Editing Trigeminal nerve (section)

====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>