Topics

Topic

design image
Accessory Nerve (Cranial nerve XI)

The accessory nerve, also known as cranial nerve XI (CN XI), is a purely motor nerve with a unique dual origin from both the brainstem and upper cervical spinal cord. It is responsible primarily for innervating the sternocleidomastoid and trapezius muscles, playing a vital role in head rotation, shoulder elevation, and neck movement. Understanding its anatomical pathway and imaging characteristics is essential for radiologists, neurologists, and surgeons, especially in the context of trauma, tumors, or surgical planning.

Synonyms

  • Cranial Nerve XI

  • CN XI

  • Spinal Accessory Nerve

  • Accessorius Nerve

Function

  • Provides motor innervation to the sternocleidomastoid muscle (turns head to opposite side, flexes neck)

  • Innervates the trapezius muscle (shoulder elevation, scapular movement)

  • Has minimal or no sensory or autonomic fibers

  • Important for movements involving head rotation and shoulder shrugging

MRI Appearance

  • T1-weighted MRI:

    • Appears as a small, low-to-intermediate signal intensity linear structure

    • Difficult to visualize unless using high-resolution imaging or in the presence of pathology

    • Best seen in the posterior cervical region or jugular foramen in thin sections

  • 3D T2-weighted MRI (e.g., FIESTA, CISS):

    • Appears as a small, linear, hypointense (dark) structure against the bright background of CSF

    • Enhanced nerve visualization at the jugular foramen and along its extracranial course

    • Useful for differentiating the nerve from adjacent vascular and soft tissue structures

CT Appearance

  • Not typically visualized directly due to its small size and soft tissue density

  • Can infer the location based on anatomical landmarks (jugular foramen, posterior triangle of neck)

  • May be indirectly identified in cases of trauma, mass effect, or post-surgical changes

  • High-resolution CT may show the bony canal at the jugular foramen through which the nerve passes

MRI images

Accessory nerve (Cranial nerve XI) MRI 3T axial image 1

MRI images

Accessory nerve (Cranial nerve XI) MRI 3T axial image