Accesory Nerve

It has 2 parts, the cranial component and the accessory component

The cranial accessory nerve arise from the nucleus ambuguus at the tegmentum of medulla oblongata posterior to the olive. It received bilateral innervation from the cortoticonuclear fibers.

The spinal accessory nerve arise from the ventral horn of  C1-6.  The SCM receives ipsilateral corticospinal innervation, whilst the Trapezius receive contralateral corticospinal innervation.

The cranial accessory nerve exit the medulla oblongata at the post olivary sulcus below vagus nerve and fluocullus and enter jugular foramen.

Within the jugular foramen it stay with vagus to form vagus accessory complex, which is separated from the glossopharyngeal nerve via a fibrous band / bony crest.

It converge with the spinal accessory nerve briefly eventually merge with vagus nerve.

The spinal accessory nerve arise from the ventral horn of C1-6. The motor nerve ascends to reach the infratentorial fossa via foramen magnum and merge with vagus nerve briefly (as just mentioned) before taking a posteroinferior course to supply the upper surface of SCM C1-2. Then it travels posterolaterally  to the posterior triangle of the neck and finally supply trapezius C2-4. 

In posterior triangle the accessory nerve is embedded in between 2 deep cervical facia within the loose connective tissue.

The spinal accessory nerve was found within 3.6 cm (mean, 1.43 cm) above Erb’s point. 

Erb’s point is where the bundle of sensory nerves from the cervical plexus emerges from the posterior border of the sternocleidomastoid muscle, midway between the mastoid process and the clavicle. 

The distance between the spinal accessory nerve entering the trapezius muscle and the clavicle was between 2 To 7 cm (mean, 4.5 cm)

Previous study  in Caucasians shows the spinal accessory nerve exits within 2 cm above the posterior border of the muscle.

In study done on thai people, the nerve was found below Erb’s point in  one neck, and at or above Erb’s point

Studies in Caucasion shows that the point where accessory nerve innervates the trapezius muscles can be found 2-4 cm away from clavicle. But in thai people, the measurement ranges 2-7cm. 

(Dr Pitchit, Chiang Mai University, 2005. Identification of Accessory Nerve in Posterior triangle)