Root Origin
- Spinal nerve roots: Spinal roots from C1-C5/C6 (ascending through the foramen magnum) plus a small cranial root from the nucleus ambiguus of the medulla
- Classification: Cranial nerve XI — despite arising primarily from the spinal cord, not the brainstem
- Type: Motor only (pure motor nerve to the SCM and trapezius; proprioceptive sensory fibers travel with it but do not supply cutaneous sensation)
Course
- Spinal cord origin. The spinal rootlets arise from the lateral portion of the anterior horn cells of C1-C5 (sometimes C6). These rootlets ascend within the spinal canal alongside the spinal cord.
- Foramen magnum. The ascending spinal rootlets enter the cranial cavity through the foramen magnum, where they briefly join the small cranial root from the medulla.
- Jugular foramen. The combined nerve exits the skull through the jugular foramen alongside cranial nerves IX (glossopharyngeal) and X (vagus). After exiting, the spinal and cranial components separate — the cranial root joins the vagus nerve. The spinal accessory nerve proper continues independently.
- Sternocleidomastoid. The nerve passes posteriorly and inferiorly, crossing either through or deep to the SCM. It sends motor branches to the SCM as it passes.
- Posterior triangle of the neck. After traversing the SCM, the nerve enters the posterior triangle — the triangular space bounded by the SCM anteriorly, the trapezius posteriorly, and the clavicle inferiorly. The nerve crosses the posterior triangle superficially, lying on the levator scapulae and covered only by skin, subcutaneous fat, and the investing layer of deep cervical fascia. This superficial course makes it extremely vulnerable to surgical injury during lymph node biopsy, neck dissection, or any procedure in the posterior triangle. The nerve crosses the triangle approximately at the junction of the upper third and lower two-thirds of the posterior border of the SCM.
- Trapezius. The nerve enters the deep surface of the trapezius approximately 5 cm above the clavicle. It travels on the deep surface of the trapezius, sending motor branches to the upper, middle, and lower trapezius from superior to inferior.
Motor Distribution
| Muscle | Action | Notes |
|---|---|---|
| anatomy/muscles/sternocleidomastoid | Unilateral: lateral flexion to same side, rotation to opposite side. Bilateral: cervical flexion, assists forced inspiration | The SCM also receives direct branches from C2-C3 (cervical plexus); accessory nerve provides the primary motor supply |
| anatomy/muscles/upper-trapezius | Scapular elevation, upward rotation, cervical extension/lateral flexion | The most commonly hypertonic muscle in clinical practice |
| anatomy/muscles/middle-trapezius | Scapular retraction | Works with the rhomboids for scapular retraction |
| anatomy/muscles/lower-trapezius | Scapular depression, upward rotation | Often inhibited in upper crossed syndrome; critical for scapular control |
Sensory Distribution
- No cutaneous sensory branches. The spinal accessory nerve is classified as a pure motor nerve. It carries proprioceptive afferents from the SCM and trapezius but does not supply any skin. Accessory nerve injury produces weakness without numbness.
Entrapment Sites
1. Posterior Triangle of the Neck
- Location: Where the nerve crosses the posterior triangle superficially, from the posterior border of the SCM to the anterior border of the trapezius
- Structure: The nerve lies only 1-2 cm deep to the skin, covered by fascia but no muscle. It is the most superficial nerve of surgical significance in the neck.
- Causes: Surgical injury during lymph node biopsy (the single most common cause), radical or modified radical neck dissection, carotid endarterectomy, posterior triangle procedures, and direct trauma to the posterior triangle.
- Presentation: Drooping shoulder on the affected side — the trapezius cannot support the shoulder girdle. Inability to shrug the shoulder (upper trap) and inability to abduct the arm above 90 degrees (trapezius is required for scapulohumeral rhythm). The shoulder appears lower on the affected side. Winging of the scapula — the scapula translates laterally due to unopposed serratus anterior protraction. Pain from chronic stretching of the brachial plexus and shoulder ligaments under the weight of the unsupported arm.
- MT relevance: Accessory nerve palsy is iatrogenic (caused by medical procedures) more often than traumatic. If a patient has shoulder drooping and cannot shrug after a neck procedure, this is almost certainly accessory nerve injury. These patients develop significant secondary muscle pain from compensatory patterns — the levator scapulae, rhomboids, and remaining cervical muscles overwork to compensate for lost trapezius function. MT can address the compensatory pain but cannot restore the nerve.
Clinical Tests
| Test | Procedure | Positive Finding | What It Tells You |
|---|---|---|---|
| Resisted shoulder shrug | Patient shrugs both shoulders against resistance. Compare sides. | Weakness or inability to shrug on the affected side. | Upper trapezius motor function via the spinal accessory nerve. The most obvious test — asymmetric shrug strength is visible and palpable. |
| Shoulder abduction above 90 degrees | Patient abducts the arm through full range. Observe from behind. | Inability to abduct past 90 degrees, with the scapula winging laterally as the arm is raised. | Trapezius function for scapular upward rotation. The deltoid can abduct to 90 degrees, but full abduction requires the trapezius to upwardly rotate the scapula. Loss of this function is the functional hallmark of accessory nerve palsy. |
| SCM strength | Patient rotates the head against resistance (turn the chin toward one shoulder while you resist). | Weakness of rotation to the opposite side. | SCM motor function. The SCM turns the head to the opposite side — resist this movement to test the nerve. Note: C2-C3 cervical plexus contributions may partially preserve SCM function even with accessory nerve injury. |
| Observation of shoulder symmetry | Patient stands with arms at sides. Observe shoulder height, scapular position, and neck contour from anterior and posterior. | Affected shoulder drops lower. Scapula translates laterally (toward the axilla). Upper trapezius contour is visibly diminished. | Trapezius denervation atrophy. In chronic cases, the upper trapezius visibly wastes, and the neck-shoulder contour is flattened on the affected side. |
Clinical Notes
- Lymph node biopsy is the most common cause of accessory nerve palsy. The posterior triangle contains a chain of lymph nodes (the posterior cervical chain) that are frequently biopsied. The nerve crosses directly through this chain. Surgeons who are not meticulous about identifying and preserving the nerve during biopsy can inadvertently cut or stretch it. The injury rate in posterior triangle lymph node biopsies ranges from 3-10%.
- Shoulder drooping creates a cascade of problems. Without the trapezius, the shoulder girdle cannot maintain its normal position. The weight of the arm pulls the shoulder down and forward, stretching the brachial plexus and developing traction symptoms. Secondary muscle overload in the levator scapulae, rhomboids, and remaining rotator cuff muscles produces chronic neck and shoulder pain. The patient's quality of life can be significantly impaired.
- Accessory nerve palsy differs from long thoracic nerve palsy. Both produce scapular dysfunction. Accessory nerve palsy: shoulder drop, lateral scapular winging (scapula moves LATERALLY), inability to abduct above 90 degrees, inability to shrug. Long thoracic nerve palsy: no shoulder drop, medial scapular winging (medial border lifts during pushing), ability to abduct fully but with winging. The direction of scapular displacement is the key visual differentiator.
- SCM hypertonicity is an MT staple — protect the nerve. When treating the SCM (one of the most commonly treated muscles), the spinal accessory nerve travels through or adjacent to the muscle. The nerve is not at significant risk from manual therapy to the SCM, but sustained heavy pressure along the posterior border of the SCM — particularly where the nerve exits to cross the posterior triangle — should be delivered with awareness.
- Torticollis assessment should include accessory nerve testing. If a patient presents with head tilt and rotation, test SCM strength bilaterally. Accessory nerve palsy can present as torticollis because the unilateral SCM weakness allows the contralateral SCM to pull the head unchallenged.
Related Nerves
- anatomy/nerves/long-thoracic-nerve — C5-C7. Supplies the serratus anterior. Both the spinal accessory (trapezius) and long thoracic (serratus anterior) nerves are essential for scapular upward rotation. Damage to either disrupts overhead arm function, but the winging pattern differs.
- anatomy/nerves/dorsal-scapular-nerve — C5. Supplies the rhomboids. The rhomboids compensate for lost trapezius retraction in accessory nerve palsy. Overloading the rhomboids can produce secondary dorsal scapular nerve irritation.
- anatomy/nerves/suprascapular-nerve — C5-C6. Supplies the rotator cuff. Shoulder drooping from accessory nerve palsy changes the mechanical load on the rotator cuff, predisposing to secondary impingement and rotator cuff overload.
Key Takeaways
- Sole motor supply to the trapezius and major supply to the SCM — palsy produces shoulder drooping, inability to shrug, and inability to abduct above 90 degrees.
- Crosses the posterior triangle of the neck superficially — iatrogenic injury during lymph node biopsy (3-10%) is the most common cause of palsy.
- Accessory nerve palsy produces lateral scapular winging (scapula moves laterally); long thoracic nerve palsy produces medial winging (medial border lifts during pushing) — the direction differentiates the two.
- Chronic palsy creates a cascade: shoulder drop, brachial plexus traction, compensatory muscle overload — MTs address the compensatory pain.