Accessory nerve

The accessory nerve (CN XI) is the eleventh of twelve paired cranial nerves. It is unique in that it has both a cranial and a spinal root. The accessory nerve is responsible for innervating the sternocleidomastoid muscle and the trapezius muscle, which are involved in movements of the head, neck, and shoulders.

Anatomy

The accessory nerve is composed of two parts: the cranial part and the spinal part.

Cranial Part

The cranial part of the accessory nerve originates from the medulla oblongata. It emerges from the medulla and briefly joins the vagus nerve (CN X) before separating again to innervate the muscles of the larynx, pharynx, and soft palate.

Spinal Part

The spinal part of the accessory nerve arises from the upper five or six segments of the spinal cord. The nerve fibers emerge from the spinal cord, ascend through the foramen magnum, and then join the cranial part for a short distance before separating again. The spinal part then travels to innervate the sternocleidomastoid muscle and the trapezius muscle.

Function

The primary function of the accessory nerve is motor innervation. It controls the movements of the sternocleidomastoid muscle, which allows for the rotation and flexion of the head, and the trapezius muscle, which facilitates shoulder elevation and adduction.

Clinical Significance

Damage to the accessory nerve can result in weakness or paralysis of the sternocleidomastoid muscle and trapezius muscle. This can lead to difficulties in head rotation, shoulder elevation, and other movements involving these muscles. Common causes of accessory nerve damage include surgical procedures in the neck region, trauma, and certain neurological conditions.

Additional Images

• 

  Plan of the cervical plexus.

See Also

• Cranial nerves
• Sternocleidomastoid muscle
• Trapezius muscle
• Medulla oblongata
• Spinal cord

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