Editing Trochlear nerve (section)

{{Short description|Cranial nerve IV, for eye movements}}
{{Infobox nerve
| Name         = Trochlear nerve
| pronounce    = 
| Latin        = nervus trochlearis
| Image        = Trochlear_nerve.png
| Caption      = The trochlear nerve entering the [[orbit (anatomy)|orbit]], seen from above, supplies the [[superior oblique muscle]]
| Image2  = Brain human normal inferior view with labels en.svg
| Caption2= The trochlear nerve ([[cranial nerve|CN IV]]) seen with other [[cranial nerves]]. It is the only cranial nerve to emerge from behind the brainstem, and curves around it to reach the front
| Innervates   = [[Superior oblique muscle]]
| BranchFrom   =
| BranchTo     =
}}

{{Cranial nerves short}}

The '''trochlear nerve''' ({{IPAc-en|ˈ|t|r|ɒ|k|l|ɪər}}),<ref>{{Cite web|url=https://www.lexico.com/definition/trochlear|archive-url=https://web.archive.org/web/20201112220327/https://www.lexico.com/definition/trochlear|url-status=dead|archive-date=November 12, 2020|title=Trochlear &#124; Definition of Trochlear by Oxford Dictionary on Lexico.com also meaning of Trochlear|website=Lexico Dictionaries &#124; English}}</ref> ([[literal translation|lit.]] ''pulley-like'' nerve) also known as the '''fourth cranial nerve''', '''cranial nerve IV''', or '''CN IV''',<!--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]] that innervates a single muscle - the [[superior oblique muscle]] of the eye (which operates through the [[pulley]]-like [[trochlea of superior oblique|trochlea]]). Unlike most other cranial nerves, the trochlear nerve is exclusively a [[motor nerve]] ([[somatic nervous system|somatic]] efferent nerve). 

The trochlear nerve is unique among the cranial nerves in several respects:
* It is the ''smallest'' nerve in terms of the number of axons it contains. 
* It has the greatest intracranial length. 
* It is the only cranial nerve that exits from the dorsal (rear) aspect of the [[brainstem]].
* It innervates a muscle, the superior oblique muscle, on the opposite side (contralateral) from its nucleus. The trochlear nerve [[Decussation|decussates]] within the brainstem before emerging on the contralateral side of the brainstem (at the level of the [[inferior colliculus]]). An injury to the trochlear nucleus in the brainstem will result in an contralateral [[superior oblique]] muscle palsy, whereas an injury to the trochlear nerve (after it has emerged from the brainstem) results in an ipsilateral [[superior oblique]] muscle palsy.

The superior oblique muscle which the trochlear nerve innervates ends in a tendon that passes through a fibrous loop, the '''trochlea''', located anteriorly on the medial aspect of the orbit.  ''Trochlea'' means “pulley” in Latin; the fourth nerve is thus also named after this structure. The words ''trochlea'' and ''trochlear'' ({{IPAc-en|ˈ|t|r|ɒ|k|l|i|ə}}, {{IPAc-en|ˈ|t|r|ɒ|k|l|i|ər}}) come from [[Ancient Greek]] {{lang|grc|[[wikt:trochlea|τροχιλέα]]}} ''trokhiléa'', “pulley; block-and-tackle equipment”.