The Accessory Nerve

 Volume 7, Number 2 (June 2012) , 125-130  Review Article  The Accessory Nerve  Rezigalla AA*, EL Ghazaly A*, Ibrahim AA*, Hag Elltayeb MK*     

http://www.sudjms.net/issues/7-2/html/10)The%20Accessory%20Nerve.htm

 ¹. De-innervations of the muscles supplied by SAN and integrated in the movements of the shoulder joint, often result in shoulder dysfunction. Usually the result is shoulder syndrome which subsequently affects the quality of life¹.   The modified radical neck dissections (MRND) and selective neck dissection (SND) intend to minimize the dysfunction of the shoulder by preserving the SAN, especially in supra-hyoid neck dissection (Level I-III±IV) and lateral neck dissection (level II-IV)^{2, 3}.  This article aims to focus on the SAN to increase the awareness during MRND and SND.

 

 

Keywords: Spinal accessory, Sternocleidomastoid, Trapezius, Cervical plexus.

 

 

The accessory nerve is a motor nerve but it is considered as containing some sensory fibres. It is formed in the posterior cranial fossa by the union of its cranial and spinal roots ^4-8 (i.e. the internal and external branches respectively^9,10) but these pass for a short distance only¹¹. The cranial root joins the vagus nerve and considered as a part of the vagus nerve, being branchial or special visceral efferent nerve^4,5,9,11. The spinal root may be considered as general somatic¹², special visceral efferent^7,13 or mixed, depending on the view taken of the embryological origin of the sternocleidomastoid and trapezius muscles which it supply¹¹.

The custom of describing the two roots as a single cranial nerve has been followed in the standard references of anatomy. The spinal root is assumed purely motor, but there is an evidence for the presence of afferent fibers (proprioceptive) provided by the occurrence of ganglion on the nerve in the prenatal and early postnatal human materials¹⁰. The nerve may communicate with the dorsal roots of the upper cervical spinal nerves, although such observations are  not confirmed in adult materials¹¹.

 

The Cranial Root:

 The cranial root is the smaller, attached to the post-olivary sulcus of the medulla oblongata (Fig.1)^8,10 and arises forms the caudal pole of the nucleus ambiguus (SVE)^{4, 7, 9} and possibly also of the dorsal vagal nucleus^{11, 14}, although both of them are connected¹¹.

The nucleus ambiguus is the column of large motor neurons that is deeply isolated in the reticular formation of the medulla oblongata¹¹, it lies midway between the spinal nucleus of the trigeminal nerve and the inferior olivary complex. The nucleus ambiguus continues downwards into the spinal nucleus of the accessory nerve. The lower part of the nucleus ambiguus gives rise to the cranial root of the accessory nerve^4,9,11,12.

The cranial root runs lateral to the jugular foramen, perhaps there is an interchanging fibers here with the spinal root¹¹, in which it unites for a short distance (Fig.1).  Also it connects with the superior vagal ganglion⁴. It traverses the jugular foramen, separates from the spinal root and continues over the inferior vagal ganglion, in which it adheres¹¹ and becomes inseparable from the vagus nerve below this level¹⁰. Usually the cranial root distributed mainly in the pharyngeal plexus, the external laryngeal nerve¹⁰, and the recurrent (inferior) laryngeal branches of the vagus nerve^{7, 9}, and this is probably the source of the vagal motor fibres which run in the pharyngeal plexus branches to the palatal muscles.

Figure.1: Shows the cranial nerves in the posterior and middle cranial fossae. (Kyle E & Lyun J. Human interactive tutorial and reference. 1995. University of Florida & Gold Standard Multimedia) ¹⁷

Some filaments continue into the vagus below the inferior vagal ganglion to be distributed with recurrent laryngeal nerve and also with the cardiac branches of the vagus¹¹.

 

The Spinal Root:

  The spinal root arises from the spinal nucleus of the accessory nerve^{4, 11}. The spinal nucleus of the accessory nerve is located laterally in the spinal anterior grey column. This nucleus extends from midlevel of the pyramidal decussation at the medulla and downwards as far as the upper fifth⁴ (or sixth) cervical segment of the spinal cord (mainly C2, 3 and 4)^4,6,9,11. Inferiorly the spinal nucleus of the accessory nerve forms the lateral processes of the anterior grey horns of the spinal cord.

  The spinal nucleus of the accessory nerve receives projections fibre from variety of sources. It is thought to receive corticospinal fibres from both cerebral hemispheres¹². The corticospinal fibres  designed for the sternocleidomastoid neurons in the cervical part of the spinal cord undergo double decussation in the brainstem. Consequently the motor cortex on one side controls the sternocleidomastoid of that side.

   Fibers arising from the nucleus pass laterally traversing the lateral white columns of the spinal cord. The fibres emerge at the lateral side of the cervical part of the spinal cord⁹between the ventral and the dorsal spinal roots^{4, 6, 11} (Fig.2), and ascend on the side of the spinal cord joining together forming a nerve trunk⁷. The nerve trunk ascends in the subarachnoid space behind the denticulate ligaments anterior to the dorsal roots of the spinal nerves to reach the foramen magnum. Here, it enters the skull through foramen magnum behind the vertebral artery¹¹ over the top of denticulate ligament^{6, 8} to reach the posterior cranial fossa. It turns upward and laterally over the lateral margin of foramen magnum ¹¹. It unites with the cranial root just medial to the jugular foramen (endocranial opening of the jugular foramen)⁶, to form the trunk of the accessory nerve (Fig.1).

  Trunk of the accessory nerve traverses the jugular foramen in the middle compartment (pars nervosa) in a single dural sheath with the vagus nerve^{4, 6}, but separated from it by a fold of arachnoid mater¹¹. At the endocranial opening of the jugular foramen, the vagus and the accessory nerves could not be distinguished as separate nerves. Even the course of the vagus and accessory nerves deep in the foramen could not be determined by dissection or on CTscan¹⁵. The two nerves travel together as they enter the foramen in positions changing from anterior to and inferior to the jugular spine of the temporal bone¹⁵. In the jugular foramen the vagus nerve may receives one or two rami from the cranial root or join it for short distance¹¹.

At its exit from the jugular foramen the two roots of the accessory nerve separate. The spinal root turns posterolaterally, usually posterior to the internal jugular vein. Here the nerve crosses the transverse process of the atlas¹⁴ and is crossed itself by the occipital artery¹¹. The accessory nerve then descends obliquely, medial to the styloid process, stylohyoid, and posterior belly of digastric muscle. With the superior sternocleidomastoid branch of the occipital artery it reaches the upper part of the sternocleidomastoid muscle, enters its deep surface⁸, between its upper two quarters¹¹, and joins branches of the second and third cervical spinal nerves⁶ (Fig.3) to supply the muscle with motor and sensory fibers respectively¹¹. The spinal root emerges from the sternocleidomastoid muscle a little above the midpoint of the muscle’s posterior border¹⁴, then crosses the posterior triangle of the neck with characteristic wavy course being adherent to the inner surface of the investing deep cervical fascia⁷. 

 

Figure.2: Shows the base of the brain. (Kyle E & Lyun J. Human interactive tutorial and reference. 1995. University of Florida & Gold Standard Multimedia).     

Here the nerve lies on levator scapulae muscle^{8, 14} and is separated from this muscle by the prevertebral layer of the deep cervical fascia and adipose connective tissue¹¹. In the posterior triangle of the neck, the spinal accessory nerve is comparatively superficial¹¹, being closely related to the superficial cervical lymph nodes (cervicales laterales^{6, 10}) and receives branches from the ventral rami of the third and fourth cervical spinal nerves. About five centimeters  above the clavicle it passes behind the anterior border of trapezius muscle^{6, 14} at the junction of the middle and lower thirds of the muscle’s anterior border and forming a plexus on its deep surface. From this plexus the trapezius muscle is innervated¹¹. 

 

Figure.3: Shows the sensory branches from the cervical plexus (C3 and4) to Accessory nerve. (Frank H & Myers H. Interactive atlas of Human anatomy. 1995, Novartis medical education).

The spinal root of the accessory nerve is the sole motor supply to the sternocleidomastoid muscle⁹. Branches from the second and third cervical spinal nerves convey proprioceptive fibers from the muscle¹¹. The spinal root of the accessory nerve supplies the descending part of the trapezius muscle, whereas the transverse and ascending parts of the muscle are innervated by both spinal accessory nerve and the cervical plexus¹⁸. But, the innervations of the trapezius muscle is less certain; some consider that the third and fourth cervical spinal nerves are purely proprioceptive fibers¹⁴, but others consider that the cervical nerves also supply motor fibres to the trapezius muscle¹¹.

Ultrasonography (Fig.4) allows visualization of the normal accessory nerve as well as changes after accessory nerve palsy¹⁹. The ultrasonography uses a 5-to12-MHz linear transducer. On High-resolution ultrasonography (HRUS), the accessory nerve appeared as a small hypoechoic oval structure in the transverse plane and as a hypoechoic linear structure in the longitudinal plane. The diameter of the nerve is approximately one mm.  It is best identified at the lateral cervical (posterior) triangle of the neck, after identification of the trapezius and sternocleidomastoid muscles, which aid as guiding structures. This appearance was previously reported by Silvestri et al.²⁰, who stated that the appearance would most probably be caused by a number of neuronal fascicles embedded in the epineurium. Furthermore, they stated that a small nerve such as the recurrent laryngeal nerve appears hypoechoic.

 

Figure.4: Shows Ultrasonography of the accessory nerve, it appears like tubular structure. The arrows show the nerve course. SCMM - Sternocleidomastoid muscle. LSM- Levator scapulae muscle.

 
 

 

Acknowledgment

The authors thank Dr. Fred J. Laine (Department of Radiology, Medical Collage, Commonwealth University. Virginia) for the preparation of the ultrasonography materials. 

 

References

1.      Van Wilgern CP, Disjkstra PU, Van Der Laaan BF, et al. Shoulder and neck morbidity in quality of life after surgery for head and neck cancer. Head Neck. 2004; 26: 839-44.

2.      Mathew L, Hinsley MD, Hartig GH. Anatomic relationship between the spinal accessory nerve and the internal jugular vein in the upper neck. Otolaryngology – Head and neck surgery. 2010; 143:239-41.

3.      Rafferty MA, Goldstain DR, Brown DH et al. The steromastoid branch of the occipital artery: Surgical landmark for the spinal accessory nerve in elective neck dissections. Otolaryngology – Head and neck surgery. 2005; 133:874-76.

4.      Ali QM. Neurological Anatomy 1^st Ed. India. Macmillan India Limited 1993; P: 88, 226-28.

5.      Carlson BM. Human Embryology and Development Biology. Mosby Publishers. 1998; P: 234, 221, 232, 213.

6.      Sinnatamby C. Last’s Anatomy Regional and Applied 10^th Ed.  Churchill Livingstone 1999; P: 388, 494-95,497-98.  

7.      Snell RS. Clinical Neuroanatomy for Medical Students 5^th Ed. NY. Lippincott Williams & Wilinks. 1995; P: 198, 199, 332, 354, 355, 362.

8.      Snell RS. Clinical Anatomy for Medical Students 6^th Ed. Lippincott Williams & Wilinks. 2000; P: 778-80, 801, 790, 652.

9.      Carpenter M, Sutin J. Human Neuroanatomy 8^th Ed. Williams & Wilkins. 1989;  P: 47, 341-42.

10.  Prives M, Lysenkov N, Bushkovich V. Human Anatomy- The Science of the Nervous System 3^rd Ed Volume II. Moscow. MIR Publisher. (1989) P: 181, 200, 306-7, 786, 277-80.

11.  Berry MM, Standring SM, Bannister LH. Nervous System: In Gray’s Anatomy 38^th Ed. Willison PL (editor). Churchill Livingstone. (1995) P: 1253. 

12.  Henary B. Anatomy of the Spinal Accessory Nerve Plexus: Relevance to Head and Neck Cancer and Atherosclerosis. Experimental Biology and Medicine. (2002) 227(8):570-578.

13.  DeMyer W. (1988). Neuroanatomy – The National Medical Series For Independent Study. U.S.A. Wiley Medical Publication. (2002). P: 139-41, 150-51.

14.  Roman GJ. Cunningham’s Manual of Practical Anatomy 15^th Ed. Oxford University Press. (1980). P: 22-5, 42, 50.

15.  Rubinstein D, Burton BS, Walker AL. The anatomy of the inferior petrosal sinus, glossopharyngeal nerve, vagus nerve, and accessory nerve in the jugular foramen.American Journal of Neuroradiology. 1995; 16(1):185 94.

16.  Prades JM, Timoshenko A, Dumollard JM  et al. High duplication of the internal jugular vein: Clinical incidence in the adults and surgical consequence, a report of three clinical cases. Journal of Surgery Radiological Anatomy. 2002; 24(2):129-32.

17.  Gardiner KJ, Irvine BW, Murry A. Anomalous relationship of the spinal accessory nerve to the internal jugular vein. Journal of Clinical Anatomy. 2002; 15(1): 62-3.

18.  Zierner AC, Zelenk I, Burian M. How do the cervical plexus and the spinal accessory nerve contributed to the innervations of the trapezius muscle? As seen from using Sihler’s stain.Arch Otolaryngology Head Neck Surgery. 2001; 127(10):1230-2.

19.  Bonder G, Harpf C, Gardetto A et al. Ultrasonography of the accessory nerve: normal and pathologic findings in cadavers and patents with iatrogenic accessory nerve palsy. J Ultrasound Med. 2002; 21 (10):1159-63.

20.  Silvestri E, Martinoli C, Derchi LE et al. Echo texture of peripheral nerves: Correlation between US and histologic findings and criteria to differentiate tendons. Radiology. 1995;197:291-296.

 

 

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*Department of Human Anatomy, Faculty of Medicine and Health sciences, Oumdurman Islamic university.