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] https://www.ncbi.nlm.nih.gov/books/NB...
[2] https://teachmeanatomy.info/head/cran...
[3] https://www.kenhub.com/en/library/ana...
[4] https://www.physio-pedia.com/Accessor...
[5] https://www.sciencedirect.com/topics/...