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 nerves 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 (abducens nerve) and anterior to cranial nerve VIII (vestibulocochlear nerve ). 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: The motor part of the facial nerve arises from the facial nerve nucleus in the pons, while the sensory and parasympathetic parts of the facial nerve arise from the intermediate nerve. From the brain stem, the motor and sensory parts of the facial nerve join together 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 very short, and ends where the facial nerve forms a bend known as the geniculum of the facial nerve ("genu" meaning knee), 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. In the temporal part of the facial canal, the nerve gives rise to the nerve 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. The facial nerve then passes through the parotid gland, which it does not innervate, to form the parotid plexus, which splits into five branches (temporal, zygomatic, buccal, marginal mandibular, and cervical) 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: Nucleus. 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. 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 rest of the hyoid is formed by the third arch). 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 (V3) of the trigeminal nerve supplies non-taste sensation (pressure, heat, texture) 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 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. 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 (superior part) 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). 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. Clinical significance. Palsy. 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 hemiplegic 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. Facial nerve decompression surgery is also sometimes carried out in certain cases of facial nerve compression. Examination. 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 upper  motor neuron 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). Lower motor neuron lesions can result in a CN VII palsy (Bell's palsy is the idiopathic form of facial nerve palsy), manifested as both upper and lower facial weakness on the same side of the lesion. Taste can be tested on the anterior two-thirds 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 nerves' 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.