M02.04.011 The accessory nerve

The accessory spinal nerve, also known as the eleventh cranial nerve (CN XI), is a motor nerve that primarily innervates certain muscles involved in head and shoulder movements. It is unique among cranial nerves because it has both a cranial (intracranial) component and a spinal (extracranial) component. Let’s explore the anatomy, origin, course, branches, function, and clinical relevance of the accessory spinal nerve:

Anatomy and Origin of the Accessory Spinal Nerve

1. Cranial Component: The cranial component of the accessory nerve arises from the nucleus ambiguous, which is located within the medulla oblongata of the brainstem. The fibers from the nucleus ambiguus join the vagus nerve (CN X) and emerge as the cranial part of the accessory nerve.
2. Spinal Component: The spinal component of the accessory nerve originates from the motor neurons in the anterior horn of the upper cervical spinal cord segments (C1-C5). These fibers exit the spinal cord and join the cranial component of the nerve as it passes through the foramen magnum.

Course and Branches of the Accessory Spinal Nerve

After its origin, the accessory spinal nerve exits the skull through the jugular foramen, where it is formed by the union of the cranial and spinal components. From there, it descends downward and innervates the following muscles:

1. Sternocleidomastoid Muscle: The accessory nerve provides motor innervation to the sternocleidomastoid muscle, which is a large muscle located on each side of the neck. This muscle is responsible for flexing the neck and rotating the head.
2. Trapezius Muscle: The accessory nerve supplies motor fibers to the trapezius muscle, which is a large triangular muscle that extends from the back of the head down to the upper back and shoulders. The trapezius muscle is involved in various movements of the scapula (shoulder blade) and head.

The function of the Accessory Spinal Nerve:

The primary function of the accessory spinal nerve is to control the sternocleidomastoid and trapezius muscles, which are essential for head and shoulder movements. The sternocleidomastoid muscle allows the head to rotate and tilt, while the trapezius muscle is involved in movements of the shoulders, such as shrugging and retracting the scapulae.

Clinical Relevance of the Accessory Spinal Nerve

1. Accessory Nerve Palsy: Damage or injury to the accessory nerve can result in accessory nerve palsy, which can lead to weakness or paralysis of the sternocleidomastoid and trapezius muscles. This can cause difficulty in turning the head, lifting the shoulders, and other movements involving these muscles.
2. Trapezius Muscle Weakness: Accessory nerve dysfunction can lead to weakness or atrophy of the trapezius muscle, resulting in shoulder asymmetry and decreased range of motion.
3. Neck and Shoulder Pain: Dysfunction of the accessory spinal nerve can contribute to neck and shoulder pain due to impaired muscle function and altered biomechanics.
4. Lymph Node Biopsy: The accessory nerve is sometimes used as a landmark for locating and preserving the spinal accessory nerve during lymph node biopsy procedures, as its course is closely related to lymph nodes in the neck.

Understanding the anatomy and function of the accessory spinal nerve is essential for diagnosing and managing conditions that affect its motor function. Neurological examination and imaging studies may be used to assess nerve function and identify the underlying cause of any associated symptoms. Treatment approaches may include physical therapy, muscle-strengthening exercises, and, in some cases, surgical intervention to repair or bypass damaged nerve pathways.