View source for Cranial nerves

{{Short description|Nerves that emerge directly from the brain}}
{{good article}}
{{Infobox brain
| Name        = Cranial nerves
| Latin       = nervus cranialis <br />(pl: nervi craniales)
| Image       = Skull_brain_human_normal.svg
| Caption     = ''Left'' View of the human brain from below, showing origins of cranial nerves. <br /> ''Right'' Juxtaposed [[skull base]] with [[Foramen|foramina]] through which many nerves exit the skull.
| Image2      = Skull_and_brainstem_inner_ear.svg
| Caption2    = Cranial nerves as they pass through the skull base to the brain
| Width       = 240
| System      =
| Artery      =
| Vein        =
}}
{{Cranial nerves short}}

'''Cranial nerves''' are  [[nerve]]s that emerge directly from the [[brain]], including the [[brainstem]]. They relay information between the brain and various parts of the body, primarily to the [[head and neck]] regions and are responsible for [[special senses]] of [[Visual perception|vision]], [[taste]], [[Olfaction|smell]], and [[hearing]].<ref name="Gray's2008">{{cite book|first1=Susan |last1=Standring |first2=Neil R. |last2=Borley|title=Gray's anatomy: the anatomical basis of clinical practice|year=2008|publisher=Churchill Livingstone/Elsevier|location=[Edinburgh]|isbn=978-0-443-06684-9|edition=40th|chapter=Overview of cranial nerves and cranial nerve nuclei}}</ref>

The cranial nerves emerge from the [[central nervous system]] above the level of the [[Atlas (anatomy)|first vertebra]] of the [[vertebral column]].<ref name="Kandel">{{cite book|last=Kandel|first=Eric R.|title=Principles of neural science|year=2013|publisher=McGraw Hill|location=Appleton and Lange|isbn=978-0-07-139011-8|pages=1019–1036|edition=5}}</ref> Each cranial nerve is paired and is present on both sides. 

There are conventionally twelve pairs of cranial nerves, which are described with [[Roman numerals]] I–XII. Some considered there to be thirteen pairs of cranial nerves, including the non-paired [[cranial nerve zero]]. The numbering of the cranial nerves is based on the order in which they emerge from the brain and brainstem, from front to back.<ref name="Kandel" />

The [[terminal nerve]]s (0), [[olfactory nerve]]s (I) and [[optic nerve]]s (II) emerge from the [[cerebrum]], and the remaining ten pairs arise from the brainstem, which is the lower part of the brain.<ref name="Vilensky">{{cite book|title=The Clinical Anatomy of the Cranial Nerves: The Nerves of "On Olympus Towering Top"|last1=Vilensky|first1=Joel|last2=Robertson|first2=Wendy|last3=Suarez-Quian|first3=Carlos|publisher=Wiley-Blackwell|year=2015|isbn=978-1-118-49201-7|location=Ames, Iowa}}</ref>

The cranial nerves are considered components of the [[peripheral nervous system]] (PNS),<ref name= Vilensky/> although on a structural level the olfactory (I), optic (II), and trigeminal (V) nerves are more accurately considered part of the central nervous system (CNS).<ref>''Board Review Series – Neuroanatomy'', Fourth Edition, Lippincott Williams & Wilkins, Maryland 2008, p. 177. {{ISBN|978-0-7817-7245-7}}.</ref>

The cranial nerves are in contrast to [[spinal nerve]]s, which emerge from segments of the [[spinal cord]].<ref name="Vilensky" />

==Anatomy==
{{See also|Table of cranial nerves}}Most typically, humans are considered to have twelve pairs of cranial nerves (I–XII), with the [[terminal nerve]] (0) more recently canonized.<ref name="Kandel" /><ref name="Sonne" /> The nerves are: the [[olfactory nerve]] (I), the [[optic nerve]] (II), [[oculomotor nerve]] (III), [[trochlear nerve]] (IV), [[trigeminal nerve]] (V), [[abducens nerve]] (VI), [[facial nerve]] (VII), [[vestibulocochlear nerve]] (VIII), [[glossopharyngeal nerve]] (IX), [[vagus nerve]] (X), [[accessory nerve]] (XI), and the [[hypoglossal nerve]] (XII).
{{Cranial nerves}}{{multiple image
| align             = right
| direction         = vertical
| width             = 250
| image1            = Human brain anterior-inferior view description.JPG
| caption1          = View of the human brain from below showing the cranial nerves on an autopsy specimen
| image2            = Brain human normal inferior view with labels en-2.svg
| caption2          = View from below of the [[Human brain|brain]] and [[brainstem]] showing the cranial nerves, numbered from olfactory to hypoglossal after the order in which they emerge
| image3            = Brain stem sagittal.svg
| caption3          = The brainstem, with [[Cranial nerve nucleus|cranial nerve nuclei]] and tracts shown in red
| total_width       = 
| alt1              = 
}}
===Terminology===

Cranial nerves are generally named according to their structure or function. For example, the olfactory nerve (I) supplies smell, and the facial nerve (VII) supplies the muscles of the face. Because [[Latin]] was the ''[[lingua franca]]'' of the study of [[anatomy]] when the nerves were first documented, recorded, and discussed, many nerves maintain Latin or [[Greek language|Greek]] names, including the trochlear nerve (IV), named according to its structure, as it supplies a muscle that attaches to a pulley ({{langx|el|trochlea}}). The trigeminal nerve (V) is named in accordance with its three components ({{langx|la|trigeminus}} meaning [[wikt:triplet|triplet]]s),<ref>{{cite web|last=Harper|first=Douglas|title=Trigeminal Nerve|url=http://www.etymonline.com/index.php?search=trigeminalnerve|work=Online Etymology Dictionary|access-date=2 May 2014}}</ref> and the vagus nerve (X) is named for its wandering course ({{langx|la|vagus}}).<ref name=Davis2014 />

Cranial nerves are numbered based on their position from front to back ([[anatomical terminology#skull and brain|rostral-caudal]]) of their position on the brain,<ref name=Marieb>{{cite book|last=Mallatt|first=Elaine N. Marieb, Patricia Brady Wilhelm, Jon|title=Human anatomy|year=2012|publisher=Benjamin Cummings|location=Boston|isbn=978-0-321-75327-4|pages=431–432|edition=6th ed. media update.}}</ref> as, when viewing the forebrain and brainstem from below, they are often visible in their numeric order. For example, the olfactory nerves (I) and optic nerves (II) arise from the base of the [[forebrain]], and the other nerves, III to XII, arise from the brainstem.<ref name=Marieb/>

Cranial nerves have paths within and outside the [[Human Skull|skull]]. The paths within the skull are called "intracranial" and the paths outside the skull are called "extracranial". There are many holes in the skull called "foramina" by which the nerves can exit the skull. All cranial nerves are ''paired'', which means they occur on both the right and left sides of the body. The muscle, skin, or additional function supplied by a nerve, on the same side of the body as the side it originates from, is an ''ipsilateral'' function. If the function is on the opposite side to the origin of the nerve, this is known as a ''contralateral'' function.<ref>{{cite book|last1 = Albert|first1=Daniel|title=Dorland's Illustrated Medical Dictionary.|date=2012|publisher=Saunders/Elsevier|location=Philadelphia, PA|isbn=978-1-4160-6257-8|edition=32nd}}</ref>

===Intracranial course===
====Nuclei====

{{main|Cranial nerve nuclei}}
[[Gross anatomy|Grossly]], all cranial nerves have a [[Nucleus (neuroanatomy)|nucleus]]. With the exception of the olfactory nerve (I) and optic nerve (II), all the nuclei are present in the brainstem.<ref name="Kandel" />

The [[midbrain]] has the nuclei of the oculomotor nerve (III) and trochlear nerve (IV); the [[pons]] has the nuclei of the trigeminal nerve (V), abducens nerve (VI), facial nerve (VII) and vestibulocochlear nerve (VIII); and the [[Medulla oblongata|medulla]] has the nuclei of the glossopharyngeal nerve (IX), vagus nerve (X), accessory nerve (XI) and hypoglossal nerve (XII).<ref name="Moore's">{{cite book|author1=Keith L. Moore |author2=Anne M.R. Agur |author3=Arthur F. Dalley|title=Clinically oriented anatomy|year=2010|publisher=Lippincott Williams & Wilkins, Wolters Kluwer|location=Philadelphia|isbn=978-1-60547-652-0|pages=1055–1082|edition=6th}}</ref> The olfactory nerve (I) emerges from the [[olfactory bulb]], and depending slightly on division the optic nerve (II) is considered to emerge from the [[lateral geniculate nucleus|lateral geniculate nuclei]].<ref name="Moore's"/>

Because each nerve may have several functions, the [[nerve fibre]]s that make up the nerve may collect in more than one [[Nucleus (neuroanatomy)|nucleus]]. For example, the trigeminal nerve (V), which has a sensory and a motor role, has [[Trigeminal nerve nuclei|at least four nuclei]].<ref name="Moore's"/><ref name="Fitzgerald"/>

====Exiting the brainstem====
With the exception of the olfactory nerve (I) and optic nerve (II), the cranial nerves emerge from the [[brainstem]]. The oculomotor nerve (III) and trochlear nerve (IV) emerge from the [[midbrain]], the trigeminal (V), abducens (VI), facial (VII) and vestibulocochlear (VIII) from the [[pons]], and the glossopharyngeal (IX), vagus (X), accessory (XI) and hypoglossal (XII) emerge from the [[Medulla oblongata|medulla]].<ref name=OBER1998>{{cite book|last2=Ober|first1=Frederic H. |last1=Martini |first2=William C.|title=Fundamentals of anatomy and physiology|year=1998|publisher=Prentice Hall International|location=London|isbn=978-0-13-010436-6|pages=474–485|edition=4th | others=coordinator, art; photographer, illustrator, Claire W. Garrison, illustrator, Kathleen Welch, clinical consultant, Ralph T. Hutchings, biomedical}}</ref>

The olfactory nerve (I) and optic nerve (II) emerge separately. The olfactory nerves emerge from the [[olfactory bulb]]s on either side of the [[crista galli]], a bony projection below the [[frontal lobe]], and the optic nerves (II) emerge from the lateral colliculus, swellings on either side of the [[temporal lobe]]s of the brain.<ref name=OBER1998 />

====Ganglia====
{{main|Cranial nerve ganglia}}
The cranial nerves give rise to a number of [[ganglia]], collections of the [[cell body|cell bodies]] of neurons in the nerves that are outside of the brain. These ganglia are both parasympathetic and sensory ganglia.<ref name="Moore's"/>

The sensory ganglia of the cranial nerves, directly correspond to the [[dorsal root ganglia]] of [[spinal nerve]]s and are known as [[cranial nerve ganglia]].<ref name="Marieb"/> Sensory ganglia exist for nerves with sensory function: V, VII, VIII, IX, X.<ref name="Kandel" /> There are also a number of [[parasympathetic ganglion|parasympathetic]] [[cranial nerve ganglion|cranial nerve ganglia]]. [[sympathetic ganglion|Sympathetic ganglia]] supplying the head and neck reside in the upper regions of the [[sympathetic trunk]], and do not belong to the cranial nerves.<ref name="Moore's"/>

The ganglion of the sensory nerves, which are similar in structure to the dorsal root ganglion of the [[spinal cord]], include:<ref name=GRAYS2005 />
* The [[trigeminal ganglia]] of the trigeminal nerve (V), which occupies a space in the [[dura mater]] called [[Meckel's cave]]. This ganglion contains only the sensory fibres of the trigeminal nerve.
* The [[geniculate ganglion]] of the facial nerve (VII), which occurs just after the nerve enters the [[facial canal]]. 
* A superior and inferior ganglia of the glossopharyngeal nerve (IX), which occurs just after it passes through the [[jugular foramen]].

Additional ganglia for nerves with [[Parasympathetic system|parasympathetic]] function exist, and include the [[ciliary ganglion]] of the oculomotor nerve (III), the [[pterygopalatine ganglion]] of the maxillary nerve (V2), the [[submandibular ganglion]] of the [[lingual nerve]], a branch of the facial nerve (VII), and the [[otic ganglion]] of the glossopharyngeal nerve (IX).<ref name="Purves" >{{cite book | author1 = Purves, Dale |author2=George J. Augustine |author3=David Fitzpatrick |author4=William C. Hall |author5=Anthony-Samuel LaMantia |author6=James O. McNamara |author7=Leonard E. White | title = Neuroscience. 4th ed. | publisher = Sinauer Associates | pages = 12–13 | year = 2008 | isbn = 978-0-87893-697-7}}</ref>

====Exiting the skull and extracranial course====
{| class="wikitable" style="float:right;margin-left:15px"
|+ Exits of cranial nerves from the skull.<ref name="Moore's"/><ref name=GRAYS2005>{{cite book|last=Drake|first=Richard L.|title=Gray's anatomy for students|year=2005|publisher=Elsevier/Churchill Livingstone|location=Philadelphia|isbn=978-0-8089-2306-0|author2=Vogl, Wayne |author3=Tibbitts, Adam W.M. Mitchell  |author4=illustrations by Richard |author5= Richardson, Paul |pages=800–807}}</ref>
|-
! Location !! Nerve
|-
| [[cribriform plate]] || Terminal nerve (0) 
|-
| [[cribriform plate]] || Olfactory nerve (I) 
|-
| [[optic foramen]] || Optic nerve (II)
|-
| [[superior orbital fissure]] || Oculomotor (III)<br/>Trochlear (IV)<br/>Abducens (VI)<br/>Trigeminal V1<br/>     ''([[Ophthalmic nerve|ophthalmic]])''
|-
| [[foramen rotundum]]|| Trigeminal V2 <br/>     ''([[maxillary nerve|maxillary]])''
|-
| [[foramen ovale (skull)|foramen ovale]]|| Trigeminal V3 <br/> ''([[Mandibular nerve|mandibular]])''
|-
| [[stylomastoid foramen]]|| Facial nerve (VII)
|-
| [[internal auditory canal]]|| Vestibulocochlear (VIII)
|-
| [[jugular foramen]] || Glossopharyngeal (IX) <br/>Vagus (X)<br/>Accessory (XI)
|-
| [[hypoglossal canal]] || Hypoglossal (XII)
|}

After emerging from the brain, the cranial nerves travel within the [[skull]], and some must leave it in order to reach their destinations.  Often the nerves pass through holes in the skull, called [[foramen|foramina]], as they travel to their destinations. Other nerves pass through bony canals, longer pathways enclosed by bone. These foramina and canals may contain more than one cranial nerve and may also contain blood vessels.<ref name=GRAYS2005 />
*The terminal nerve (0) is a thin network of fibers associated with the dura and lamina terminalis running rostral to the olfactory nerve, with projections through the cribriform plate.
* The olfactory nerve (I) passes through perforations in the [[cribriform plate]] part of the [[ethmoid bone]].  The nerve fibres end in the upper nasal cavity.
* The optic nerve (II) passes through the [[optic foramen]] in the [[sphenoid bone]] as it travels to the eye.
* The oculomotor nerve (III), trochlear nerve (IV), abducens nerve (VI) and the ophthalmic branch of the trigeminal nerve (V1) travel through the [[cavernous sinus]] into the [[superior orbital fissure]], passing out of the skull into the [[Orbit (anatomy)|orbit]].
* The maxillary division of the trigeminal nerve (V2) passes through [[foramen rotundum]] in the sphenoid bone.
* The mandibular division of the trigeminal nerve (V3) passes through [[foramen ovale (skull)|foramen ovale]] of the sphenoid bone.
* The facial nerve (VII) and vestibulocochlear nerve (VIII) both enter the [[internal auditory canal]] in the [[temporal bone]].  The facial nerve then reaches the side of the face by using the stylomastoid foramen, also in the temporal bone.  Its fibers then spread out to reach and control all of the muscles of facial expression.  The vestibulocochlear nerve reaches the organs that control balance and hearing in the temporal bone and therefore does not reach the external surface of the skull.
* The glossopharyngeal (IX), vagus (X) and accessory nerve (XI) all leave the skull via the jugular foramen to enter the neck. The glossopharyngeal nerve provides sensation to the upper throat and the back of the tongue, the vagus supplies the muscles in the [[larynx]] and continues downward to supply parasympathetic supply to the chest and abdomen.  The accessory nerve controls the trapezius and sternocleidomastoid muscles in the neck and shoulder.
*[[File:Cranial Nerves Anatomy in Schematic 3D.ogg|thumb|Schematic 3D model of the cranial nerves]]The hypoglossal nerve (XII) exits the skull using the hypoglossal canal in the [[occipital bone]].

==Development==

The cranial nerves are formed from the contribution of two specialized embryonic cell populations, [[cranial neural crest]] and ectodermal placodes. The components of the sensory nervous system of the head are derived from the neural crest and from an embryonic cell population developing in close proximity, the cranial sensory placodes (the olfactory, lens, otic, trigeminal, epibranchial and paratympanic placodes). The dual origin cranial nerves are summarized in the following table:<ref>{{Cite journal|last1=Méndez-Maldonado|first1=Karla|last2=Vega-López|first2=Guillermo A.|last3=Aybar|first3=Manuel J.|last4=Velasco|first4=Iván|date=2020-08-07|title=Neurogenesis From Neural Crest Cells: Molecular Mechanisms in the Formation of Cranial Nerves and Ganglia|journal=Frontiers in Cell and Developmental Biology|volume=8|page=635|doi=10.3389/fcell.2020.00635|pmid=32850790|pmc=7427511|issn=2296-634X|doi-access=free}}</ref>

'''Contributions of neural crest cells and placodes to ganglia and cranial nerves'''
{| class="wikitable"
! Cranial nerve
! Ganglion and type
! Origin of neurons
|-
|'''CNI – olfactory'''

(Ensheating glia of olfactory nerves)
|
|Telencephalon/olfactory  placode; NCCs at forebrain
|-
|'''CNIII – oculomotor'''

(m)
|Ciliary,  visceral efferent
|NCCs  at forebrain-midbrain junction (caudal diencephalon and the anterior  mesencephalon)
|-
|'''CNV – trigeminal'''

(mix)
|Trigeminal,  general afferent
|NCCs  at forebrain-midbrain junction (from r2 into 1st PA), trigeminal  placode
|-
|'''CNVII – facial'''

(mix)
|<nowiki>-Superior,  general and special afferent</nowiki>

-Inferior:  geniculate, general and special afferent

-Sphenopalatine,  visceral efferent

-Submandibular,  visceral efferent
|<nowiki>-Hindbrain  NCCs (from r4 into 2</nowiki><sup>nd</sup> PA), 1st epibranchial placode

-1st  epibranchial placode (geniculate)

-Hindbrain  NCCs (2nd PA)

-Hindbrain  NCCs (2nd PA)
|-
|'''CNVIII – Vestibulocochlear'''

(s)
|<nowiki>-Acoustic:  cochlear, special afferent; and vestibular, special afferent</nowiki>
|<nowiki>-Otic  placode and hindbrain (from r4) NCCs</nowiki>
|-
|'''CNIX – glossopharyngeal'''

(mix)
|<nowiki>-Superior,  general and special afferent</nowiki>

-Inferior,  petrosal, general and special afferent

-Otic,  visceral efferent
|<nowiki>-Hindbrain  NCCs (from r6 into 3</nowiki><sup>rd</sup> PA)

-2nd  epibranchial placode (petrosal)

-Hindbrain  NCCs (from r6 into 3rd PA)
|-
|'''CNX – vagus'''

(mix)

Superior  laryngeal branch; and recurrent laryngeal branch 
|<nowiki>-Superior,  general afferent</nowiki>

-Inferior:  nodose, general and special afferent

-Vagal:  parasympathetic, visceral efferent
|<nowiki>-Hindbrain  NCCs (from r7-r8 to 4</nowiki><sup>th</sup> & 6th PA)

-Hindbrain  NCCs (4th& 6th PA); 3rd (nodose) and 4th  epibranchial placodes

-Hindbrain  NCCs (4th & 6th PA)
|-
|'''CNXI – accessory'''

(m)
|No  ganglion *
|Hindbrain  (from r7-r8 to PA 4); NCCs (4th PA)
|}
Abbreviations: CN, cranial nerve; m, purely motor nerve; mix, mixed nerve (sensory and motor); NC, neural crest; PA, pharyngeal (branchial) arch; r, rhombomere; s, purely sensory nerve. * There is no known ganglion of the accessory nerve. The cranial part of the accessory nerve sends occasional branches to the superior ganglion of the vagus nerve.{{See also|List of foramina of the human body}}

==Function==
The cranial nerves provide motor and sensory supply mainly to the structures within the head and neck.  The sensory supply includes both "general" sensation such as temperature and touch, and "special" senses such as [[taste]], [[Visual perception|vision]], [[Olfaction|smell]], balance and [[hearing]].<ref name=Fitzgerald>{{cite book|last=Mtui|first=M.J. Turlough FitzGerald, Gregory Gruener, Estomih|title=Clinical neuroanatomy and neuroscience|year=2012|publisher=Saunders/Elsevier|location=[Edinburgh?]|isbn=978-0-7020-3738-2|page=198|edition=6th}}</ref> The vagus nerve (X) provides sensory and autonomic (parasympathetic) supply to structures in the neck and also to most of the organs in the chest and abdomen.<ref name = Vilensky/><ref name="Kandel" />

===Terminal nerve (0)===
The [[terminal nerve]] (0) may not have a role in humans,<ref name = Vilensky/> although it has been implicated in hormonal responses to smell, sexual response and mate selection.<ref name = Sonne>{{cite journal |last1=Sonne |first1=J |last2=Lopez-Ojeda |first2=W |title=Neuroanatomy, Cranial Nerve 0 (Terminal Nerve) |journal=StatPearls [Internet] |date=January 2019 |pmid=29083731}}</ref>

===Smell (I)===
The [[olfactory nerve]] (I) conveys information giving rise to the sense of smell.<ref name=TC2018 />

Damage to the olfactory nerve (I) can cause an inability to smell ([[anosmia]]), a distortion in the sense of smell ([[parosmia]]), or a distortion or lack of taste.<ref name=TC2018 /><ref name="Kandel Appendix B">{{cite book|last=Kandel|first=Eric R.|title=Principles of neural science|year=2013|publisher=McGraw Hill|location=Appleton and Lange|isbn=978-0-07-139011-8|pages=1533–1549|edition=5.}}</ref>

===Vision (II) ===
The [[optic nerve]] (II) transmits visual information.<ref name="Kandel"/>

Damage to the optic nerve (II) affects specific aspects of vision that depend on the location of the damage. A person may not be able to see objects on their left or right sides ([[homonymous hemianopsia]]), or may have difficulty seeing objects from their outer visual fields ([[bitemporal hemianopsia]]) if the [[optic chiasm]] is involved. Inflammation ([[optic neuritis]]) may impact the sharpness of vision or color detection<ref name=TC2018 />

===Eye movement (III, IV, VI)===
[[File:Lawrence 1960 14.13.png|thumb|The oculomotor (III), troclear (IV) and abducens (VI) nerves supply the muscle of the eye. Damage will affect the movement of the eye in various ways, shown here.|alt=]]

The [[oculomotor nerve]] (III), [[trochlear nerve]] (IV) and [[abducens nerve]] (VI) coordinate [[eye movement]]. The oculomotor nerve (III) controls all muscles of the eye except for the [[superior oblique muscle]] controlled by the trochlear nerve (IV), and the [[lateral rectus muscle]] controlled by the abducens nerve (VI). This means the ability of the eye to look down and inwards is controlled by the trochlear nerve (IV), the ability to look outwards is controlled by the abducens nerve (VI), and all other movements are controlled by the oculomotor nerve (III)<ref name=TC2018 />

Damage to these nerves may affect the movement of the eye. Damage may result in double vision ([[diplopia]]) because the movements of the eyes are not synchronized. Abnormalities of visual movement may also be seen on examination, such as jittering ([[nystagmus]]).<ref name="Kandel Appendix B" />

Damage to the oculomotor nerve (III) can cause double vision and inability to coordinate the movements of both eyes ([[strabismus]]), also eyelid drooping ([[ptosis (eyelid)|ptosis]]) and pupil dilation ([[mydriasis]]).<ref name=Netter2007>{{cite book|first=Neil |last=Norton|title=Netter's head and neck anatomy for dentistry|year=2007|publisher=Saunders Elsevier|location=Philadelphia, Pa.|isbn=978-1-929007-88-2|page=78}}</ref> Lesions may also lead to inability to open the eye due to paralysis of the [[levator palpebrae]] muscle. Individuals suffering from a lesion to the oculomotor nerve, may compensate by tilting their heads to alleviate symptoms due to paralysis of one or more of the eye muscles it controls.<ref name="Kandel Appendix B" />

Damage to the trochlear nerve (IV) can also cause double vision with the eye adducted and elevated.<ref name=Netter2007/> The result will be an eye which can not move downwards properly (especially downwards when in an inward position). This is due to impairment in the superior oblique muscle.<ref name="Kandel Appendix B" />

Damage to the abducens nerve (VI) can also result in double vision.<ref name=Netter2007 /> This is due to impairment in the lateral rectus muscle, supplied by the abducens nerve.<ref name="Kandel Appendix B" />

===Trigeminal nerve (V)===
The [[trigeminal nerve]] (V) and its three main branches the ophthalmic (V1), maxillary (V2), and mandibular (V3) provide sensation to the skin of the face and also controls the muscles of [[mastication|chewing]].<ref name=TC2018 />

{{Trigeminal nerve}}

Damage to the trigeminal nerve leads to loss of sensation in an affected area. Other conditions affecting the trigeminal nerve (V) include [[trigeminal neuralgia]], herpes zoster, sinusitis pain, presence of a [[dental abscess]], and [[cluster headache]]s.<ref name=Nesbitt2012>{{cite journal|author=Nesbitt AD, Goadsby PJ|title=Cluster headache|journal=BMJ (Clinical Research Ed.)|date=Apr 11, 2012|volume=344|article-number=e2407|pmid=22496300 |type=Review|doi=10.1136/bmj.e2407|s2cid=5479248}}</ref><ref name=TC2018 />
{{clear}}
[[File:Bellspalsy.JPG|alt=|thumb|The facial nerve (VII) supplies the muscles of facial expression. Damage to the nerve causes a lack of muscle tone on the affected side, as can be seen on the right side of the face here.]]

===Facial expression (VII)===
The [[facial nerve]] (VII) controls most muscles of facial expression, supplies the sensation of taste from the front two-thirds of the tongue, and controls the [[stapedius muscle]].<ref name=TC2018 /> Most muscles are supplied by the cortex on the opposite side of the brain; the exception is the [[frontalis muscle]] of the forehead, in which the left and the right side of the muscle both receive inputs from both sides of the brain.<ref name=TC2018 />

Damage to the facial nerve (VII) may cause [[facial palsy]]. This is where a person is unable to move the muscles on one or both sides of their face.<ref name=TC2018 /> The most common cause of this is [[Bell's palsy]], the ultimate cause of which is unknown.<ref name=TC2018 /> Patients with Bell's palsy often have a drooping mouth on the affected side and often have trouble chewing because the [[buccinator muscle]] is affected.<ref name="Vilensky" /> The facial nerve is also the most commonly affected cranial nerve in [[blunt trauma]].<ref name="pmid29861376">{{cite journal |vauthors=Cools MJ, Carneiro KA |title=Facial nerve palsy following mild mastoid trauma on trampoline |journal=Am J Emerg Med |volume= 36|issue= 8|pages= 1522.e1–1522.e3|date=April 2018 |pmid=29861376 |doi=10.1016/j.ajem.2018.04.034 |s2cid=44106089 }}</ref>

===Hearing and balance (VIII)===
The [[vestibulocochlear nerve]] (VIII) supplies information relating to balance and hearing via its two branches, the [[vestibular nerve|vestibular]] and [[cochlear nerve]]s. The vestibular part is responsible for supplying sensation from the [[vestibules (inner ear)|vestibules]] and [[semicircular canal]] of the [[inner ear]], including information about [[balance (ability)|balance]], and is an important component of the [[vestibuloocular reflex]], which keeps the head stable and allows the eyes to track moving objects. The cochlear nerve transmits information from the [[cochlea]], allowing sound to be heard.<ref name="Kandel"/>

When damaged, the vestibular nerve may give rise to the sensation of spinning and dizziness ([[vertigo]]).  Function of the vestibular nerve may be tested by putting cold and warm water in the ears and watching eye movements [[caloric stimulation]].<ref name = Vilensky/><ref name="Kandel Appendix B"/> Damage to the vestibulocochlear nerve can also present as repetitive and involuntary eye movements ([[nystagmus]]), particularly when the eye is moving horizontally.<ref name="Kandel Appendix B" /> Damage to the cochlear nerve will cause partial or complete [[deafness]] in the affected ear.<ref name="Kandel Appendix B"/>

===Oral sensation, taste, and salivation (IX)===
[[File:Lawrence 1960 16.9.png|thumb|164x164px|A damaged glossopharyngeal nerve (IX) may cause the uvula to deviate to the affected side.|alt=]]
The [[glossopharyngeal nerve]] (IX) supplies the [[stylopharyngeus muscle]] and provides sensation to the [[oropharynx]] and back of the tongue.<ref name = Vilensky/> The glossopharyngeal nerve also provides parasympathetic input to the [[parotid gland]].<ref name = Vilensky />

Damage to the nerve may cause failure of the [[gag reflex]]; a failure may also be seen in damage to the vagus nerve (X).<ref name=TC2018 />

===Vagus nerve (X)===
The [[vagus nerve]] (X) provides sensory and parasympathetic supply to structures in the neck and also to most of the organs in the chest and abdomen.<ref name="Kandel" />

Loss of function of the vagus nerve (X) will lead to a loss of parasympathetic supply to a very large number of structures. Major effects of damage to the vagus nerve may include a rise in blood pressure and heart rate. Isolated dysfunction of only the vagus nerve is rare, but – if the lesion is located above the point at which the vagus first branches off – can be indicated by a hoarse voice, due to dysfunction of one of its branches, the [[recurrent laryngeal nerve]].<ref name="Moore's"/>

Damage to this nerve may result in difficulties swallowing.<ref name="Kandel Appendix B"/>

===Shoulder elevation and head-turning (XI)===
{{multiple image
| image1            = Lawrence 1960 14.14.png
| caption1          = The accessory nerve (XI) supplies the sternocleidomastoid and trapezius muscles. Damage to the nerve may cause a [[winged scapula]], shown here.
| image2            = Unilateral hypoglossal nerve injury.jpeg
| caption2          = The hypoglossal nerve (XII) supplies the muscles of the tongue. A damaged hypoglossal nerve will result in an inability to stick the tongue out straight; here seen in an injury resulting from [[branchial cyst]] surgery.<ref>{{Cite journal|title = A case with unilateral hypoglossal nerve injury in branchial cyst surgery|journal = Journal of Brachial Plexus and Peripheral Nerve Injury|date = 2014-01-01|pmc = 3395866|pmid = 22296879|volume = 7|issue = 1|doi = 10.1186/1749-7221-7-2|first1 = Sudipta|last1 = Mukherjee|first2 = Chandra|last2 = Gowshami|first3 = Abdus|last3 = Salam|first4 = Ruhul|last4 = Kuddus|first5 = Mohshin|last5 = Farazi|first6 = Jahid|last6 = Baksh|page=2 | doi-access=free }}</ref>
| align             = right
| direction         = horizontal
| height1           = 200
}}

The [[accessory nerve]] (XI) supplies the [[sternocleidomastoid muscle|sternocleidomastoid]] and [[trapezius muscle]]s.<ref name=TC2018 />

Damage to the accessory nerve (XI) will lead to weakness in the trapezius muscle on the same side as the damage. The trapezius lifts the shoulder when [[shrug]]ging, so the affected shoulder will not be able to shrug and the shoulder blade ([[scapula]]) will protrude into a [[winged scapula|winged]] position.<ref name = Vilensky /> Depending on the location of the lesion there may also be weakness present in the sternocleidomastoid muscle, which acts to turn the head so that the face points to the opposite side.<ref name=TC2018 />

===Tongue movement (XII)===
The [[hypoglossal nerve]] (XII) supplies the intrinsic muscles of the tongue, controlling tongue movement.<ref name=TC2018 /> The hypoglossal nerve (XII) is unique in that it is supplied by the [[motor cortex|motor cortices]] of both hemispheres of the brain.<ref name="Kandel Appendix B"/>

Damage to the nerve may lead to fasciculations or wasting ([[atrophy]]) of the muscles of the tongue. This will lead to weakness of tongue movement on that side. When damaged and extended, the tongue will move towards the weaker or damaged side, as shown in the image.<ref name="Kandel Appendix B"/> The fasciculations of the tongue are sometimes said to look like a "bag of worms". Damage to the nerve tract or nucleus will not lead to atrophy or fasciculations, but only weakness of the muscles on the same side as the damage.<ref name="Kandel Appendix B"/>

==Clinical significance==

<!-- This section refers only to conditions which damage the nerves themselves, not symptoms of damage (see 'Function')-->

===Examination===
{{Main|Cranial nerve examination}}
Doctors, [[neurologist]]s and other medical professionals may conduct a [[cranial nerve examination]] as part of a [[neurological examination]] to examine the cranial nerves. This is a highly formalised series of steps involving specific tests for each nerve.<ref name=TC2018 >{{cite book |last1=Talley |first1=Nicholas J. |last2=O'Connor |first2=Simon |title=Clinical examination|year=2018 |publisher=Elsevier |location=Chatswood |isbn=978-0-7295-4286-9 |edition=8th|chapter=Chapter 32. The neurological examination: general signs and the cranial nerves|pages=500–539}}</ref> Dysfunction of a nerve identified during testing may point to a problem with the nerve or of a part of the brain.<ref name=TC2018 />

A cranial nerve exam starts with observation of the patient, as some cranial nerve lesions may affect the symmetry of the eyes or face.<ref name=TC2018 /> Vision may be tested by examining the [[visual field]]s, or by examining the [[retina]] with an [[ophthalmoscope]], using a process known as [[funduscopy]]. Visual field testing may be used to pin-point structural lesions in the optic nerve, or further along the visual pathways.<ref name="Kandel Appendix B" /> Eye movement is tested and abnormalities such as [[nystagmus]] are observed for. The sensation of the face is tested, and patients are asked to perform different facial movements, such as puffing out of the cheeks. Hearing is checked by voice and [[tuning fork]]s. The patient's [[uvula]] is examined. After performing a shrug and head turn, the patient's tongue function is assessed by various tongue movements.<ref name=TC2018 />

Smell is not routinely tested, but if there is suspicion of a change in the sense of smell, each nostril is tested with substances of known odors such as coffee or soap.  Intensely smelling substances, for example [[ammonia]], may lead to the activation of [[Nociceptor|pain receptors]] of the trigeminal nerve (V) located in the nasal cavity and this can confound olfactory testing.<ref name="TC2018" /><ref name="Kandel Appendix B" />

===Damage===

====Compression====
Nerves may be compressed because of increased [[intracranial pressure]], a [[Mass effect (medicine)|mass effect]] of an [[intracerebral haemorrhage]], or tumour that presses against the nerves and interferes with the transmission of impulses along the nerve.<ref name=DAVIDSONS2010C>{{cite book |editor=Nicki R. Colledge |editor2=Brian R. Walker |editor3=Stuart H. Ralston |title=Davidson's principles and practice of medicine.|year=2010|publisher=Churchill Livingstone/Elsevier|location=Edinburgh|isbn=978-0-7020-3085-7|pages=787, 1215–1217|edition=21st}}</ref>  Loss of function of a cranial nerve may sometimes be the first symptom of an [[cranium|intracranial]] or [[skull base]] cancer.<ref name="Robbins">{{cite book|last=Kumar ()|first=Vinay|title=Robbins and Cotran pathologic basis of disease|year=2010|publisher=Saunders/Elsevier|location=Philadelphia, PA|isbn=978-1-4160-3121-5|page=1266|edition=8th|display-authors=etal }}</ref>

An increase in intracranial pressure may lead to impairment of the optic nerves (II) due to compression of the surrounding veins and capillaries, causing swelling of the eyeball ([[papilloedema]]).<ref>{{cite book |editor=Nicki R. Colledge |editor2=Brian R. Walker |editor3=Stuart H. Ralston |title=Davidson's principles and practice of medicine|year=2010|publisher=Churchill Livingstone/Elsevier|location=Edinburgh|isbn=978-0-7020-3085-7|page=1166|edition=21st}}</ref> A cancer, such as an [[optic nerve glioma]], may also impact the optic nerve (II). A [[pituitary tumour]] may compress the optic tracts or the [[optic chiasm]] of the optic nerve (II), leading to visual field loss. A pituitary tumour may also extend into the cavernous sinus, compressing the oculomotor nerve (III), trochlear nerve (IV) and abducens nerve (VI), leading to double-vision and [[strabismus]]. These nerves may also be affected by herniation of the [[temporal lobe]]s of the brain through the [[falx cerebri]].<ref name=DAVIDSONS2010C />

The cause of [[trigeminal neuralgia]], in which one side of the face is exquisitely painful, is thought to be compression of the nerve by an artery as the nerve emerges from the brain stem.<ref name=DAVIDSONS2010C/> An [[acoustic neuroma]], particularly at the junction between the pons and medulla, may compress the facial nerve (VII) and vestibulocochlear nerve (VIII), leading to hearing and sensory loss on the affected side.<ref name=DAVIDSONS2010C /><ref name=HARRISONS2008CN />

====Stroke====
Occlusion of blood vessels that supply the nerves or their nuclei, an [[ischemia|ischemic]] [[stroke]], may cause specific signs and symptoms relating to the damaged area. If there is a stroke of the [[midbrain]], [[pons]] or [[Medulla oblongata|medulla]], various cranial nerves may be damaged, resulting in dysfunction and symptoms of [[:Template:Lesions of spinal cord and brain|a number of different syndromes]].<ref>{{cite book|editor1=Anthony S. Fauci |editor2=T. R. Harrison |display-editors=etal|title=Harrison's principles of internal medicine|url=https://archive.org/details/harrisonsprincip00asfa |url-access=limited |year=2008|publisher=McGraw-Hill Medical|location=New York [etc.]|isbn=978-0-07-147693-5|pages=[https://archive.org/details/harrisonsprincip00asfa/page/n2564 2526]–2531|edition=17th}}</ref> [[Thrombosis]], such as a [[cavernous sinus thrombosis]], refers to a clot ([[thrombus]]) affecting the venous drainage from the [[cavernous sinus]], affects the optic (II), oculomotor (III), trochlear (IV), ophthalmic branch of the trigeminal nerve (V1) and the abducens nerve (VI).<ref name=HARRISONS2008CN>{{cite book|editor1=Anthony S. Fauci |editor2=T. R. Harrison |display-editors=etal |title=Harrison's principles of internal medicine|url=https://archive.org/details/harrisonsprincip00asfa |url-access=limited |year=2008|publisher=McGraw-Hill Medical|location=New York [etc.]|isbn=978-0-07-147693-5|pages=[https://archive.org/details/harrisonsprincip00asfa/page/n2621 2583]–2587|edition=17th}}</ref>

====Inflammation====
Inflammation of a cranial nerve can occur as a result of infection, such as viral causes like reactivated [[herpes simplex virus]], or can occur spontaneously. Inflammation of the facial nerve (VII) may result in [[Bell's palsy]].<ref name=DAVIDSONS2010>{{cite book |editor=Nicki R. Colledge |editor2=Brian R. Walker |editor3=Stuart H. Ralston |title=Davidson's principles and practice of medicine|year=2010|publisher=Churchill Livingstone/Elsevier|location=Edinburgh|isbn=978-0-7020-3085-7|pages=1164–1170, 1192–1193|edition=21st}}</ref>

[[Multiple sclerosis]], an inflammatory process resulting in a loss of the [[myelin]] sheathes which surround the cranial nerves, may cause a variety of shifting symptoms affecting multiple cranial nerves. Inflammation may also affect other cranial nerves.<ref name=DAVIDSONS2010/> Other rarer inflammatory causes affecting the function of multiple cranial nerves include [[sarcoidosis]], [[miliary tuberculosis]], and [[vasculitis|inflammation of arteries]], such as [[granulomatosis with polyangiitis]].<ref name=HARRISONS2008CN />

====Other====
Trauma to the skull, disease of bone, such as [[Paget's disease of bone|Paget's disease]], and injury to nerves during surgery are other causes of nerve damage.<ref name=HARRISONS2008CN />

==History==
The Graeco-Roman anatomist [[Galen]] (AD 129–210) named seven pairs of cranial nerves.<ref name=Davis2014>{{Cite journal|last1=Davis|first1=Matthew C.|last2=Griessenauer|first2=Christoph J.|last3=Bosmia|first3=Anand N.|last4=Tubbs|first4=R. Shane|last5=Shoja|first5=Mohammadali M.|year=2014|title=The naming of the cranial nerves: A historical review|journal=Clinical Anatomy|language=en|volume=27|issue=1|pages=14–19|doi=10.1002/ca.22345|pmid=24323823|s2cid=15242391|issn=1098-2353}}</ref> Much later, in 1664, English anatomist Sir [[Thomas Willis]] suggested that there were actually 9 pairs of nerves.  Finally, in 1778, German anatomist [[Samuel Thomas von Sömmerring|Samuel Soemmering]] named the 12 pairs of nerves that are generally accepted today.<ref name=Davis2014 /> However, because many of the nerves emerge from the brain stem as rootlets, there is continual debate as to how many nerves there actually are, and how they should be grouped.<ref name=Davis2014 /> For example, there is reason to consider both the olfactory (I) and optic (II) nerves to be brain tracts, rather than cranial nerves.<ref name=Davis2014 />

==Other animals==
{{multiple image
| align = right
| direction = vertical
| width = 200
| image1 = Lawrence 1960 1.3 A.png
| width1 = 200
| alt1 = 
| image2 = Lawrence 1960 1.4 A.png
| width2 = 200
| alt2 = 
| footer = [[Dogfish sharks|Dog-fish]] brain in two projections. <br />top; ''ventral'' bottom; ''lateral''<br />The accessory nerve (XI) and hypoglossal nerve (XII) cannot be seen, as they are not always present in all vertebrates.
| footer_align = center
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| caption1 = 
| caption2 = 
}}
Cranial nerves are also present in other [[vertebrate]]s. Other [[amniote]]s (non-[[amphibian]] [[tetrapod]]s) have cranial nerves similar to those of humans.  In [[anamniote]]s (fishes and amphibians), the accessory nerve (XI) and hypoglossal nerve (XII) do not exist, with the accessory nerve (XI) being an integral part of the vagus nerve (X); the hypoglossal nerve (XII) is represented by a variable number of spinal nerves emerging from vertebral segments fused into the occiput. These two nerves only became discrete nerves in the ancestors of amniotes.<ref name=Quiring>{{cite book|last=Quiring|first=Daniel Paul|title=Functional anatomy of the vertebrates.|series=McGraw-Hill publications in the zoological sciences |year=1950|publisher=McGraw-Hill|location=New York|page=249|url=http://babel.hathitrust.org/cgi/pt?id=mdp.39015001909962;view=1up;seq=265}}</ref> The very small [[terminal nerve]] (nerve N or O) exists in humans but may not be functional. In other animals, it appears to be important to sexual receptivity based on perceptions of [[pheromones]].<ref name = Vilensky/><ref>{{cite journal | pmid = 22836597 | doi=10.1002/ca.22130 | volume=27 | issue=1 | title=The neglected cranial nerve: nervus terminalis (cranial nerve N) | date=January 2014 | journal=Clin Anat | pages=46–53 | last1 = Vilensky | first1 = JA| s2cid=21454488 }}</ref>

<gallery>
File:Quiring 1950 146.png|The cranial nerves in the horse
File:Cranial nerve sheep ventral.png|Ventral view of a [[sheep]]'s brain. The exits of the various cranial nerves are marked with red.
</gallery>

{{clear}}

==See also==
* [[List of anatomy mnemonics#Cranial nerves|Cranial nerve mnemonics]]
* [[Spinal nerve]]
* [[Plexus]]
* [[Nerve plexus]]
:* [[Brachial plexus]]

==References==
{{reflist|30em}}

==External links==
{{Commons}}[https://www.youtube.com/watch?v=CU5Mz18U3jQ Cranial Nerve Exam - by UBC Medicine - Educational Media]{{Nervous system}}
{{Cranial nerves}}

{{Authority control}}

[[Category:Cranial nerves| ]]