.
Extracranial branches
Distal to stylomastoid foramen, the following nerves branch off the facial nerve:- Posterior auricular nerve which controls movements of some of the scalp muscles around the ear
- Branch to posterior belly of digastric muscle
- Branch of the stylohyoid muscle
- Five terminal facial branches – from superior to inferior:
- * Temporal branch
- * Zygomatic branch
- * Buccal branch
- * Mandibular branch
- * Cervical branch
- At the tip of tragus where the nerve is 1 cm deep and inferior
- At the posterior belly of digastric by tracing this backwards to the tympanic plate, the nerve can be found between these two structures
- By locating the posterior facial vein at the inferior aspect of the gland where the marginal branch would be seen crossing it.
- Lateral semicircular canal
- Foot of incus
Nucleus
Development
The facial nerve is developmentally derived from the second pharyngeal arch, or branchial arch. The second arch is called the hyoid arch because it contributes to the formation of the lesser horn and upper body of the hyoid bone. The facial nerve supplies motor and sensory innervation to the muscles formed by the second pharyngeal arch, including the muscles of facial expression, the posterior belly of the digastric, stylohyoid, and stapedius. 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.Although the anterior two thirds of the tongue are derived from the first pharyngeal arch, which gives rise to the trigeminal nerve, not all innervation of the tongue is supplied by it. The lingual branch of the mandibular division of the trigeminal nerve supplies non-taste sensation to the anterior part of the tongue via general somatic afferent fibers. Nerve fibers for taste are supplied by the chorda tympani branch of the facial nerve via special visceral afferent fibers.
Function
Facial expression
The main function of the facial nerve is motor control of all 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. These skeletal muscles are developed from the second pharyngeal arch.Facial sensation
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. These sensory and taste fibers travel together as the lingual nerve briefly before the chorda tympani leaves the lingual nerve to enter the tympanic cavity 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.
Other
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 parasympathetic fibers that travel in the facial nerve originate in the superior salivatory nucleus.The facial nerve also functions as the efferent limb of the corneal reflex.
Functional components
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, and lacrimal glands, also mucosa of nasal cavity.
Axons of type SVE, special visceral efferent, innervate muscles of facial expression, stapedius, the posterior belly of digastric, and the stylohyoid.
The axons of type SVA, special visceral afferent, provide taste to the anterior two-thirds of tongue via chorda tympani.