Жылқы бездің XI жұлындық тамыры: Бас және иық қозғалыстары

Cranial nerve XI, for head and shoulder movements

The accessory nerve, also known as the eleventh cranial nerve, cranial nerve XI, or simply CN XI, is a cranial nerve that supplies the sternocleidomastoid and trapezius muscles. It is classified as the eleventh of twelve pairs of cranial nerves because part of it was formerly believed to originate in the brain. The sternocleidomastoid muscle tilts and rotates the head, whereas the trapezius muscle, connecting to the scapula, acts to shrug the shoulder. Traditional descriptions of the accessory nerve divide it into a spinal part and a cranial part. The cranial component rapidly joins the vagus nerve, and there is ongoing debate about whether the cranial part should be considered part of the accessory nerve proper. Consequently, the term "accessory nerve" usually refers only to nerve supplying the sternocleidomastoid and trapezius muscles, also called the spinal accessory nerve. Injury can cause wasting of the shoulder muscles, winging of the scapula, and weakness of shoulder abduction and external rotation. These fibres join together to form rootlets, roots, and finally the spinal accessory nerve itself. The formed nerve enters the skull through the foramen magnum, the large opening at the skull's base. The nerve travels along the inner wall of the skull towards the jugular foramen. Leaving the skull, the nerve travels through the jugular foramen with the glossopharyngeal and vagus nerves. After leaving the skull, the cranial component detaches from the spinal component. The spinal accessory nerve continues alone and heads backwards and downwards. In the neck, the accessory nerve crosses the internal jugular vein around the level of the posterior belly of digastric muscle. As it courses downwards, the nerve pierces through the sternocleidomastoid muscle (approximately 1cm above Erb's point) while sending it motor branches, then continues down until it reaches the trapezius muscle (entering at the junction of the middle and lower third of the anterior border of the trapezius) to provide motor innervation to its upper part.

The fibres that form the spinal accessory nerve are formed by lower motor neurons located in the upper segments of the spinal cord. This cluster of neurons, called the spinal accessory nucleus, is located in the lateral aspect of the anterior horn of the spinal cord, and stretches from where the spinal cord begins (at the junction with the medulla) through to the level of about C6. The lateral horn of high cervical segments appears to be continuous with the nucleus ambiguus of the medulla oblongata, from which the cranial component of the accessory nerve is derived. Traditionally, the accessory nerve is described as having a small cranial component that descends from the medulla and briefly connects with the spinal accessory component before branching off of the nerve to join the vagus nerve. A study, published in 2007, of twelve subjects suggests that in the majority of individuals, this cranial component does not make any distinct connection to the spinal component; the roots of these distinct components were separated by a fibrous sheath in all but one subject.

The spinal component of the accessory nerve provides motor control of the sternocleidomastoid and trapezius muscles. The nerve fibres supplying sternocleidomastoid, however, are thought to change sides (decussate) twice. This means that the sternocleidomastoid is controlled by the brain on the same side of the body. Contraction of the stenocleidomastoid fibres turns the head to the opposite side, the net effect meaning that the head is turned to the side of the brain receiving visual information from that area. This is in line with the observation that the spinal accessory nucleus appears to be continuous with the nucleus ambiguus of the medulla. Others consider the spinal accessory nerve to carry general somatic efferent (GSE) information. Still others believe it is reasonable to conclude that the spinal accessory nerve contains both SVE and GSE components.

The accessory nerve is tested by evaluating the function of the trapezius and sternocleidomastoid muscles. The trapezius muscle is tested by asking the patient to shrug their shoulders with and without resistance. The sternocleidomastoid muscle is tested by asking the patient to turn their head to the left or right against resistance. Damage at any point along the nerve's course will affect the function of the nerve. The nerve is intentionally removed in "radical" neck dissections, which are attempts at exploring the neck surgically for the presence and extent of cancer. Attempts are made to spare it in other forms of less aggressive dissection. Surgical management includes neurolysis, nerve end to end suturing, and surgical replacement of affected trapezius muscle segments with other muscle groups, such as the Eden Lange procedure.

English anatomist Thomas Willis in 1664 first described the accessory nerve, choosing to use "accessory" (described in Latin as nervus accessorius) meaning in association with the vagus nerve. In 1848, Jones Quain described the nerve as the "spinal nerve accessory to the vagus", recognizing that while a minor component of the nerve joins with the larger vagus nerve, the majority of accessory nerve fibres originate in the spinal cord. In 1893 it was recognised that the heretofore named nerve fibres "accessory" to the vagus originated from the same nucleus in the medulla oblongata, and it came to pass that these fibres were increasingly viewed as part of the vagus nerve itself.