Accessory Nerve || Cranial Nerve 11, Pathway, Nerve Lesion

The accessory nerve, also known as the spinal accessory nerve or cranial nerve XI, is a mixed nerve with both motor and sensory functions. It has two components: the spinal portion and the cranial portion.
🧠 Anatomical Course
Spinal Component
- Arises from neurons in the upper spinal cord (C1-C5/C6 spinal nerve roots)
- Enters the cranial cavity through the foramen magnum
- Traverses the posterior cranial fossa and exits the skull through the jugular foramen
- Descends along the internal carotid artery to innervate the sternocleidomastoid muscle
- Crosses the posterior triangle of the neck to supply the trapezius muscle
Cranial Component
- Arises from the lateral aspect of the medulla oblongata
- Exits the cranium through the jugular foramen
- Combines with the vagus nerve (CN X) at the inferior ganglion of the vagus nerve
- Distributed through the vagus nerve, considered part of CN X
💪 Motor Function
The accessory nerve innervates two muscles:
1. **Sternocleidomastoid**: Lateral flexion and rotation of the neck when acting unilaterally, extension of the neck when acting bilaterally
2. **Trapezius**: Elevation and rotation of the scapula during arm abduction (upper fibers), retraction of the scapula (middle fibers), and depression of the scapula (lower fibers)
🏥 Clinical Relevance
🩺 Examination of the Accessory Nerve
- Ask the patient to rotate their head and shrug their shoulders, both normally and against resistance
- Observe for muscle wasting in the sternocleidomastoid and trapezius in cases of long-standing nerve damage
🚑 Accessory Nerve Lesions
- Most commonly caused by iatrogenic injury during procedures such as cervical lymph node excision, biopsy, or central line insertion
- Clinical features include muscle wasting and partial paralysis of the sternocleidomastoid, resulting in inability to rotate the head or weakness in shoulder shrugging
- Damage to the muscles may cause an asymmetrical neckline
Citations:
[1] www.ncbi.nlm.n...
[2] teachmeanatomy...
[3] www.kenhub.com...
[4] www.physio-ped...
[5] www.sciencedir...

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