Which cranial nerve controls neck muscle strength?

Cranial Nerve Control of Neck Muscle Strength

The accessory nerve (CN XI) is the primary cranial nerve that controls neck muscle strength, specifically innervating the sternocleidomastoid and trapezius muscles. 1

Anatomy and Function of the Accessory Nerve (CN XI)

The accessory nerve consists of two components:

• Cranial root: Originates from the nucleus ambiguus within the medulla oblongata
• Spinal root: Originates from the ventral horn of the spinal cord, between C1 and C5 levels

These two components join and enter the jugular foramen through its pars vascularis 1. After exiting the skull, the accessory nerve provides motor innervation to:

• Sternocleidomastoid muscle: Responsible for head rotation and flexion
• Trapezius muscle (upper portion): Controls shoulder elevation, scapular retraction, and neck extension

Clinical Significance

Accessory nerve palsy presents with:

• Weakness and atrophy of the sternocleidomastoid and trapezius muscles
• Decreased shoulder abduction
• Shoulder pain
• Cosmetic disfiguration
• Functional disability 1

Common Causes of Accessory Nerve Palsy

1. Iatrogenic injury: Most common cause of isolated accessory nerve palsy

   • Surgical procedures in the neck region
   • Internal jugular vein cannulation

2. Trauma: Direct injury to the nerve in the posterior triangle of the neck

3. Neoplasms: Tumors affecting the nerve along its course

4. Combined syndromes: Accessory nerve palsy can occur alongside palsies of other lower cranial nerves (CN IX, X, XII) due to lesions in the brainstem or jugular foramen 1

Diagnostic Imaging

For suspected accessory nerve pathology, the preferred imaging modalities are:

• MRI of the neck and posterior fossa: Offers superior visualization of the entire course of the accessory nerve

  • Provides excellent soft tissue contrast
  • Can directly visualize features of neuritis or nerve sheath tumors
  • Can demonstrate denervation changes in affected muscles
  • Can identify the cranial segment in 88% of cases and the spinal segment in 93% of cases using thin-cut high-resolution heavily T2-weighted imaging 1

• CT of the neck: Complementary to MRI

  • Useful for characterizing lesions along the extracranial course of CN XI
  • Can demonstrate atrophy of the trapezius or sternocleidomastoid muscles
  • Provides detailed information on bony structures and foramina 1

• Ultrasound: Can directly visualize the accessory nerve within the posterior cervical triangle

  • Useful for diagnosing spinal accessory nerve injuries
  • Sensitivity may be user-dependent 1

Clinical Pearls and Pitfalls

• The path of the accessory nerve in the neck is highly variable, making it vulnerable to iatrogenic injury during neck surgery 2
• In patients with accessory nerve palsy, MRI can reveal atrophy and denervation signal changes in the trapezius muscle 1
• The accessory nerve has a specific cranio-caudal motor organization:
  • C1 rootlets primarily innervate the sternal part of the sternocleidomastoid
  • C2 rootlets primarily innervate the clavicular part of the sternocleidomastoid
  • C3 rootlets primarily innervate the superior part of the trapezius
  • Lower cervical rootlets innervate the middle part of the trapezius 3
• Anatomical variations exist, including rare cases of duplicated accessory nerves (1.8%) 4

Remember that while the accessory nerve (CN XI) is the primary controller of neck muscle strength, the hypoglossal nerve (CN XII) controls tongue strength and is critical for functions like speech and swallowing 1.