The accessory nerve, also known as the eleventh cranial nerve (CN XI), is a unique cranial nerve that has both cranial and spinal roots. It is primarily a motor nerve, supplying muscles involved in movements of the neck and shoulders.
Structure
- Components: The accessory nerve is primarily composed of motor fibers. It has two main components: the spinal accessory nerve, arising from the cervical spinal cord, and the cranial accessory nerve, originating from the medulla oblongata in the brainstem.
- Cranial and Spinal Roots: The cranial part originates in the medulla and is often considered a minor part of the nerve. The spinal part arises from the upper segments of the cervical spinal cord (usually C1-C5) and is the major contributor to the accessory nerve.
- Anastomosis: The cranial component of the accessory nerve often joins with fibers from the vagus nerve (CN X) and is believed to contribute to the innervation of muscles in the pharynx and larynx, though this is a subject of anatomical variability.
Location
- Brainstem and Spinal Cord: The cranial part of the nerve originates from the medulla oblongata, while the spinal part has its origins in the anterior horn cells of the cervical spinal cord.
- Exit from Skull: The spinal root fibers ascend through the foramen magnum to exit the skull via the jugular foramen, often alongside the vagus and glossopharyngeal nerves.
- Course in the Neck: After exiting the jugular foramen, the accessory nerve travels down the neck, typically running in the posterior cervical triangle.
- Muscular Innervation: The accessory nerve supplies the sternocleidomastoid muscle, which is involved in rotating and flexing the neck, and the trapezius muscle, which controls various shoulder movements like shrugging.
- Confluence with Vagus: The cranial component of the accessory nerve typically anastomoses with the vagus nerve and contributes to the pharyngeal plexus, though this can vary between individuals.
Functions
The accessory nerve is primarily a motor nerve, and its functions are mainly concerned with supplying specific muscles in the neck and shoulder region.
Motor Functions
Sternocleidomastoid Muscle: One of the major muscles innervated by the accessory nerve, the sternocleidomastoid, is responsible for rotating the head to the opposite side and aiding in flexion of the neck. When both sternocleidomastoids act together, they assist in flexing the neck forward.
Trapezius Muscle: The accessory nerve also supplies the trapezius muscle, which has various functions related to the movement of the shoulder blades. These include elevation (shrugging), retraction (pulling the shoulder blades together), and depression (pulling the shoulder blades down).
Integrated Functions
- Head Movement: Through its innervation of the sternocleidomastoid, the accessory nerve plays a role in various head movements like nodding and turning.
- Upper Body Posture: By controlling the trapezius muscle, the accessory nerve is involved in maintaining proper posture of the upper body.
- Shoulder Mobility: The trapezius muscle, innervated by the accessory nerve, is instrumental for various shoulder movements like lifting, pulling, and reaching.
- Breathing: Although not its primary function, the accessory nerve indirectly aids in deep breathing when the neck is stabilized by the trapezius and sternocleidomastoid muscles, thereby allowing for greater chest expansion.
Specialized Functions
The cranial component of the accessory nerve often contributes fibers to the vagus nerve, and through this, it may play a minor role in speech and swallowing, although this is not its primary function.
Clinical significance
The accessory nerve (CN XI) holds substantial clinical significance due to its primary role in controlling movements of the neck and shoulder.
Neurological Evaluation
- Shoulder Droop: Damage to the accessory nerve often presents as drooping of the shoulder on the affected side, a symptom frequently evaluated during neurological examinations.
- Head Rotation: Impairment can also affect the ability to rotate the head against resistance, another key component in neurological and orthopedic assessments.
Surgical Risks
- Neck Surgeries: The nerve is at risk during various surgical procedures in the neck, such as lymph node biopsies or dissections, and thyroid surgeries. Surgeons must be aware of its course to avoid inadvertent damage.
- Postoperative Complications: Damage to the nerve during surgery can lead to long-term complications, including muscle weakness, impaired shoulder movement, and chronic pain.
Trauma
Physical Injury: Trauma to the neck or upper chest can also damage the accessory nerve, leading to functional impairment.
Rehabilitative Concerns
Physical Therapy: In cases of accessory nerve damage, physical therapy may be necessary to improve shoulder mobility and strength, as well as to address postural issues.
Orthotic Devices: Some patients may require orthotic devices to aid in shoulder stability if nerve damage is severe.
Diagnostic Importance
Electromyography (EMG): EMG studies may be used to confirm suspected cases of accessory nerve dysfunction, particularly when symptoms like shoulder weakness or muscle wasting are present.
