CN VII The facial nerve is the seventh cranial nerve, or simply cranial nerve VII. 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 parasymphatetic fibers to several head and neck ganglia. Cranial nerve VII Structure 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 nervus intermedius. 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 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. The nerves of the scalp, face, and side of neck. Intracranial branches 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. Extracranial branches 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 Tolong Zip Baju Makcik Cepat): Temporal branch of the facial nerve Zygomatic branch of the facial nerve Buccal branch of the facial nerve Marginal mandibular branch of the facial nerve Cervical branch of the facial nerve Intra operatively the facial nerve is recognized at 3 constant land marks: 1. At the tip of tragal cartilage where the nerve is 1cm deep and inferior 2. At the posterior belly of digastric by tracing this backwards to the tympanic plate the nerve can be found between these two structures 3. By locating the posterior facial vein at the inferior aspect of the gland where the marginal branch would be seen crossing it. 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. Embryology 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. Function Facial expression 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. 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 (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 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 ("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. 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 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). Lower motor neuron lesions can result in a CNVII 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 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.