Anatomy of the spinal accessory (CN XI) and hypoglossal (CN XII) nerves

Chapters:

Watch video only

Introduction0:00–0:28

The spinal accessory nerve, or the eleventh cranial nerve, and the hypoglossal nerve, or the twelfth cranial nerve, are two purely motor cranial nerves that innervate muscles in the neck, as well as the tongue.
Specifically, the spinal accessory nerve provides somatic motor innervation to the sternocleidomastoid and trapezius muscles, while the hypoglossal nerve provides somatic motor innervation to almost all the muscles of the tongue.Let’s start with the spinal accessory nerve.

CN XI0:28–1:38

The somatic motor pathway of the spinal accessory nerve starts in the cerebral cortex. Then, the axons travel to the spinal accessory nucleus, which is located in the cervical spinal cord, specifically in the posterolateral aspect of the anterior horn.
The axons travel through the rootlets from the first five or six cervical segments of the spinal cord and form the spinal accessory nerve.
The spinal accessory nerve ascends through the foramen magnum, enters the posterior cranial fossa, and then exits through the jugular foramen.
It continues its journey down along the internal carotid artery and runs deep to the sternocleidomastoid muscle, which it supplies.
Then, it emerges from the sternocleidomastoid muscle through its posterior border, and continues inferiorly in the posterior triangle of the neck to reach and supply the trapezius muscle through multiple branches.
The spinal accessory nerve is located superficially in the posterior triangle of the neck, and you can identify it using two landmarks: one third of the way down from the posterior border of the sternocleidomastoid muscle and one third of the way up from the anterior border of the trapezius muscle.

CN XII1:38–3:13

Now, let’s switch gears and look at the hypoglossal nerve, which supplies all the muscles of the tongue, except the palatoglossus.
The somatic motor pathway of the hypoglossal nerve starts in the cerebral cortex. Then, the axons travel to the hypoglossal nucleus, located in the medulla, and arise as the hypoglossal nerve from rootlets along the medulla and exit the cranium through the hypoglossal canal.
After exiting the cranium, the hypoglossal nerve runs inferiorly, posterior to the mandible and loops around a branch from the external carotid artery called the occipital artery.
Around the angle of the mandible, it curves anteriorly and travels deep to the posterior belly of the digastric muscle and enters the oral cavity.
Once in the oral cavity, it runs on the floor lateral to the tongue and can be found running with the lingual vein, lateral to the hyoglossus muscle.
Along its path in the oral cavity, it gives branches to supply the muscles of the tongue, with the exception of the palatoglossus muscle, which is innervated by the vagus nerve instead.A final note about the hypoglossal nerve.
During its course within the neck, nerve fibres from the C1 spinal nerve hitch a ride with the hypoglossal nerve to supply muscles associated with the hyoid bone.
Most of these fibres form part of the ansa cervicalis which is a nerve loop that is part of the cervical plexus, and it innervates most of the infrahyoid muscles.
A separate branch of the C1 spinal nerve hitchhikes along CN XII to innervate the thyrohyoid and geniohyoid muscles.All right, as a quick recap...

Review3:13–3:42

The spinal accessory nerve is the eleventh cranial nerve and it has only a motor function to supply the sternocleidomastoid and trapezius muscles.
The hypoglossal nerve is the twelfth cranial nerve and it has only a motor function to supply all the muscles of the tongue, except the palatoglossus muscle.
C1 fibers also travel with it to supply the infrahyoid muscles and one suprahyoid muscle.