View source for Accessory nerve

{{Short description|Cranial nerve XI, for head and shoulder movements}}
{{Use dmy dates|date=April 2017}}
{{Infobox nerve
| Name        = Accessory nerve
| Latin       = nervus accessorius
| Image       = Brain human normal inferior view with labels en.svg
| Caption     = View of the [[human brain]] from below. The accessory nerve emerges from the [[medulla oblongata|medulla]] of the [[brainstem]], and is visible at the bottom of the image in blue.
| Innervates  = [[Sternocleidomastoid muscle]], [[trapezius muscle]]
| BranchFrom  =
| BranchTo    =
}}
{{Cranial nerves short}}

The '''accessory nerve''', also known as the '''eleventh cranial nerve''', '''cranial nerve XI''', or simply '''CN XI''',<!--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 supplies the [[sternocleidomastoid]] and [[trapezius muscle]]s. It is classified as the eleventh of twelve pairs of cranial nerves because part of it was formerly believed to originate in the brain. The sternocleidomastoid muscle tilts and rotates the head, whereas the trapezius muscle, connecting to the [[scapula]], acts to shrug the shoulder.

Traditional descriptions of the accessory nerve divide it into a spinal part and a cranial part.{{sfn|Gray's Anatomy|2008|p=459}} The cranial component rapidly joins the [[vagus nerve]], and there is ongoing debate about whether the cranial part should be considered part of the accessory nerve proper.<ref name="luc">{{cite web |url=http://www.meddean.luc.edu/lumen/MedED/grossanatomy/h_n/cn/cn1/cn11.htm |title=Spinal Accessory Nerve |access-date=2007-06-17 |work=Structure of the Human body, Loyola University Medical Education Network| archive-url= https://web.archive.org/web/20070616051402/http://www.meddean.luc.edu/lumen/meded/grossanatomy/h_n/cn/cn1/cn11.htm| archive-date= 16 June 2007 | url-status= live}}</ref>{{sfn|Gray's Anatomy|2008|p=459}} Consequently, the term "accessory nerve" usually refers only to nerve supplying the sternocleidomastoid and trapezius muscles, also called the [[Spinal root of accessory nerve|spinal accessory nerve]].<ref name=ANTHONEY1993 />

Strength testing of these muscles can be measured during a [[neurological examination]] to assess function of the spinal accessory nerve. Poor strength or limited movement are suggestive of damage, which can result from a variety of causes. Injury to the spinal accessory nerve is most commonly caused by medical procedures that involve the head and neck.<ref name="london">{{cite journal |vauthors=London J, London NJ, Kay SP |title=Iatrogenic accessory nerve injury |journal=Annals of the Royal College of Surgeons of England |volume=78 |issue=2 |pages=146–50 |year=1996 |pmid=8678450 |pmc=2502542}}</ref> Injury can cause wasting of the [[shoulder]] muscles, [[winged scapula|winging of the scapula]], and weakness of shoulder abduction and [[external rotation]].<ref name=KELLEY2008/>

The accessory nerve is derived from the [[Basal plate (neural tube)|basal plate]] of the [[embryo]]nic spinal segments C1–C6.<ref>{{Cite journal |last=Weglowski |first=M. |last2=Woźniak |first2=W. |last3=Piotrowski |first3=A. |last4=Bruska |first4=M. |last5=Weglowska |first5=J. |last6=Sobański |first6=J. |last7=Grzymisławska |first7=M. |last8=Łupicka |first8=J. |date=2015-05-28 |title=Early development of the facial nerve in human embryos at stages 13–15 |url=http://dx.doi.org/10.5603/fm.2015.0039 |journal=Folia Morphologica |volume=74 |issue=2 |pages=252–257 |doi=10.5603/fm.2015.0039 |issn=1644-3284|doi-access=free }}</ref>

==Structure==
{{See also|Spinal root of accessory nerve|Cranial root of accessory nerve}}
The fibres of the spinal accessory nerve originate solely in [[neuron]]s situated in the upper [[spinal cord]], from where the spinal cord begins at the junction with the [[medulla oblongata]], to the level of about C6.{{sfn|Gray's Anatomy|2008|p=459}}<ref name="ryan">{{cite journal |vauthors=Ryan S, Blyth P, Duggan N, Wild M, Al-Ali S |title=Is the cranial accessory nerve really a portion of the accessory nerve? Anatomy of the cranial nerves in the jugular foramen |journal=Anatomical Science International |volume=82 |issue=1 |pages=1–7 |year=2007 |pmid=17370444 |doi=10.1111/j.1447-073X.2006.00154.x|s2cid=25032538 }}</ref> These fibres join to form rootlets, roots, and finally the spinal accessory nerve itself. The formed nerve enters the skull through the [[foramen magnum]], the large opening at the skull's base.{{sfn|Gray's Anatomy|2008|p=459}} The nerve travels along the inner wall of the skull towards the [[jugular foramen]].{{sfn|Gray's Anatomy|2008|p=459}} Leaving the skull, the nerve travels through the jugular foramen with the [[glossopharyngeal nerve|glossopharyngeal]] and [[vagus nerve]]s.<ref name=TC2014 /> The spinal accessory nerve is notable for being the only cranial nerve to both enter and exit the skull. This is due to it being unique among the cranial nerves in having neurons in the spinal cord.<ref name="Finsterer2015">{{cite journal|last1=Finsterer|first1=Josef|last2=Grisold|first2=Wolfgang|title=Disorders of the lower cranial nerves|journal=Journal of Neurosciences in Rural Practice|date=2015|volume=6|issue=3|pages=377–91|doi=10.4103/0976-3147.158768|pmc=4481793|pmid=26167022 |doi-access=free }}</ref>

After leaving the skull, the cranial component detaches from the spinal component. The spinal accessory nerve continues alone and heads backwards and downwards. In the neck, the accessory nerve crosses the internal jugular vein around the level of the posterior belly of digastric muscle. As it courses downwards, the nerve pierces through the [[sternocleidomastoid muscle]] (approximately 1&nbsp;cm above [[Erb's point (neurology)|Erb's point]]) while sending it [[Motor neuron|motor]] branches, then continues down until it reaches the [[trapezius muscle]] (entering at the junction of the middle and lower third of the anterior border of the [[trapezius]]) to provide motor innervation to its upper part.{{sfn|Gray's Anatomy|2008|p=460}}

===Nucleus===
{{Main|Spinal accessory nucleus}}

The fibres that form the spinal accessory nerve are formed by [[lower motor neuron]]s located in the upper segments of the [[spinal cord]]. This [[nucleus (neuroanatomy)|cluster of neurons]], called the [[spinal accessory nucleus]], is located in the lateral aspect of the anterior horn of the spinal cord, and stretches from where the spinal cord begins (at the junction with the medulla) through to the level of about C6.{{sfn|Gray's Anatomy|2008|p=459}} The lateral horn of high cervical segments appears to be continuous with the [[nucleus ambiguus]] of the [[medulla oblongata]], from which the cranial component of the accessory nerve is derived.<ref name="Finsterer2015" />

=== Variation ===
In the neck, the accessory nerve crosses the internal jugular vein around the level of the posterior belly of [[digastric muscle]], in front of the vein in about 80% of people, and behind it in about 20%,<ref name="Finsterer2015" /> and in one reported case, piercing the vein.<ref name="saman">{{cite journal|year=2010|title=Anatomic relationship between the spinal accessory nerve and the jugular vein: a cadaveric study|journal=Surgical and Radiologic Anatomy|volume=33|issue=2|pages=175–179|doi=10.1007/s00276-010-0737-y|pmid=20959982|vauthors=Saman M, Etebari P, Pakdaman MN, Urken ML|s2cid=24202845}}</ref>

Traditionally, the accessory nerve is described as having a small cranial component that descends from the [[medulla oblongata|medulla]] and briefly connects with the spinal accessory component before branching off of the nerve to join the vagus nerve.{{sfn|Gray's Anatomy|2008|p=459}} A study, published in 2007, of twelve subjects suggests that in the majority of individuals, this cranial component does not make any distinct connection to the spinal component; the roots of these distinct components were separated by a fibrous sheath in all but one subject.<ref name="ryan" />

===Development===
The accessory nerve is derived from the [[basal plate (neural tube)|basal plate]] of the embryonic spinal segments C1–C6.<ref>{{cite journal|last1=Skórzewska|first1=A|last2=Bruska|first2=M|last3=Woźniak|first3=W|title=The development of the spinal accessory nerve in human embryos during 5th week (stages 14 and 15).|journal=Folia Morphologica|date=1994|volume=53|issue=3|pages=177–84|pmid=7883243}}</ref>

==Function==
[[File:1610 Muscles Controlled by the Accessory Nerve-02.jpg|thumb|The accessory nerve supplies the [[sternocleidomastoid muscle|sternocleidomastoid]] and [[trapezius muscle]]s|alt=Image showing the head with two muscles highlighted.]]
The spinal component of the accessory nerve provides motor control of the [[sternocleidomastoid]] and [[trapezius]] [[muscle]]s.<ref name=TC2014 /> The trapezius muscle controls the action of shrugging the shoulders, and the sternocleidomastoid the action of turning the head.<ref name=TC2014 /> Like most muscles, control of the trapezius muscle arises from the opposite side of the brain.<ref name=TC2014 />  Contraction of the upper part of the trapezius muscle elevates the scapula.<ref>{{cite journal|last1=Kelley|first1=Martin J.|last2=Kane|first2=Thomas E.|last3=Leggin|first3=Brian G.|title=Spinal Accessory Nerve Palsy: Associated Signs and Symptoms|journal=Journal of Orthopaedic & Sports Physical Therapy|date=February 2008|volume=38|issue=2|pages=78–86|doi=10.2519/jospt.2008.2454|pmid=18560187|quote="The upper trapezius elevates, the middle trapezius retracts, and the lower trapezius depresses. In unison, the pri- mary function of the trapezius is to up- wardly rotate the scapula during shoulder elevation, forming a force couple with the serratus anterior"}}</ref> The nerve fibres supplying sternocleidomastoid, however, are thought to change sides ({{langx|la|decussate}}) twice. This means that the sternocleidomastoid is controlled by the brain on the same side of the body. Contraction of the sternocleidomastoid fibres turns the head to the opposite side, the net effect meaning that the head is turned to the side of the brain receiving visual information from that area.<ref name=TC2014 /> The cranial component of the accessory nerve, on the other hand, provides motor control to the muscles of the soft palate, larynx and pharynx.

===Classification===
Among researchers there is disagreement regarding the terminology used to describe the type of information carried by the accessory nerve. As the trapezius and sternocleidomastoid muscles are derived from the [[pharyngeal arch]]es, some researchers believe the spinal accessory nerve that innervates them must carry specific [[special visceral efferent]] (SVE) information.<ref>{{cite book |author =William T. Mosenthal |title=A Textbook of Neuroanatomy: With Atlas and Dissection Guide |publisher=Taylor & Francis |location=Washington, DC |year= 1995|page=12 |isbn=1-85070-587-9 }}</ref> This is in line with the observation that the spinal accessory nucleus appears to be continuous with the [[nucleus ambiguus]] of the medulla. Others consider the spinal accessory nerve to carry [[General somatic efferent fibers|general somatic efferent]] (GSE) information.<ref>{{cite book |author=Duane E. Haines |title=Neuroanatomy: an atlas of structures, sections, and systems |publisher=Lippincott Williams & Wilkins |location=Hagerstown, MD |year=2004 |isbn=0-7817-4677-9 |url-access=registration |url=https://archive.org/details/neuroanatomyatla00hain }}</ref> Still others believe it is reasonable to conclude that the spinal accessory nerve contains both SVE and GSE components.<ref>{{cite journal |vauthors=Joshi SS, Joshi SD |title=Muscle Dorso-Fascialis — A Case Report |journal=Journal of the Anatomical Society of India |volume=50 |issue=2 |pages=159–160 |year=2001 }}</ref>

==Clinical significance==
===Examination===
The accessory nerve is tested by evaluating the function of the [[trapezius]] and [[sternocleidomastoid muscle]]s.<ref name=TC2014>{{Cite book|title=Clinical Examination|last=Talley|first=Nicholas|publisher=Churchill Livingstone|year=2014|isbn=978-0-7295-4198-5|pages=424–5}}</ref> The trapezius muscle is tested by asking the patient to shrug their shoulders with and without resistance. The sternocleidomastoid muscle is tested by asking the patient to turn their head to the left or right against resistance.<ref name=TC2014 />

One-sided weakness of the trapezius may indicate injury to the nerve on the same side of an injury to the spinal accessory nerve on the same side ({{langx|la|ipsilateral|link=no}}) of the body being assessed.<ref name=TC2014 /> Weakness in head-turning suggests injury to the contralateral spinal accessory nerve: a weak leftward turn is indicative of a weak right sternocleidomastoid muscle (and thus right spinal accessory nerve injury), while a weak rightward turn is indicative of a weak left sternocleidomastoid muscle (and thus left spinal accessory nerve).<ref name=TC2014 />

Hence, weakness of shrug on one side and head-turning on the other side may indicate damage to the accessory nerve on the side of the shrug weakness, or damage along the nerve pathway at the other side of the brain. Causes of damage may include trauma, surgery, tumours, and compression at the jugular foramen.<ref name=TC2014 /> Weakness in both muscles may point to a more general disease process such as [[amyotrophic lateral sclerosis]], [[Guillain–Barré syndrome]] or [[poliomyelitis]].<ref name=TC2014 />

=== Injury ===
{{Main|Accessory nerve disorder}}
Injury to the spinal accessory nerve commonly occurs during neck surgery, including [[neck dissection]] and [[lymph node]] excision. It can also occur as a result of blunt or penetrating trauma, and in some causes spontaneously.<ref name=MARTIN2007 /><ref name="KELLEY2008">{{cite journal|last1=Kelley|first1=Martin J.|last2=Kane|first2=Thomas E.|last3=Leggin|first3=Brian G.|title=Spinal Accessory Nerve Palsy: Associated Signs and Symptoms|journal=Journal of Orthopaedic & Sports Physical Therapy|date=February 2008|volume=38|issue=2|pages=78–86|doi=10.2519/jospt.2008.2454|pmid=18560187}}</ref> Damage at any point along the nerve's course will affect the function of the nerve.{{sfn|Gray's Anatomy|2008|p=460}} The nerve is intentionally removed in "radical" neck dissections, which are attempts at exploring the neck surgically for the presence and extent of cancer. Attempts are made to spare it in other forms of less aggressive dissection.<ref name=KELLEY2008 />

Injury to the accessory nerve can result in neck pain and weakness of the trapezius muscle. Symptoms will depend on at what point along its length the nerve was severed.<ref name=KELLEY2008 /> Injury to the nerve can result in shoulder girdle depression, atrophy, abnormal movement, a protruding scapula, and weakened [[abduction (anatomy)|abduction]].<ref name=KELLEY2008 /> Weakness of the shoulder girdle can lead to traction injury of the [[brachial plexus]].{{sfn|Gray's Anatomy|2008|p=460}} Because diagnosis is difficult, [[electromyogram]] or [[nerve conduction studies]] may be needed to confirm a suspected injury.<ref name=KELLEY2008 /> Outcomes with surgical treatment appear to be better than conservative management, which entails physiotherapy and pain relief.<ref name="MARTIN2007">{{cite journal|last1=Martin|first1=Ryan M.|last2=Fish|first2=David E.|title=Scapular winging: anatomical review, diagnosis, and treatments|journal=Current Reviews in Musculoskeletal Medicine|date=2 November 2007|volume=1|issue=1|pages=1–11|doi=10.1007/s12178-007-9000-5|pmc=2684151|pmid=19468892}}</ref> Surgical management includes [[neurolysis]], nerve end-to-end suturing, and surgical replacement of affected trapezius muscle segments with other muscle groups, such as the [[Eden-Lange procedure]].<ref name=MARTIN2007/>

Damage to the nerve can cause [[torticollis]].<ref name=":0">{{Cite journal|last=Tomczak|first=K|date=2013|title=Torticollis|journal=Journal of Child Neurology|doi=10.1177/0883073812469294|pmid=23271760|volume=28|issue=3|pages=365–378|s2cid=216099695}}</ref>

==History==
English anatomist [[Thomas Willis]] in 1664 first described the accessory nerve, choosing to use "accessory" (described in [[Latin]] as ''nervus accessorius'') meaning in association with the vagus nerve.<ref name=NAMING2014>{{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.|title=The naming of the cranial nerves: A historical review|journal=Clinical Anatomy|date=January 2014|volume=27|issue=1|pages=14–19|doi=10.1002/ca.22345|pmid=24323823|s2cid=15242391}}</ref>

In 1848, [[Jones Quain]] described the nerve as the "spinal nerve accessory to the vagus", recognizing that while a minor component of the nerve joins with the larger vagus nerve, the majority of accessory nerve fibres originate in the spinal cord.<ref name=ANTHONEY1993 /><ref>{{cite book |author =Jones Quain |editor1=Richard Quain |editor2=William Sharpey |title=Elements of Anatomy |publisher=Taylor, Walton, and Maberly |location=London |year=1848 |page=812 |edition=5th |volume=2 |url=https://books.google.com/books?id=wSAEAAAAQAAJ}}</ref> In 1893 it was recognised that the heretofore named nerve fibres "accessory" to the vagus originated from the same nucleus in the [[medulla oblongata]], and it came to pass that these fibres were increasingly viewed as part of the vagus nerve itself.<ref name=ANTHONEY1993 /> Consequently, the term "accessory nerve" was and is increasingly used to denote only fibres from the spinal cord; the fact that only the spinal portion could be tested clinically lent weight to this opinion.<ref name=ANTHONEY1993 >{{cite book |author =Terence R. Anthoney |title=Neuroanatomy and the Neurologic Exam: A Thesaurus of Synonyms, Similar-Sounding Non-Synonyms, and Terms of Variable Meanings |publisher=CRC-Press |location=Boca Raton |year=1993 |pages=69–73 |isbn=0-8493-8631-4 }}</ref>

== See also ==
* [[List of anatomy mnemonics#Cranial nerves]]

==Additional images==
<gallery>
File:Gray793.png|Course and distribution of the [[glossopharyngeal]], [[vagus]], and accessory nerves. The accessory nerve (top left) travels down through the jugular foramen with the other two nerves, and then passes down, usually over the internal jugular vein, to supply the sternocleidomastoid and trapezius muscles
File:Gray1210.png|Side of the neck, with accessory nerve seen between the sternocleidomastoid and trapezius muscles
File:Slide2ZEO.JPG|The brain and upper spinal cord in a [[cadaver]] specimen. The accessory nerve can be seen as a number of rootlets arising from the medulla.
</gallery>

==References==
{{Reflist}}

;Books
* {{cite book|others=editor-in-chief, Susan Standring|title=Gray's anatomy : the anatomical basis of clinical practice|date=2008|publisher=Churchill Livingstone|location=London|isbn=978-0-8089-2371-8|edition=40th|ref={{harvid|Gray's Anatomy|2008}}}}

==External links==
{{Commons category}}
* {{LoyolaMedEd|GrossAnatomy/h_n/cn/cn1/cn11.htm}}
* {{NormanAnatomy|lesson6}}
* {{NormanAnatomy|cranialnerves}} ({{NormanAnatomyFig|XI}})
* {{SUNYAnatomyLabs|28|13|01|15}}
* {{cite web|url=http://www.yale.edu/cnerves/cn11/cn11_1.html |title=11-1 |publisher=[[Yale School of Medicine]] |work=Cranial Nerves |archive-url=https://web.archive.org/web/20160303222022/http://www.yale.edu/cnerves/cn11/cn11_1.html |archive-date=3 March 2016 |url-status=dead }}

{{Cranial nerves}}
{{Authority control}}
{{Good article}}

[[Category:Accessory nerve| ]]
[[Category:Cranial nerves]]
[[Category:Good articles missing image alternative text]]
[[Category:Otorhinolaryngology]]
[[Category:Human head and neck]]
[[Category:Nervous system]]
[[Category:Neurology]]
[[Category:Nerves of the head and neck]]