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# Accessory nerve (XI)

The accessory nerve (cranial nerve XI) moves the sternocleidomastoid and trapezius muscles, which turn the head and shrug the shoulders.

Official name: Accessory nerve

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## What it is

A nerve that runs across the back triangle of the neck to the trapezius.

## What it does

Moves the sternocleidomastoid and trapezius (shrugging); easily injured in neck biopsies.

## Key points

- Most often injured in surgery in the posterior triangle (lymph node biopsy, neck dissection): [trapezius](https://durgical.com/en/anatomy/trapezius-muscle/) weakness with a drooping shoulder, shoulder pain and difficulty raising the arm overhead
- In the posterior triangle it lies just under the investing fascia, so it is easy to cut; it emerges from the posterior border of the sternocleidomastoid a little above the great auricular nerve
- An injury in the posterior triangle spares the sternocleidomastoid, whose branch leaves higher up, so only the trapezius weakens
- Trapezius palsy gives lateral winging of the [scapula](https://durgical.com/en/anatomy/scapula/) on abduction, unlike the medial winging of [serratus anterior](https://durgical.com/en/anatomy/serratus-anterior/) ([long thoracic nerve](https://durgical.com/en/anatomy/long-thoracic-nerve/)) palsy

## Key facts

**Roots**
Spinal accessory nucleus (C1–C5 cord); rises through the foramen magnum, exits the jugular foramen; classic texts also describe a cranial root (caudal nucleus ambiguus) that joins the vagus as the internal branch and supplies the larynx

**Moves**
Sternocleidomastoid, trapezius

**Feeling from**
None

## Relations

**Course**
Rootlets from the upper cervical [spinal cord](https://durgical.com/en/anatomy/spinal-cord/) join and rise through the [foramen magnum](https://durgical.com/en/anatomy/foramen-magnum/), then leave through the [jugular foramen](https://durgical.com/en/anatomy/jugular-foramen/) with the [vagus nerve](https://durgical.com/en/anatomy/vagus-nerve/) (X); the nerve passes in front of the [internal jugular vein](https://durgical.com/en/anatomy/internal-jugular-vein/), supplies and usually pierces the [sternocleidomastoid](https://durgical.com/en/anatomy/sternocleidomastoid-muscle/), crosses the posterior triangle on the [levator scapulae](https://durgical.com/en/anatomy/levator-scapulae/) and reaches the anterior border of the [upper trapezius](https://durgical.com/en/anatomy/descending-part-of-trapezius-muscle/)

**Deep**
In the posterior triangle, the levator scapulae and the [splenius](https://durgical.com/en/anatomy/splenius-capitis-muscle/) in its floor

**Superficial**
Only the investing layer of deep [cervical fascia](https://durgical.com/en/anatomy/superficial-investing-cervical-fascia/) and the [skin](https://durgical.com/en/anatomy/skin/); superficial [neck lymph nodes](https://durgical.com/en/anatomy/lymph-nodes-of-neck/) lie along it

## How to find it

Its surface line runs from about a third of the way down the posterior border of the sternocleidomastoid, across the posterior triangle, to the anterior border of the trapezius in the lower neck; lymph nodes along this line are where it is at risk.

## Examination

- Shoulder shrug against resistance: weakness on one side points to the trapezius supply
- Head turning against the examiner's hand on the chin: weakness turning to one side means a weak sternocleidomastoid on the opposite side (the left muscle turns the face to the right)
- Raising the arm above the head: shoulder droop and lateral winging of the scapula point to trapezius weakness

## On imaging

Not seen on radiographs. High-resolution ultrasound can follow it across the posterior triangle just under the investing fascia, from the posterior border of the sternocleidomastoid, and show a cut nerve, neuroma or scar entrapment after node biopsy. Old injury leaves a thin trapezius on CT or MRI. For skull base tumours, MRI checks the jugular foramen, where it exits.

## Clinical notes

- The injury can go unnoticed at first; shoulder aching and weakness appear in the following weeks as the trapezius wastes
- Modified radical neck dissection preserves it where possible, because sacrificing it leaves lasting shoulder dysfunction
- Jugular foramen lesions affect it together with IX and X (Vernet syndrome)

## Anatomical variants

- It usually passes in front of the internal jugular vein at the top of the neck, less often behind it
- It may pass deep to the sternocleidomastoid instead of piercing it

## Related conditions

- [Cervical lymphadenopathy](https://durgical.com/en/conditions/cervical-lymphadenopathy/)

## Test yourself

Think of the answer, then tap the question to see it.

## Where is the accessory nerve (XI) most often injured, and what is the result?

In posterior triangle surgery such as lymph node biopsy: trapezius weakness with a drooping shoulder, shoulder pain and difficulty raising the arm.

## Why does an accessory nerve injury in the posterior triangle spare the sternocleidomastoid?

The branch to the sternocleidomastoid leaves higher up, so only the trapezius weakens.

## How does scapular winging from accessory nerve palsy differ from long thoracic nerve palsy?

Trapezius palsy gives lateral winging on abduction; serratus anterior (long thoracic) palsy gives medial winging.

## How is the accessory nerve tested at the bedside?

Shoulder shrug against resistance (trapezius) and head turning against the hand (sternocleidomastoid turns the face to the opposite side).

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Educational information, not medical advice. For any symptom, see a health professional.

## Its names

**Official Latin (TA2)**
nervus accessorius TA2 6352

**Official Spanish (DeCS)**
Nervio Accesorio DeCS D000055

**English (TA2)**
Accessory nerve

**Also called**
spinal accessory nerve · cranial nerve XI · nerve of Willis

## Related

- [Jugular foramen](https://durgical.com/en/anatomy/jugular-foramen/)
- [Neck lymph nodes](https://durgical.com/en/anatomy/lymph-nodes-of-neck/)
- [Sternocleidomastoid](https://durgical.com/en/anatomy/sternocleidomastoid-muscle/)
- [Upper trapezius](https://durgical.com/en/anatomy/descending-part-of-trapezius-muscle/)

Sources: Terminologia Anatomica (TA2); [DeCS](https://decs.bvsalud.org/es/ths/resource/?id=60); [Neuroanatomy, Cranial Nerve 11 (Accessory)](https://www.ncbi.nlm.nih.gov/books/NBK507722/).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/anatomy/accessory-nerve/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/accessory-nerve/