Parts and functions

The accessory nerve has two parts: cranial and spinal. Oddly enough, when clinicians refer to the eleventh cranial nerve, or accessory nerve, they almost always mean spinal accessory, which is not really a cranial nerve at all!

Cranial accessory

This arises from a caudal extension of the nucleus ambiguus by rootlets below and in series with those of IX and X. It joins the vagus, from which it is functionally indistinguishable (its name: accessory vagus). Some people hold that the muscles of the larynx and pharynx are innervated by the cranial accessory, leaving the vagus ‘proper’ with parasympathetic fibres only, but this is not certain. Clinically, such distinctions are unnecessary in any case, since when something goes wrong, it tends to affect a large area of the brain stem such that X and XI are likely to be affected along with other nerves. This book considers the cranial accessory no more.

Spinal accessory

Note: This is the one to remember.

This is motor to the muscles bounding the posterior triangle of the neck: sternocleidomastoid and trapezius.

Origin and course of spinal accessory (Fig. 16.1)

• Rootlets from upper four or five segments of spinal cord continue series of rootlets of IX, X and cranial XI.

• Emerge between ventral and dorsal spinal nerve roots, just behind denticulate ligament.

• Ascends through foramen magnum to enter posterior cranial fossa.

• Briefly runs with cranial XI before emerging through jugular foramen (middle compartment).

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