Accessory Nerve — Course in Posterior Triangle
Overview
The accessory nerve (CN XI) has two components:
- Cranial root: Joins vagus nerve — supplies pharynx and larynx
- Spinal root: Supplies SCM and trapezius — this is the component relevant to the posterior triangle
Course Through the Posterior Triangle
| Segment | Details |
|---|---|
| Entry into posterior triangle | Nerve emerges from deep surface of SCM (after supplying it), piercing the investing layer of deep cervical fascia at Erb’s point (junction of upper 1/3 and lower 2/3 of posterior border of SCM) |
| Course in triangle | Runs obliquely downward and backward across the floor of the posterior triangle, lying on levator scapulae, covered by investing fascia and fatty areolar tissue |
| Position | Superficial — vulnerable to surgical injury |
| Accompaniment | Branches from C3 and C4 (not motor — proprioceptive/sensory to trapezius) run alongside |
| Exit | Passes deep to the anterior border of trapezius, approximately 5 cm above the clavicle |
| Terminal supply | Motor supply to trapezius |
Relations in Posterior Triangle
- Superficial to: Levator scapulae (floor muscle)
- Deep to: Investing fascia and skin
- Accompanied by: C3, C4 branches (proprioceptive)
- Near: Occipital lymph nodes and cutaneous nerves of posterior triangle
Applied Anatomy
| Condition | Cause | Effect |
|---|---|---|
| CN XI palsy (iatrogenic) | Lymph node biopsy, radical neck dissection, cervical surgery | Trapezius paralysis: shoulder drop, winging of scapula (inferior angle rotated laterally), inability to abduct arm above 90° |
| Penetrating trauma | Stab/gunshot in posterior triangle | Same as above |
| Shoulder droop deformity | Chronic CN XI palsy | Visible on inspection |
The accessory nerve is the most commonly injured nerve in the posterior triangle during surgical procedures.
Differentiation: Winging of Scapula
| Nerve | Muscle Paralysed | Winging Pattern |
|---|---|---|
| Long thoracic nerve (C5,6,7) | Serratus anterior | Medial border of scapula wings out (worse on pushing against wall) |
| Accessory nerve (CN XI) | Trapezius | Inferior angle of scapula rotates laterally (winging with arm abduction) |
Posterior Triangle of Neck | Nerves of Posterior Triangle of Neck