Overview
Gross Anatomy
The
accessory nerve is also known as cranial nerve 11. It arises from the
brainstem, the medulla to be precise, below the glossopharyngeal and vagus
nerve. It emerges lateral to the olive (an ovaloid structure responsible for
pathways of motor learning and hearing).
The nerve
has a spinal division and a cranial division, with the division occurring close
to its origin. The cranial division runs with the vagus nerve (and leaves via
the jugular foramen) and is distributed with the superior laryngeal and
pharyngeal branches of the vagus nerve. The nerve leaves the skull through the
jugular foramen, along with cranial nerves 9 (Glossopharyngeal) and 10 (Vagus).
It also innervates the palatoglossus, which is a muscle that initiates
swallowing; it also prevents saliva spillage into the oropharynx (this is
achieved by maintaining the palatoglossal arch). It is innervated by this
nerve, and is the only tongue muscle that is not innervated by the 12th
nerve.
The spinal
accessory nerve arises from the fibers of the ventral horn cells between C1 and
C5 of the cervical plexus. The nerve then ascends through the foramen magnum
and leaves through the jugular foramen. It leaves the skull via the jugular
foramen and innervates the sternocleidomastoid (rotates the head), and the
trapezius (shrugs the shoulder). It passes in the same sheath of the vagus
nerve, which it is separated from it by a fold of arachnoid. As the nerve exits
the jugular foramen, it passes posterolaterally and runs behind the digastric
and stylohyoid muscles, to reach the superior portion of the
sternocleidomastoid. As the nerve passes behind the muscle, it unites with the
C2 spinal nerve. It unites with the C3 spinal nerve in the posterior triangle
of the neck, and unites with the C4 spinal nerve deep to trapezius and forms a
plexus of nerves. After the nerve has pierced the muscle, it runs across the
posterior triangle of the neck, and terminates on the deep surface of the
trapezius muscle.
Clinical Anatomy
11th nerve palsy- The
accessory nerve is vulnerable to damage due to its relatively superficial
course in the cervical region. Injuries to the nerve result in weakness of the
trapezius and sternocleidomastoid muscles. Trapezius denervation causes
drooping of the shoulder, winged scapula and weakness of shoulder flexion.
Sternocleidomastoid denervation results in weakness of neck rotation, and a
asymmetrical neckline. Damage occurs following lymph node biopsy, carotid
vessel surgery or facelift procedures.
Quick Anatomy
Key Facts
Developmental precursor
Myelencephalon (secondary brain
vesicle)
Muscles
Spinal division Sternocleidomastoid,
Trapezius
Cranial division- Palatoglossus,
muscles of the pharynx
Aide-Memoire
When the anatomy professor asks the
student what the accessory nerve does, he shakes his head and shrugs his
shoulders. The professor says ‘correct!’
These are the actions of the
sternocleidomastoid and trapezius muscles, both innervated by the accessory
nerve.
Summary
The
accessory nerve has a spinal and cranial division. The spinal division
innervates the sternocleidomastoid and trapezius, and the cranial division
joins the vagus nerve and pharyngeal plexus, supplying the muscles of the
region.
References
1.
Frank H.Netter MD: Atlas of Human Anatomy, 5th Edition, Elsevier Saunders, Chapter 1 Head
and Neck
2.
Chummy S.Sinnatamby: Last’s Anatomy Regional and
Applied, 12th Edition, Churchill
Livingstone Elsevier
3.
Richard L. Drake, A. Wayne Vogl, Adam. W.M. Mitchell: Gray’s Anatomy for Students, 2nd Edition, Churchill Livingstone Elsevier
4.
Elliiot L.Manchell: Gray's
Clinical Neuroanatomy:
The Anatomic Basis for Clinical Neuroscience
5.
The Definitive Neurological Surgery Board
Review
By
Shawn
P. Moore, 2005
6.
Human Neuroanatomy
By
James R. Augustine, 2008