Editing Trigeminal nerve (section)

==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.