Editing Trigeminal nerve

{{Short description|Cranial nerve responsible for the face's senses and motor functions}}
{{Distinguish|text=other nerves that innervate the face, such as the [[facial nerve]]}}
{{Redirect|Trigeminal|the artery|Trigeminal artery}}
{{More footnotes|date=October 2014}}
{{Infobox nerve
| Name        = Trigeminal nerve
| Latin       = nervus trigeminus
| Image       = Grant 1962 654.png
| Caption     = Schematic illustration of the trigeminal nerve and the organs (or structures) it supplies
| Width       = 300
| Image2      = Brain human normal inferior view with labels en.svg
| Caption2    = [[Anatomical terms of location#Superior and inferior|Inferior view]] of the [[human brain]], with cranial nerves labelled
| Innervates  = ''Motor:'' [[Muscles of mastication]], [[tensor tympani]], [[tensor veli palatini]], [[mylohyoid muscle|mylohyoid]], [[Digastric muscle#Anterior belly|anterior belly of the digastric]]<br/>''Sensory:'' [[Face]], [[mouth]], [[temporomandibular joint]]
| BranchFrom  =
| BranchTo    = [[Ophthalmic nerve]]<br/>[[Maxillary nerve]]<br/>[[Mandibular nerve]]
}}

{{Cranial nerves short}}

In [[neuroanatomy]], the '''trigeminal nerve''' ([[literal translation|lit.]] ''triplet'' nerve), also known as the '''fifth cranial nerve''', '''cranial nerve V''', or simply '''CN V''',<!--Each CN article, CN 0 plus CN I to CN XII, should not fail to have the standard, established synonyms at outset, nor be styled substantially differently from the others; do not change this aspect of any particular one of them without first discussing the same change for all of them.--> is a [[cranial nerve]] responsible for [[Sense|sensation]] in the [[face]] and [[motor functions]] such as [[biting]] and [[chewing]]; it is the most complex of the [[cranial nerves]]. Its name (''trigeminal'', {{ety|la|tri-|three||-geminus|twin}}<ref>[[American Heritage Dictionary]], 1969.</ref>) derives from each of the two nerves (one on each side of the [[pons]]) having three major branches: the [[ophthalmic nerve]] (V{{sub|1}}), the [[maxillary nerve]] (V{{sub|2}}), and the [[mandibular nerve]] (V{{sub|3}}). The ophthalmic and maxillary nerves are purely sensory, whereas the mandibular nerve supplies motor as well as sensory (or "[[cutaneous]]") functions.<ref>{{cite web | vauthors = Pazhaniappan N |title=The Trigeminal Nerve (CN V) |url=https://teachmeanatomy.info/head/cranial-nerves/trigeminal-nerve/ |publisher=TeachMeAnatomy |access-date=5 April 2021 |date=15 August 2020}}</ref> Adding to the complexity of this nerve is that [[Autonomic nervous system|autonomic nerve fibers]] as well as special [[sensory fibers]] ([[taste]]) are contained within it.

The motor division of the trigeminal nerve derives from the [[Basal plate (neural tube)|basal plate]] of the [[embryo]]nic [[pons]], and the sensory division originates in the [[cranial neural crest]]. Sensory information from the face and body is processed by parallel pathways in the [[central nervous system]].

== Structure ==

=== Origin ===
From the trigeminal ganglion, a single, large sensory root  enters the [[brainstem]] at the level of the [[pons]]. Immediately adjacent to the sensory root, a smaller motor root emerges from the pons<ref name="pmid29354310">{{cite journal |vauthors=Kontzialis M, Kocak M |date=2017 |title=Imaging evaluation of trigeminal neuralgia |journal=Journal of Istanbul University Faculty of Dentistry |volume=51 |issue=3 Suppl 1 |pages=S62–S68 |doi=10.17096/jiufd.27242 |pmc=5750829 |pmid=29354310}}</ref> slightly rostrally and medially to the sensory root.'''<ref name=":022">{{Cite book |last=Sinnatamby |first=Chummy S. |title=Last's Anatomy |publisher= Elsevier Australia|year=2011 |isbn=978-0-7295-3752-0 |edition=12th |pages=478}}</ref>'''

Motor fibers pass through the trigeminal ganglion without synapsing on their way to peripheral muscles, their cell bodies being located in the nucleus of the fifth nerve, deep within the pons.

=== Trigeminal ganglion ===
The three major branches of the trigeminal nerve—the [[ophthalmic nerve]] (V<sub>1</sub>), the [[maxillary nerve]] (V<sub>2</sub>) and the [[mandibular nerve]] (V<sub>3</sub>)—converge on the [[trigeminal ganglion]] (also called the semilunar ganglion or gasserian ganglion), located within [[Meckel's cave]] and containing the cell bodies of incoming sensory-nerve fibers. The trigeminal ganglion is analogous to the [[dorsal root]] ganglia of the spinal cord, which contain the cell bodies of incoming sensory fibers from the rest of the body.[[File:Gray784.png|thumb|alt=Drawing of the head, with areas served by specific nerves color-coded|[[Dermatome (anatomy)|Dermatome]] distribution of the trigeminal nerve]]

===Sensory branches===
[[File:Trig innervation.svg|thumb|left|alt=Profile of the head, with the three sub-nerves color-coded|Dermatome distribution of the trigeminal nerve]]
The ophthalmic, maxillary and mandibular branches leave the skull through three separate [[Foramina of the skull|foramina]]: the [[superior orbital fissure]], the [[foramen rotundum]] and the [[foramen ovale (skull)|foramen ovale]], respectively. The ophthalmic nerve (V<sub>1</sub>) carries sensory information from the scalp and forehead, the upper eyelid, the conjunctiva and cornea of the eye, the nose (including the tip of the nose, except alae nasi), the nasal mucosa, the frontal sinuses and parts of the [[meninges]] (the [[Dura mater|dura]] and blood vessels). The maxillary nerve (V<sub>2</sub>) carries sensory information from the lower eyelid and cheek, the [[nares]] and upper lip, the upper teeth and gums, the nasal mucosa, the palate and roof of the pharynx, the maxillary, ethmoid and sphenoid sinuses and parts of the meninges. The mandibular nerve (V<sub>3</sub>) carries sensory information from the lower lip, the lower teeth and gums, the chin and jaw (except the angle of the jaw, which is supplied by C2-C3), parts of the external ear and parts of the meninges. The mandibular nerve carries touch-position and pain-temperature sensations from the mouth. Although it does not carry taste sensation (the [[chorda tympani]] is responsible for taste), one of its branches—the [[lingual nerve]]—carries sensation from the tongue.

The peripheral processes of mesencephalic nucleus of V neurons run in the motor root of the trigeminal nerve and terminate in the muscle spindles in the muscles of mastication. They are proprioceptive fibers, conveying information regarding the location of the masticatory muscles. The central processes of mesencephalic V neurons synapse in the motor nucleus V.

=== Dermatomes ===
The areas of cutaneous distribution ([[Dermatome (anatomy)|dermatome]]s) of the three sensory branches of the trigeminal nerve have sharp borders with relatively little overlap (unlike dermatomes in the rest of the body, which have considerable overlap). The injection of a [[local anesthetic]], such as [[lidocaine]], results in the complete loss of sensation from well-defined areas of the face and mouth. For example, teeth on one side of the jaw can be numbed by injecting the mandibular nerve. Occasionally, injury or disease processes may affect two (or all three) branches of the trigeminal nerve; in these cases, the involved branches may be termed:
* '''V1/V2 distribution''' – Referring to the ophthalmic and maxillary branches
* '''V2/V3 distribution''' – Referring to the maxillary and mandibular branches
* '''V1-V3 distribution''' – Referring to all three branches

Nerves on the left side of the jaw slightly outnumber the nerves on the right side of the jaw.

== Function ==
The sensory function of the trigeminal nerve is to provide tactile, [[proprioceptive]], and [[Nociception|nociceptive]] afference to the face and mouth. Its motor function activates the [[muscles of mastication]], the [[tensor tympani]], [[tensor veli palatini]], [[mylohyoid muscle|mylohyoid]] and the [[Digastric muscle#Anterior belly|anterior belly of the digastric]].

The trigeminal nerve carries [[general somatic afferent fibers]] (GSA), which innervate the skin of the face via ophthalmic (V1), maxillary (V2) and mandibular (V3) divisions. The trigeminal nerve also carries [[special visceral efferent]] (SVE) [[axon]]s, which innervate the muscles of mastication via the mandibular (V3) division.

===Muscles===
The motor component of the mandibular division (V3) of the trigeminal nerve controls the movement of eight muscles, including the four [[muscles of mastication]]: the [[Masseter muscle|masseter]], the [[temporal muscle]], and the [[Medial pterygoid muscle|medial]] and [[Lateral pterygoid muscle|lateral pterygoids]]. The other four muscles are the [[tensor veli palatini]], the [[Mylohyoid muscle|mylohyoid]], the anterior belly of the [[digastric muscle|digastric]] and the [[tensor tympani]]. 

With the exception of the tensor tympani, all these muscles are involved in biting, chewing and swallowing and all have bilateral [[Cortex (anatomy)|cortical]] representation.  A unilateral central lesion (for example, a [[stroke]]), no matter how large, is unlikely to produce an observable deficit. Injury to a peripheral nerve can cause paralysis of muscles on one side of the jaw, with the jaw deviating towards the paralyzed side when it opens. This direction of the mandible is due to the action of the functioning pterygoids on the opposite side.

===Sensation===
{{Main|Somatosensory system}}
The two basic types of sensation are touch-position and pain-temperature. Touch-position input comes to attention immediately, but pain-temperature input reaches the level of consciousness after a delay; when a person steps on a pin, the awareness of stepping on something is immediate but the pain associated with it is delayed.

Touch-position information is generally carried by myelinated (fast-conducting) nerve fibers, and pain-temperature information by unmyelinated (slow-conducting) fibers. The primary sensory receptors for touch-position ([[Meissner’s corpuscles|Meissner's corpuscles]], [[Merkel's receptors]], [[Pacinian corpuscles]], [[Ruffini’s corpuscles|Ruffini's corpuscles]], [[hair receptors]], [[muscle spindle organs]] and [[Golgi tendon organs]]) are structurally more complex than those for pain-temperature, which are nerve endings.

Sensation in this context refers to the conscious perception of touch-position and pain-temperature information, rather than the [[special senses]] (smell, sight, taste, hearing and balance) processed by different cranial nerves and sent to the cerebral cortex through different pathways. The perception of magnetic fields, electrical fields, low-frequency vibrations and infrared radiation by some nonhuman vertebrates is processed by their equivalent of the fifth cranial nerve.

Touch in this context refers to the perception of detailed, localized tactile information, such as [[two-point discrimination]] (the difference between touching one point and two closely spaced points) or the difference between coarse, medium or fine sandpaper. People without touch-position perception can feel the surface of their bodies and perceive touch in a broad sense, but they lack perceptual detail.

Position, in this context, refers to conscious [[proprioception]]. Proprioceptors (muscle spindle and Golgi tendon organs) provide information about joint position and muscle movement. Although much of this information is processed at an unconscious level (primarily by the [[cerebellum]] and the [[Vestibular system|vestibular]] nuclei), some is available at a conscious level.

Touch-position and pain-temperature sensations are processed by different pathways in the central nervous system. This hard-wired distinction is maintained up to the cerebral cortex. Within the cerebral cortex, sensations are linked with other cortical areas.

===Sensory pathways===
Sensory pathways from the periphery to the cortex are separate for touch-position and pain-temperature sensations. All sensory information is sent to specific nuclei in the [[thalamus]]. Thalamic nuclei, in turn, send information to specific areas in the [[cerebral cortex]]. Each pathway consists of three bundles of nerve fibers connected in series:

[[File:Sensory Pathways III.png|center|alt=Flow chart from sensory receptors to the cerebral cortex]]
The secondary neurons in each pathway [[decussate]] (cross the spinal cord or brainstem), because the spinal cord develops in segments. Decussated fibers later reach and connect these segments with the higher centers. The [[optic chiasm]] is the primary cause of decussation; nasal fibers of the optic nerve cross (so each cerebral hemisphere receives contralateral—opposite—vision) to keep the [[interneuron]]al connections responsible for processing information short. All sensory and motor pathways converge and diverge to the contralateral hemisphere.<ref>Excerpt from [https://archive.org/details/cunninghamstextb00cunn ''Cunningham's Textbook of Anatomy'']</ref>

Although sensory pathways are often depicted as chains of individual neurons connected in series, this is an oversimplification. Sensory information is processed and modified at each level in the chain by interneurons and input from other areas of the nervous system. For example, cells in the main trigeminal nucleus (Main V in the diagram below) receive input from the [[reticular formation]] and cerebellar cortex. This information contributes to the final output of the cells in Main V to the thalamus.

[[File:Touch Pain Pathways.png|thumb|600px|center|alt=Text-and-line diagram of sensory-nerve pathways|C = Cervical segment, S = Sacral segment, VPL = [[Ventral posterolateral nucleus]], SI = [[Primary somatosensory cortex]], VM = [[Ventromedial prefrontal cortex]], MD = [[Medial dorsal thalamic nucleus]], IL = [[Intralaminar nucleus]], VPM = [[Ventral posteromedial nucleus]], Main V = [[Main trigeminal nucleus]], Spinal V = [[Spinal trigeminal nucleus]]]]

Touch-position information from the body is carried to the thalamus by the [[medial lemniscus]], and from the face by the [[trigeminal lemniscus]] (both the anterior and posterior trigeminothalamic tracts). Pain-temperature information from the body is carried to the thalamus by the [[spinothalamic tract]], and from the face by the anterior division of the [[trigeminal lemniscus]] (also called the [[anterior trigeminothalamic tract]]).

Pathways for touch-position and pain-temperature sensations from the face and body merge in the brainstem, and touch-position and pain-temperature sensory maps of the entire body are projected onto the thalamus. From the thalamus, touch-position and pain-temperature information is projected onto the cerebral cortex.

===Summary===
The complex processing of pain-temperature information in the thalamus and cerebral cortex (as opposed to the relatively simple, straightforward processing of touch-position information) reflects a [[Phylogenetics|phylogenetically]] older, more primitive sensory system. The detailed information received from peripheral touch-position receptors is superimposed on a background of awareness, memory and emotions partially set by peripheral pain-temperature receptors.

Although thresholds for touch-position perception are relatively easy to measure, those for pain-temperature perception are difficult to define and measure. "Touch" is an objective sensation, but "pain" is an individualized sensation which varies among different people and is conditioned by memory and emotion. Anatomical differences between the pathways for touch-position perception and pain-temperature sensation help explain why pain, especially chronic pain, is difficult to manage.

==Trigeminal nuclei==
[[File:Gray696 Trigeminal.svg|thumb|left|alt=Diagram of the brainstem|Brainstem nuclei: Red = Motor; Blue = Sensory; Dark blue = Trigeminal nucleus]]
All sensory information from the face, both touch-position and pain-temperature, is sent to the [[trigeminal nucleus]]. In classical anatomy most sensory information from the face is carried by the fifth nerve, but sensation from parts of the mouth, parts of the ear and parts of the [[meninges]] is carried by [[general somatic afferent fibers]] in cranial nerves VII (the [[facial nerve]]), IX (the [[glossopharyngeal nerve]]) and X (the [[vagus nerve]]).

All sensory fibers from these nerves terminate in the trigeminal nucleus. On entering the brainstem, sensory fibers from V, VII, IX and X are sorted and sent to the trigeminal nucleus (which contains a sensory map of the face and mouth). The spinal counterparts of the trigeminal nucleus (cells in the [[posterior horn of spinal cord|dorsal horn]] and [[dorsal column]] nuclei of the spinal cord) contain a sensory map of the rest of the body.

The trigeminal nucleus extends throughout the brainstem, from the midbrain to the medulla, continuing into the cervical cord (where it merges with the dorsal horn cells of the spinal cord). The nucleus is divided into three parts, visible in microscopic sections of the brainstem. From caudal to rostral (ascending from the medulla to the midbrain), they are the [[spinal trigeminal nucleus|spinal trigeminal]], the [[principal sensory nucleus of trigeminal nerve|principal sensory]] and the [[mesencephalic nucleus of trigeminal nerve|mesencephalic]] nuclei. The parts of the trigeminal nucleus receive different types of sensory information; the spinal trigeminal nucleus receives pain-temperature fibers, the principal sensory nucleus receives touch-position fibers and the mesencephalic nucleus receives proprioceptor and mechanoreceptor fibers from the jaws and teeth.

===Spinal trigeminal nucleus===
The [[spinal trigeminal nucleus]] represents pain-temperature sensation from the face. Pain-temperature fibers from peripheral [[nociceptors]] are carried in cranial nerves V, VII, IX and X. On entering the brainstem, sensory fibers are grouped and sent to the spinal trigeminal nucleus. This bundle of incoming fibers can be identified in cross-sections of the pons and medulla as the spinal tract of the trigeminal nucleus, which parallels the spinal trigeminal nucleus. The spinal tract of V is analogous to, and continuous with, [[Lissauer's tract]] in the spinal cord.

The spinal trigeminal nucleus contains a pain-temperature sensory map of the face and mouth. From the spinal trigeminal nucleus, secondary fibers cross the midline and ascend in the trigeminothalamic (quintothalamic) tract to the contralateral thalamus. Pain-temperature fibers are sent to multiple thalamic nuclei. The central processing of pain-temperature information differs from the processing of touch-position information.

====Somatotopic representation====
[[File:Onionskinddistribution.svg|thumb|alt=The head in profile, with trigeminal-nerve distribution illustrated|Onion-skin distribution of the trigeminal nerve]]
Exactly how pain-temperature fibers from the face are distributed to the spinal trigeminal nucleus is disputed. The present general understanding is that pain-temperature information from all areas of the human body is represented in the spinal cord and brainstem in an ascending, [[Anatomical terms of location#Anterior and posterior|caudal-to-rostral]] fashion. Information from the lower extremities is represented in the lumbar cord, and that from the upper extremities in the thoracic cord. Information from the neck and the back of the head is represented in the cervical cord, and that from the face and mouth in the spinal trigeminal nucleus.

Within the spinal trigeminal nucleus, information is represented in a layered, or "onion-skin" fashion. The lowest levels of the nucleus (in the upper cervical cord and lower medulla) represent peripheral areas of the face (the scalp, ears and chin). Higher levels (in the upper medulla) represent central areas (nose, cheeks and lips). The highest levels (in the pons) represent the mouth, teeth and pharyngeal cavity. 

The onion skin distribution differs from the dermatome distribution of the peripheral branches of the fifth nerve. Lesions which destroy lower areas of the spinal trigeminal nucleus (but spare higher areas) preserve pain-temperature sensation in the nose (V<sub>1</sub>), upper lip (V<sub>2</sub>) and mouth (V<sub>3</sub>) and remove pain-temperature sensation from the forehead (V<sub>1</sub>), cheeks (V<sub>2</sub>) and chin (V<sub>3</sub>). Although analgesia in this distribution is "nonphysiologic" in the traditional sense (because it crosses several dermatomes), this analgesia is found in humans after surgical sectioning of the spinal tract of the trigeminal nucleus.

The spinal trigeminal nucleus sends pain-temperature information to the [[thalamus]] and sends information to the [[mesencephalon]] and the [[reticular formation]] of the brainstem. The latter pathways are analogous to the spinomesencephalic and spinoreticular tracts of the spinal cord, which send pain-temperature information from the rest of the body to the same areas. The mesencephalon modulates painful input before it reaches the level of consciousness. The reticular formation is responsible for the automatic (unconscious) orientation of the body to painful stimuli.  Incidentally, [[Sulfur]]-containing compounds found in plants in the [[onion]] family stimulate receptors found in trigeminal ganglia, bypassing the [[olfactory system]].<ref>{{cite journal | vauthors = Lübbert M, Kyereme J, Schöbel N, Beltrán L, Wetzel CH, Hatt H | title = Transient receptor potential channels encode volatile chemicals sensed by rat trigeminal ganglion neurons | journal = PLOS ONE | volume = 8 | issue = 10 | pages = e77998 | date = October 21, 2013 | pmid = 24205061 | pmc = 3804614 | doi = 10.1371/journal.pone.0077998 | doi-access = free | bibcode = 2013PLoSO...877998L }}</ref>

==={{anchor|Main trigeminal nucleus}}Principal nucleus===
The [[principal sensory nucleus of trigeminal nerve|principal nucleus]] represents touch-pressure sensation from the face. It is located in the pons, near the entrance for the fifth nerve. Fibers carrying touch-position information from the face and mouth via cranial nerves V, VII, IX, and X are sent to this nucleus when they enter the brainstem.

The principal nucleus contains a touch-position sensory map of the face and mouth, just as the spinal trigeminal nucleus contains a complete pain-temperature map. This nucleus is analogous to the [[dorsal column]] nuclei (the [[Gracile nucleus|gracile]] and [[cuneate nucleus|cuneate]] nuclei) of the spinal cord, which contain a touch-position map of the rest of the body.

From the principal nucleus, secondary fibers cross the midline and ascend in the [[Anterior trigeminothalamic tract|ventral trigeminothalamic tract]] to the contralateral [[thalamus]]. The ventral trigeminothalamic tract runs parallel to the [[medial lemniscus]], which carries touch-position information from the rest of the body to the thalamus.

Some sensory information from the teeth and jaws is sent from the principal nucleus to the [[Anatomical terms of location#Other directional terms|ipsilateral]] thalamus via the small [[dorsal trigeminal tract]]. Touch-position information from the teeth and jaws of one side of the face is represented bilaterally in the thalamus and cortex.

==={{anchor|Mesencephalic trigeminal nucleus}}Mesencephalic nucleus===
The [[Mesencephalic nucleus of trigeminal nerve|mesencephalic nucleus]] is not a true [[Nucleus (neuroanatomy)|nucleus]]; it is a sensory [[ganglion]] (like the [[trigeminal ganglion]]) embedded in the brainstem{{Citation needed|date=January 2012}} and the sole exception to the rule that sensory information passes through peripheral sensory ganglia before entering the central nervous system. It has been found in all [[vertebrates]] except [[lampreys]] and [[hagfishes]]. They are the only vertebrates without jaws and have specific cells in their brainstems. These "internal ganglion" cells were discovered in the late 19th century by medical student [[Sigmund Freud]].

Two types of sensory fibers have cell bodies in the mesencephalic nucleus: [[proprioception|proprioceptor]] fibers from the jaw and mechanoreceptor fibers from the teeth. Some of these incoming fibers go to the motor nucleus of the trigeminal nerve (V), bypassing the pathways for conscious perception. The [[jaw jerk reflex]] is an example; tapping the jaw elicits a reflex closure of the jaw in the same way that tapping the knee elicits a reflex kick of the lower leg. Other incoming fibers from the teeth and jaws go to the main nucleus of V. This information is projected bilaterally to the thalamus and available for conscious perception.

Activities such as biting, chewing and swallowing require symmetrical, simultaneous coordination of both sides of the body. They are automatic activities, requiring little conscious attention and involving a sensory component (feedback about touch-position) processed at the unconscious level in the mesencephalic nucleus.

==={{anchor|Pathways to the thalamus and the cortex}}Pathways to the thalamus and cortex===
Sensation has been defined as the conscious perception of touch-position and pain-temperature information. With the exception of smell, all sensory input (touch-position, pain-temperature, sight, taste, hearing and balance) is sent to the thalamus and then the cortex. The thalamus is anatomically subdivided into nuclei.

==={{anchor|Touch/position sensation}}Touch-position sensation===
[[File:Sensory Homunculus.png|thumb|right|250px|alt=Diagram of functions controlled by the cerebral cortex|Cortical homunculus]]
Touch-position information from the body is sent to the [[ventral posterolateral nucleus]] (VPL) of the thalamus. Touch-position information from the face is sent to the [[ventral posteromedial nucleus]] (VPM) of the thalamus. From the VPL and VPM, information is projected to the [[Postcentral gyrus|primary somatosensory cortex]] (SI) in the [[parietal lobe]].

The representation of sensory information in the postcentral gyrus is organized [[Somatotopic arrangement|somatotopically]]. Adjacent areas of the body are represented by adjacent areas in the cortex. When body parts are drawn in proportion to the density of their innervation, the result is a "little man": the [[cortical homunculus]].

Many textbooks have reproduced the outdated [[Wilder Penfield|Penfield]]-Rasmussen diagram [ref?], with the toes and genitals on the [[Dental terminology#Mesial|mesial]] surface of the cortex when they are actually represented on the convexity.<ref name="pmid15976087">{{cite journal | vauthors = Kell CA, von Kriegstein K, Rösler A, Kleinschmidt A, Laufs H | title = The sensory cortical representation of the human penis: revisiting somatotopy in the male homunculus | journal = The Journal of Neuroscience | volume = 25 | issue = 25 | pages = 5984–5987 | date = June 2005 | pmid = 15976087 | pmc = 6724806 | doi = 10.1523/JNEUROSCI.0712-05.2005 | doi-access = free }}</ref> The classic diagram implies a single primary sensory map of the body, when there are multiple primary maps. At least four separate, anatomically distinct sensory homunculi have been identified in the postcentral gyrus. They represent combinations of input from surface and deep receptors and rapidly  and slowly adapting peripheral receptors; smooth objects will activate certain cells, and rough objects will activate other cells.

Information from all four maps in SI is sent to the secondary sensory cortex (SII) in the parietal lobe. SII contains two more sensory homunculi. Information from one side of the body is generally represented on the opposite side in SI, but on both sides in SII. Functional MRI imaging of a defined stimulus (for example, stroking the skin with a toothbrush) "lights up" a single focus in SI and two foci in SII.

==={{anchor|Pain/temperature sensation}}Pain-temperature sensation===
Pain-temperature information is sent to the VPL (body) and VPM (face) of the thalamus (the same nuclei which receive touch-position information). From the thalamus, pain-temperature and touch-position information is projected onto SI.

Unlike touch-position information, however, pain-temperature information is also sent to other thalamic nuclei and projected onto additional areas of the cerebral cortex. Some pain-temperature fibers are sent to the [[medial dorsal thalamic nucleus]] (MD), which projects to the anterior [[cingulate cortex]]. Other fibers are sent to the ventromedial (VM) nucleus of the thalamus, which projects to the [[insular cortex]]. Finally, some fibers are sent to the [[intralaminar nucleus]] (IL) of the thalamus via the [[reticular formation]]. The IL projects diffusely to all parts of the cerebral cortex.

The insular and cingulate cortices are parts of the brain which represent touch-position and pain-temperature in the context of other simultaneous perceptions (sight, smell, taste, hearing and balance) in the context of memory and emotional state. Peripheral pain-temperature information is channeled directly to the brain at a deep level, without prior processing. Touch-position information is handled differently. Diffuse thalamic projections from the IL and other thalamic nuclei are responsible for a given level of consciousness, with the thalamus and reticular formation "activating" the brain; peripheral pain-temperature information also feeds directly into this system.

==Clinical significance==
*[[Trigeminal neuralgia]]
*[[Cluster headache]]
*[[Migraine]]
===Lateral medullary syndrome===
{{Main|Lateral medullary syndrome}}
[[Lateral medullary syndrome]] (Wallenberg syndrome) is a clinical demonstration of the anatomy of the trigeminal nerve, summarizing how it processes sensory information. A stroke usually affects only one side of the body; loss of sensation due to a stroke will be lateralized to the right or the left side of the body. The only exceptions to this rule are certain spinal-cord lesions and the medullary syndromes, of which Wallenberg syndrome is the best-known example. In this syndrome, a stroke causes a loss of pain-temperature sensation from one side of the face and the other side of the body.

This is explained by the anatomy of the brainstem. In the medulla, the ascending [[spinothalamic tract]] (which carries pain-temperature information from the opposite side of the body) is adjacent to the ascending spinal tract of the trigeminal nerve (which carries pain-temperature information from the same side of the face). A stroke which cuts off the blood supply to this area (for example, a clot in the posterior inferior cerebellar artery) destroys both tracts simultaneously. The result is a loss of pain-temperature (but not touch-position) sensation in a "checkerboard" pattern (ipsilateral face, contralateral body), facilitating diagnosis.

===Sensory neuronopathy===
[[Sensory neuronopathy]] (also known as sensory ganglionopathy) is a type of peripheral neuropathy in which sensory nerve cell bodies in the [[dorsal root ganglia]], commonly including the [[trigeminal ganglion]] of the trigeminal nerve, are damaged due to a variety of mechanisms leading to sensory symptoms such as [[parasthesias]], [[dysesthesia]]s, or [[hyperalgesia]] in the affected nerve distribution including the distribution of the trigeminal nerve.<ref name="Amato 2020">{{cite journal |last1=Amato |first1=Anthony A. |last2=Ropper |first2=Allan H. |title=Sensory Ganglionopathy |journal=New England Journal of Medicine |date=22 October 2020 |volume=383 |issue=17 |pages=1657–1662 |doi=10.1056/NEJMra2023935|pmid=33085862 }}</ref>

==Additional images==
{{Multiple image
|align=center
| width  = 200
| image1 = Grant 1962 655.png
| alt1   = Anatomical diagram of the upper jaw
| image2 = Grant 1962 656a.png
| alt2   = Nerves of the jaw
| image3 = Grant 1962 656b.png
| alt3   = Nerves to separate upper and lower jaws
| footer = Distribution schemes of the trigeminal nerve
|footer_align=center
}}
{{Gallery
| width = 160|height=160
|align=center
|File:Grant 1962 470.png
| alt1 = See caption
|Diagram of facial sensory nerves (front view)
|Image:Gray778 Trigeminal.png
| alt2 = Facial anatomy from the side
|Trigeminal nerve in yellow
|File:Trigeminal ganglion.jpg
| alt3 = Diagram of the brain
|Trigeminal ganglion
|File:Slide2cuc.JPG
| alt4 = Dissected brain, with parts labeled
|Cerebrum (deep inferior dissection)
}}

== See also ==
{{Anatomy-terms}}
*[[List of mnemonics#Anatomy]]
*[[Trigeminovascular system]]
* [[Alveolar nerve]] ([[Dental nerve]])

== References ==
{{Reflist}}

==Sources==
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* {{cite book | vauthors = Carpenter MB, Sutin J | title = Human Neuroanatomy | edition = 8th | publisher = Williams and Wilkins | date = 1983 }}
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* {{cite book | vauthors = Kandel ER, Schwartz JH, Jessell TM | title = Principles of Neural Science | edition = 4th | publisher = McGraw-Hill | date = 2000 }}
* {{cite book | vauthors = Martin JH | title = Neuroanatomy Text and Atlas | edition = 3rd | publisher = McGraw-Hill | date = 2003 }}
* {{cite book | vauthors = Patten J | title = Neurological Differential Diagnosis | edition = 2nd | publisher = Springer | date = 1996 }}
* {{cite book | vauthors = Ropper AH, Brown RH | title = Adam's and Victor's Principles of Neurology | edition = 8th | publisher = McGraw-Hill | date = 2001 }}
* {{cite book | vauthors = Wilson-Pauwels L, Akesson EJ, Stewart PA | title = Cranial Nerves: Anatomy and Clinical Comments | publisher = B. C. Decker | date = 1998 }}
{{refend}}

== External links ==
{{Commons category}}
* [http://faculty.washington.edu/chudler/pmagnet.html Pigeons Detect Magnetic Fields] An experiment indicating that the trigeminal nerve in ''Columba livia'' may be the mechanism through which "homing pigeons" detect magnetic fields
* {{NormanAnatomy|cranialnerves}} ({{NormanAnatomyFig|V}})
* [https://www.youtube.com/watch?v=iMT1bHbw6y0#t=22s Trigeminal nerve anatomy, part 1] and [https://www.youtube.com/watch?v=8gqEloRF69U#t=14s part 2] on [[YouTube]]
* [http://www.mountsinai.org/patient-care/health-library/diseases-and-conditions/trigeminal-neuralgia Trigeminal neuralgia]
{{Cranial nerves}}
{{Trigeminal nerve}}
{{Authority control}}

[[Category:Trigeminal nerve| ]]
[[Category:Cranial nerves]]
[[Category:Innervation of the face]]
[[Category:Medical mnemonics]]
[[Category:Otorhinolaryngology]]
[[Category:Neurology]]
[[Category:Nervous system]]
[[Category:Nerves of the head and neck]]
[[Category:Human head and neck]]