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Accessory nerve
Accessory nerve
Accessory nerve
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Accessory nerve
View of the human brain from below. The accessory nerve emerges from the medulla of the brainstem, and is visible at the bottom of the image in blue.
Details
InnervatesSternocleidomastoid muscle, trapezius muscle
Identifiers
Latinnervus accessorius
MeSHD000055
TA98A14.2.01.184
A14.1.02.112
TA26352
FMA6720
Anatomical terms of neuroanatomy
Cranial nerves

The accessory nerve, also known as the eleventh cranial nerve, cranial nerve XI, or simply CN XI, is a cranial nerve that supplies the sternocleidomastoid and trapezius muscles. 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.[1] 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.[2][1] Consequently, the term "accessory nerve" usually refers only to nerve supplying the sternocleidomastoid and trapezius muscles, also called the spinal accessory nerve.[3]

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.[4] Injury can cause wasting of the shoulder muscles, winging of the scapula, and weakness of shoulder abduction and external rotation.[5]

The accessory nerve is derived from the basal plate of the embryonic spinal segments C1–C6.[6]

Structure

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See also: Spinal root of accessory nerve and Cranial root of accessory nerve

The fibres of the spinal accessory nerve originate solely in neurons 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.[1][7] 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.[1] The nerve travels along the inner wall of the skull towards the jugular foramen.[1] Leaving the skull, the nerve travels through the jugular foramen with the glossopharyngeal and vagus nerves.[8] 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.[9]

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 cm above Erb's point) while sending it 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.[10]

Nucleus

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Main article: Spinal accessory nucleus

The fibres that form the spinal accessory nerve are formed by lower motor neurons located in the upper segments of the spinal cord. This 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.[1] 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.[9]

Variation

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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%,[9] and in one reported case, piercing the vein.[11]

Traditionally, the accessory nerve is described as having a small cranial component that descends from the medulla and briefly connects with the spinal accessory component before branching off of the nerve to join the vagus nerve.[1] 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.[7]

Development

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The accessory nerve is derived from the basal plate of the embryonic spinal segments C1–C6.[12]

Function

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Image showing the head with two muscles highlighted.
The accessory nerve supplies the sternocleidomastoid and trapezius muscles

The spinal component of the accessory nerve provides motor control of the sternocleidomastoid and trapezius muscles.[8] The trapezius muscle controls the action of shrugging the shoulders, and the sternocleidomastoid the action of turning the head.[8] Like most muscles, control of the trapezius muscle arises from the opposite side of the brain.[8] Contraction of the upper part of the trapezius muscle elevates the scapula.[13] The nerve fibres supplying sternocleidomastoid, however, are thought to change sides (Latin: 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.[8] 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

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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 arches, some researchers believe the spinal accessory nerve that innervates them must carry specific special visceral efferent (SVE) information.[14] 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 (GSE) information.[15] Still others believe it is reasonable to conclude that the spinal accessory nerve contains both SVE and GSE components.[16]

Clinical significance

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Examination

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The accessory nerve is tested by evaluating the function of the trapezius and sternocleidomastoid muscles.[8] 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.[8]

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 (Latin: ipsilateral) of the body being assessed.[8] 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).[8]

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.[8] Weakness in both muscles may point to a more general disease process such as amyotrophic lateral sclerosis, Guillain–Barré syndrome or poliomyelitis.[8]

Injury

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Main article: 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.[17][5] Damage at any point along the nerve's course will affect the function of the nerve.[10] 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.[5]

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.[5] Injury to the nerve can result in shoulder girdle depression, atrophy, abnormal movement, a protruding scapula, and weakened abduction.[5] Weakness of the shoulder girdle can lead to traction injury of the brachial plexus.[10] Because diagnosis is difficult, electromyogram or nerve conduction studies may be needed to confirm a suspected injury.[5] Outcomes with surgical treatment appear to be better than conservative management, which entails physiotherapy and pain relief.[17] 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.[17]

Damage to the nerve can cause torticollis.[18]

History

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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.[19]

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.[3][20] 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.[3] 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.[3]

See also

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Additional images

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  • 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
    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
  • Side of the neck, with accessory nerve seen between the sternocleidomastoid and trapezius muscles
    Side of the neck, with accessory nerve seen between the sternocleidomastoid and trapezius muscles
  • 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.
    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.

References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Accessory nerve

View on Grokipedia
from Grokipedia
The accessory nerve, also known as cranial nerve XI or the spinal accessory nerve, is a purely motor cranial nerve that originates from both cranial and spinal roots to provide innervation primarily to the sternocleidomastoid and trapezius muscles, enabling key movements of the head, neck, and shoulders. Its spinal root arises from motor neurons in the spinal accessory nucleus located in the upper five or six cervical segments (C1–C5 or C6) of the spinal cord, while the cranial root emerges from the nucleus ambiguus in the medulla oblongata and typically merges with the vagus nerve (cranial nerve X) to contribute to pharyngeal and laryngeal functions. The nerve exits the skull through the jugular foramen, descends in the neck posterior to the internal jugular vein in most cases, and branches to supply the sternocleidomastoid muscle (for ipsilateral head rotation and neck flexion) before penetrating the trapezius muscle to facilitate shoulder elevation, retraction, and head stabilization. Anatomically, the accessory nerve exhibits notable variations, including occasional anastomoses with cervical roots (such as the C1 ventral rootlet in 3–7% of cases) and differences in its relationship to the , where it may course anteriorly (56–96%), posteriorly (57.4%), or even through a venous fenestration (2.8%). These variations, along with potential duplication of the or atypical fiber contributions from upper cervical nerves, underscore its complex morphology and the need for precise identification during surgical procedures. Functionally, the 's motor fibers to the sternocleidomastoid allow unilateral contraction to turn the head contralaterally and bilateral action for flexion, while innervation supports scapular elevation against resistance and ipsilateral head tilting when the is fixed. Although primarily motor, limited sensory components, including nociceptive fibers, have been identified, potentially contributing to in cases of dysfunction. Clinically, injury to the accessory nerve—often iatrogenic from neck surgeries like biopsies or radical neck dissections—results in atrophy, droop, winging of the , and impaired arm abduction, with sternocleidomastoid leading to head tilt and rotation deficits. Such lesions can also arise from tumors, trauma, or , manifesting as signs like fasciculations and atrophy, in contrast to milder, often transient effects from involvement. The nerve's vulnerability highlights its surgical importance, including its use in nerve transfer techniques for restoring function in injuries.

Anatomy

Origin and nucleus

The accessory nerve, or cranial nerve XI, consists of two distinct components: a cranial root and a spinal root, each arising from separate nuclear origins in the central nervous system. The cranial root originates from the nucleus ambiguus, a column of motor neurons located in the reticular formation of the medulla oblongata. This nucleus provides special visceral efferent fibers that are primarily involved in innervating branchiomeric muscles, though in humans, these fibers largely merge with the vagus nerve (cranial nerve X) rather than forming an independent pathway. The rootlets of the cranial component, typically numbering 2 to 9, emerge from the caudal third of the nucleus ambiguus and exit the lateral aspect of the medulla oblongata via the postolivary sulcus, positioned inferior to the rootlets of the glossopharyngeal (CN IX) and vagus (CN X) nerves. In contrast, the spinal root arises from the spinal accessory nucleus, a specialized group of motor neurons situated in the lateral aspect of the anterior (ventral) horn of the upper cervical spinal cord, spanning segments C1 to C5 (occasionally extending to C6 or C7). This nucleus receives inputs primarily from the , which originates in the of the and descends through the to synapse on these lower motor neurons, enabling voluntary control of muscles. The spinal rootlets, usually 6 to 12 in number, protrude from the anterolateral surface of the spinal cord between the dorsal and ventral roots at these cervical levels, then ascend as a cohesive trunk through the vertebral canal, passing dorsal to the denticulate . Microscopically, the spinal accessory nucleus forms a distinct, elongated column of large multipolar motor neurons, measuring approximately 4-6 mm in craniocaudal length, with axons that are myelinated and contribute to the nerve's somatic motor function. These neurons are and exhibit bilateral innervation patterns from cortical inputs, distinguishing the nucleus from adjacent cervical motor pools. The cranial root's nuclear contribution from the is more integrated with vagal efferents, lacking the isolated columnar structure seen in the spinal component.

Course and relations

The spinal root of the accessory nerve, arising from rootlets at cervical levels C1 to C5, forms a trunk that enters the cranium through the and pierces the to enter the subarachnoid space. It then ascends superolaterally across the floor of the , positioned dorsal to the denticulate ligament, ventral to the dorsal spinal rootlets, posterior to the , and adjacent to the . This trunk joins the cranial root, which originates from the , within the , through which the combined accessory nerve exits the skull alongside the glossopharyngeal and vagus nerves. Extracranially, the accessory nerve emerges from the jugular foramen posterior to the styloid process and descends within the carotid sheath, initially positioned between the internal carotid artery and internal jugular vein. The cranial root typically detaches to merge with the vagus nerve, while the spinal root continues as the principal external branch, crossing anterior to the internal jugular vein in 56-96% of cases. It then pierces the sternocleidomastoid muscle near its upper third, approximately 4-9 cm inferior to the mastoid process, supplying motor branches either by penetrating the muscle or coursing along its deep surface. Emerging from the posterior border of the sternocleidomastoid about 7.5-9 cm superior to the clavicle, the nerve traverses the posterior triangle of the neck obliquely inferiorly and laterally, passing deep to the posterior belly of the digastric muscle and superficial to the levator scapulae and stylohyoid muscles. Throughout its extracranial path, the accessory nerve maintains close relations to several structures in the neck, including the within the and at the , branches of the (C1-C4), and the sympathetic chain in the posterior triangle. It also lies in proximity to the indirectly through shared cervical segmental contributions, though the phrenic courses more anteriorly along the . The nerve enters the anterior border of the trapezius muscle approximately 1.5 cm medial to the midclavicular line and 4.5 cm superior to the , distributing motor fibers within the muscle. The total extracranial length of the spinal accessory nerve averages 12.02 cm (range 7-18.5 cm), with a of approximately 2 mm containing 1,700-2,000 myelinated fibers.

Variations

The accessory nerve exhibits several anatomical variations in its origin, course, and branching patterns, which have been documented through cadaveric dissections and surgical observations. The spinal root typically arises from the upper five cervical segments (C1–C5), but in some cases includes contributions from C6, extending the caudal limit of its origin. The cranial root is present in approximately 80% of individuals and emerges from the medulla oblongata; its fibers typically fuse with the spinal root or merge with the vagus nerve for laryngeal innervation. Branching anomalies are common, particularly in the innervation of the muscle, where dual supply from the accessory nerve and branches of the (primarily C3–C4) occurs in about 40–50% of cases, based on intraoperative monitoring and studies. Occasional fusions occur between the accessory nerve and the vagus or hypoglossal nerves extracranially, though these are less frequent and often involve the cranial root joining the vagus trunk. Cadaveric studies show variable rootlet inputs from C2–C4 to trapezius innervation (zero cervical rootlets in ~38%, one in 25%, and two or more in 37%). Rare variants include intracranial anastomoses, such as connections between the accessory nerve rootlets and the within the , observed in a minority of dissections. Duplicated spinal roots, where the splits and reunites intradurally or extracranially, have also been reported sporadically in cadaveric and surgical cases, potentially altering the 's vulnerability during procedures.

Development

The accessory nerve develops from distinct embryonic origins for its cranial and spinal roots. The cranial root arises from the caudal portion of the in the , deriving from the 4th and providing special visceral efferent (branchiomotor) innervation to pharyngeal and laryngeal muscles, with fibers typically integrating into the (CN X). In contrast, the spinal root originates from motor neurons in the spinal accessory nucleus, formed from cells in the upper cervical spinal cord (levels C1–C5) and influenced by occipital and trunk somites, which contribute to the development of the shoulder girdle muscles such as the sternocleidomastoid and . Both roots share an initial common structure with the , emerging from the ganglionic crest of the during early neural development. The developmental timeline begins around week 3 of gestation (approximately day 20), when the ganglionic crest lateralizes to form a dorsal bridge, and the accessory nerve bundle emerges from segments C4–C6. By the end of week 4, the nerve extends and curves around the developing vagus, penetrating the mesodermal precursor of the sternocleidomastoid muscle. The spinal accessory nucleus forms by week 5, with the nerve lying medial to dorsal rootlets and connecting irregularly to the spinal cord; around day 30, ganglia appear among the rootlets. Axons of the spinal root migrate rostrally to consolidate at C1–C5 by approximately week 8, while the nerve elongates during weeks 5–12, separating its cranial (sensory-leaning) and caudal (motor) components. The jugular foramen, through which the nerve exits the skull, develops by week 10 as part of skull base ossification. Molecular mechanisms guide the nerve's formation and . establish segmental identity in the and , patterning populations that contribute to the accessory nerve's origins and ensuring proper axial specification for innervation of and muscles. Guidance cues such as netrins, acting through receptors like DCC, promote dorsal migration of s, while UNC5C mediates repulsion from the ventral midline to direct pathfinding; disruptions in these, as seen in UNC5C mutants, lead to abnormal clustering of cell bodies. Additionally, transcription factors like Nkx2.9 and Gli2 regulate initial axon extension and growth, with Nkx2.9-null models exhibiting shorter and thinner spinal root axons. Postnatally, myelination of the accessory nerve, formed by Schwann cells in nervous system, largely completes by birth, following initiation in the sixth fetal month, which supports efficient motor conduction to target muscles. Minor increases in nerve diameter occur during infancy, accompanying overall growth and refinement of neuromuscular connections.

Function

Motor innervation

The accessory nerve, also known as cranial nerve XI, provides primarily somatic motor (efferent) innervation to two key muscles involved in head and shoulder movements: the sternocleidomastoid (SCM) and the muscles. These fibers originate from the spinal nucleus in the upper cervical spinal cord (segments C1–C5) and travel via the spinal root of the nerve, which is considered the primary functional component of CN XI for motor purposes. Although traditionally viewed as purely motor, recent evidence suggests the presence of minor sensory (nociceptive) fibers. The SCM muscle receives innervation from a branch of the spinal root of the accessory nerve, which enters the muscle deep to its anterior border. This innervation enables the SCM to perform ipsilateral lateral flexion of the (tilting the head toward the same side) and contralateral rotation (turning the head to the opposite side). When both SCM muscles contract bilaterally, they assist in flexion and can serve as accessory muscles for inspiration by elevating the . The accessory nerve provides the predominant motor supply to the SCM, although minor contributions from the upper cervical nerves (C2–C3) may augment its function in some individuals. The muscle is innervated by the spinal root of the accessory nerve, which pierces the muscle's deep surface after crossing the . This allows the trapezius to coordinate scapular movements essential for shoulder stability and upper limb positioning. Specifically, the upper fibers elevate the (as in shrugging), the middle fibers retract the scapula toward the spinal column, and the lower fibers depress the scapula while aiding in its upward during arm abduction. The accessory nerve supplies the majority of motor fibers to the trapezius, working in synergy with branches from the (primarily C3–C4) to ensure comprehensive function across the muscle's three functional divisions; disruption of this coordination can impair overall mobility.

Classification

The accessory nerve is traditionally classified as the eleventh cranial nerve (CN XI), a designation rooted in its anatomical exit from the skull via the alongside the (CN X). However, this classification is nuanced due to its dual composition: a cranial root originating from the in the and a spinal root arising from motor neurons in the upper cervical (segments C1 to C5). The spinal root's origin below the has led some anatomists to argue that it functions more akin to a cervical than a true cranial nerve, particularly since it ascends through the to join the cranial root before diverging. Functionally, the accessory nerve is categorized as primarily efferent, with distinct subtypes for each root. The cranial root carries special visceral efferent (SVE) fibers that innervate branchial arch-derived muscles, such as those in the , though these fibers typically merge with the and are not independent. In contrast, the spinal root conveys general somatic efferent (GSE) fibers to skeletal muscles of the and , specifically the sternocleidomastoid and muscles, enabling head and shoulder elevation. This hybrid efferent profile—SVE for the cranial component and GSE for the spinal—distinguishes it from purely somatic efferent nerves like the (CN XII), which lacks any visceral component or sensory fibers and solely innervates intrinsic tongue muscles. Similarly, unlike brainstem-origin motor nerves such as the (CN III), which is entirely GSE and confined to ocular movements, the accessory nerve's spinal contribution imparts a unique extracranial trajectory and mixed heritage. Debates persist regarding the precise taxonomic status of the spinal accessory nerve, particularly whether it qualifies as a genuine cranial nerve or an "accessory spinal nerve" based on embryological development and anatomical exit pathways. Embryologically, the spinal root derives from somatic motor columns in the cervical spinal cord, resembling spinal nerves in its ventral rootlet formation and axon guidance, yet it adopts a cranial-like dorsolateral exit and fuses transiently with brainstem efferents during development. Proponents of a transitional classification highlight this blend, noting that the nerve's evolution from gnathostome-specific repatterning of spinal motoneurons challenges strict categorization as either SVE or GSE, positioning it as a bridge between cranial and spinal systems. The cranial root's frequent incorporation into the vagus further complicates matters, with some sources advocating its reclassification as part of CN X rather than an independent entity in CN XI. These discussions underscore the accessory nerve's atypical position among the cranial nerves, informed by both classical anatomy and modern neurodevelopmental studies.

Clinical significance

Examination

The integrity of the accessory nerve is primarily assessed through clinical tests targeting its motor innervation to the sternocleidomastoid (SCM) and muscles. To evaluate the SCM, the patient is asked to turn their head against resistance applied by the examiner's hand placed on the contralateral , while the muscle is palpated for contraction strength. Weakness in this maneuver indicates an ipsilateral lesion, as the SCM rotates the head to the opposite side. For the , the patient performs a shoulder shrug against downward resistance from the examiner's hands on both shoulders, assessing elevation and retraction; reduced power suggests accessory nerve dysfunction. Observation during these tests reveals signs of paresis or chronic damage. Trapezius weakness often manifests as scapular winging, drooping of the affected shoulder, or asymmetry in shoulder contour when viewed from behind. SCM paresis may cause the head to tilt toward the affected side during resisted or result in an uneven neckline due to in longstanding cases. Adjunctive electrophysiological studies, such as (EMG), confirm patterns in the trapezius and SCM by recording spontaneous activity or reduced potentials, particularly useful when clinical findings are subtle. Routine imaging is not indicated for functional assessment, but (MRI) may be employed if involvement is suspected, revealing or edema. Normal examination demonstrates symmetric muscle strength graded as 5/5 on the scale for both SCM rotation and elevation, with no evidence of , fasciculations, or asymmetry.

Injury

Injury to the accessory nerve, also known as the spinal accessory nerve (SAN), most commonly arises from iatrogenic causes during surgical procedures in the posterior cervical triangle. biopsies carry a 3-8% risk of SAN injury, while neck dissections show higher rates: 46.7% in radical procedures, 42.5% in selective dissections, and 25% in modified radical neck dissections. Traumatic etiologies include penetrating injuries such as gunshot or knife wounds and like whiplash or sports-related impacts (e.g., blows). Tumors, particularly schwannomas originating from the nerve sheath or lesions causing syndromes like Collet-Sicard or Vernet, represent rarer compressive causes. Symptoms typically manifest as trapezius muscle paralysis, leading to shoulder drop, drooping of the affected side, and scapular winging due to unopposed serratus anterior pull. Patients often experience severe shoulder pain (mean visual analog scale score of 7), radiating to the neck, upper back, or arm, alongside weakness in shrugging or overhead activities. Sternocleidomastoid (SCM) weakness may cause head tilt or torticollis, though less prominent than trapezius involvement. In chronic cases, adhesive capsulitis (frozen shoulder) develops, limiting abduction (30-140°) and flexion (50-180°), with trapezius atrophy exacerbating functional deficits. The highest injury rates occur during radical neck surgery for cancer, where up to 80% of cases result in severe dysfunction if the is sacrificed. Congenital are rare and usually stem from birth trauma, presenting similarly but with early onset. varies by injury severity and intervention timing, with partial recovery in 77-90% of repaired cases achieving at least grade 3 motor function (on the Louisiana State University Health Sciences Center scale) within 4-12 months. grafting or end-to-end repair yields 85-90% success rates for functional restoration, though longer grafts (>2-3 cm) or delays beyond 6 months reduce outcomes due to fiber misdirection. Spinal accessory , characterized by persistent and scapulohumeral dysfunction, affects chronic cases, often requiring multidisciplinary management for pain relief and rehabilitation.

Surgical considerations

During neck surgeries involving the posterior triangle, such as modified radical for head and neck malignancies, the accessory nerve is identified by its course exiting the posterior border of the , typically 1-2 cm deep to its surface and posterior to the posterior belly of the . Reliable landmarks include the greater auricular nerve point or the sternocleidomastoid branch of the occipital artery to facilitate precise localization and minimize traction injury. Intraoperative nerve monitoring, particularly () of the muscle, is widely employed to confirm nerve integrity and guide dissection, with studies showing its potential to reduce iatrogenic damage in selective and modified s. Protection of the accessory nerve is prioritized in modified radical dissections through techniques that spare non-lymphatic structures, allowing preservation in approximately 90% of cases while achieving oncologic clearance comparable to radical procedures. In procedures like with central compartment dissection, the nerve is routinely avoided by limiting dissection to anterior levels, resulting in very low injury rates. During posterior access, surgeons employ gentle mobilization and avoid excessive retraction to prevent neuropraxia, with comprehensive dissections incorporating nerve-sparing approaches to maintain function. If intraoperative injury occurs, reconstruction techniques depend on the defect size: end-to-end neurorrhaphy is preferred for partial transections or gaps under 2 cm, while autografts are used for larger defects exceeding 2 cm to bridge the gap and restore innervation to the . Postoperative outcomes following these repairs show functional recovery in 60-80% of patients, with strength reaching Medical Research Council grades M3-M4 and improved shoulder abduction, particularly when surgery occurs within 6-12 months of . Recent advances include robotic-assisted , which enhances visualization and dexterity through magnified 3D imaging and tremor filtration, with comparable safety to conventional techniques regarding . Emerging non-surgical approaches, such as isotonic saline injections at nerve entry points to the sternocleidomastoid and muscles, show potential in reducing associated with accessory nerve dysfunction.

History

Early descriptions

The earliest references to what is now known as the accessory nerve appear in the works of (c. 129–210 AD), the Greco-Roman physician whose anatomical observations dominated medical thought for over a . Based on dissections of animals such as oxen and apes, Galen identified seven pairs of but did not recognize the accessory nerve as a distinct structure; instead, he incorporated it into his sixth pair, grouping it with the glossopharyngeal and vagus nerves. He briefly noted its role in innervating the "muscle of the ," an early allusion to its motor function in the shoulder region, though without detailed separation from the vagus complex. During the , significant progress in human came from (1514–1564) in his seminal 1543 text De humani corporis fabrica. Vesalius employed direct dissections of human cadavers to illustrate the more accurately than his predecessors, depicting seven pairs similar to Galen's scheme. He provided one of the first explicit descriptions of a branching to the , referring to it as an accessory branch (nervus accessorius), though he did not fully distinguish it as an independent or trace its spinal contributions. This observation marked a shift toward precise topographic , highlighting the 's extracranial path and muscular innervation. The 17th century brought clearer recognition of the accessory nerve's unique identity through the work of (1621–1675), an English physician and anatomist. In his 1664 publication Cerebri anatome, Willis proposed a system of nine cerebral nerves and was the first to designate the spinal accessory as a separate entity, naming it the "accessory nerve" (nervus accessorius) due to its supplementary role relative to the vagus. He accurately linked its origins to the upper , describing how its roots ascend from the cervical region to join the cranial outflow, and illustrated its path using detailed engravings by . This conceptualization solidified the nerve's dual cranial-spinal nature and its primary motor supply to the and sternocleidomastoid muscles. By the late , experimental and histological approaches further validated the accessory nerve's dual origins and functions. Studies, including those by Ramón y Cajal, confirmed the cranial roots from the and spinal roots from upper cervical motoneurons, establishing the nerve's predominantly spinal motor pathway and its essential role in and movement, paving the way for its formal classification as cranial nerve XI.

Nomenclature evolution

The accessory nerve was first accurately described and named by Thomas Willis in his 1664 work Cerebri anatome, where he referred to it as the nervus accessorium Willisii, recognizing its association with the vagus nerve while distinguishing its unique course from the brainstem and spinal cord. This Latin term, meaning "accessory nerve," reflected its perceived supplementary role to the vagus, though Willis grouped it with the glossopharyngeal and vagus as part of the eighth pair of cranial nerves. In the 19th century, particularly in German anatomical literature, the nerve was frequently emphasized for its spinal origin and termed nervus accessorius spinalis or simply nervus spinalis, highlighting the contributions from upper cervical spinal segments over its cranial components, as seen in detailed dissections by anatomists like Friedrich Arnold. The early 20th century brought standardization through the Basle Nomina Anatomica (BNA) in 1895, which fixed the name as nervus accessorius for , adopting Samuel Thomas von Soemmerring's 1778 classification of 12 and separating it definitively from the vagus. However, debates persisted regarding its dual nature; editions of from the late 19th to mid-20th century reflected evolving views, with earlier versions (e.g., 1858) describing both cranial and spinal roots as integral to the accessory nerve, while later ones (e.g., post-1900) increasingly argued that the cranial root was functionally part of the vagus, based on its merger and shared innervation of pharyngeal muscles. Embryological investigations, notably George L. Streeter's 1905 study on cranial and spinal nerve development in embryos, provided critical of the nerve's hybrid origins—cranial roots from the and spinal roots from C1-C5 segments—resolving some ambiguities and influencing classifications by underscoring its transitional characteristics. In contemporary usage, the (1998, with revisions in 2019 by the Federative International Programme on Anatomical Terminology) maintains nervus accessorius [XI] as the official Latin term, specifying subclauses for radix cranialis nervi accessorii (cranial root, often merging with the vagus) and radix spinalis nervi accessorii (spinal root, innervating sternocleidomastoid and trapezius muscles). Despite its predominantly spinal motor function and lack of sensory components typical of most , it remains classified as cranial nerve XI due to the cranial root's emergence from the , a convention solidified by 20th-century phylogenetic and developmental studies that affirm its evolutionary position between cranial and spinal systems. This nomenclature balances historical precedent with anatomical precision, avoiding reclassification as a purely .

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