Description

Accessory Nerve Overview

The accessory nerve provides motor function to the sternocleidomastoid and the trapezius. It also joins the vagus nerve to innervate muscles in the pharynx, larynx, and soft palate.

Nucleus and Course

• Nucleus ambiguus:
  • Gives off the cranial root (radix cranialis).
  • Located in the medulla oblongata.
• Spinal nucleus of the accessory nerve (nucleus spinalis nervi accessorii):
  • Spinal roots (radix spinalis) arise from spinal segments C1–C6 lateral to the anterior horn, in the lateral funiculus.
  • These roots ascend laterally to the spinal cord between the motor and sensory roots of the spinal nerve.
  • They reach the posterior cranial fossa via the foramen magnum.

When the cranial root and the spinal root meet, they form the trunk of the accessory nerve (truncus nervi accessorii), which leaves the cranium through the jugular foramen.

Branches

• Internal branch (ramus internus):
  • Joins the vagus nerve and participates in the pharyngeal branch and the recurrent laryngeal nerve of the vagus nerve.
  • Innervates the pharynx, larynx, and muscles of the soft palate.
• External branch (ramus externus):
  • Receives fibers from the anterior ramus of spinal nerves C2, C3, and C4 (of the cervical plexus).
  • Innervates the sternocleidomastoid and trapezius muscles.

Sources

• Singh, I. (2017). Human Neuroanatomy (10th ed.).
• Kozlowski, T. (2017). Memorix Anatomy: The Complete Study Guide. 2nd ed. Thieme Medical Publishers.

Programs

• Complete Anatomy
• Biorender
• PowerPoint
• Camtasia 2023

Pictures and Visuals

Used under licensed permission.

Transcript

Introduction
0:06
What’s up, Taim Talks Med here. Let’s continue our Cranial nerve series. Cranial nerves are
0:11
twelve pairs of nerves that exit the brain and the brainstem, and in this segment,
0:16
we’ll talk detailed about the eleventh cranial nerve, the accessory nerve.
0:21
And we’ll do that by first drawing a quick scheme of the accessory nerve pathway.
0:25
We will talk about the distribution of nuclei and the course of this nerve. And then go through the
0:30
two main branches that this nerve give off. Awesome, so the accessory nerve is purely a
Accessory Nerve Scheme
0:38
motor nerve. It provides motor functions to the sternocleidomastoid and the trapezius,
0:44
and it goes together with the vagus nerve to innervate muscles in the pharynx, larynx and
0:49
soft palate. So it’s accessory to the vagus nerve. Now, this nerve has two main nuclei. Just like the
0:58
glossopharyngeal and the vagus nerve, this nerve is also has motor fibers coming from the nucleus
1:03
ambiguous, located in the medulla oblongata, And, we have the spinal accessory nuclei.
1:10
These are nuclei that lie in the spinal cord, in the 6 upper segments of the spinal cord,
1:15
lateral to the anterior horns of the grey matter. The nucleus ambiguous give rise to the cranial
1:22
root of the accessory nerve, while the spinal accessory nucleus give rise to the spinal root
1:27
of the accessory nerve. The cranial root is going to go out from the brainstem through
1:32
the retro-olivary groove, while the spinal root is going to go up, enter the cranium through foramen
1:39
magnus, and fuse with the cranial root of the accessory nerve, which then form the trunk of
1:45
the accessory nerve. The trunk of this nerve is then going to go through the jugular foramen,
1:51
and split into an internal branch and an external branch. If you remember from the previous video
1:57
when we talked about the vagus nerve. Remember the vagus nerve has the superior ganglion of the vagus
2:03
nerve, and the inferior ganglion of the vagus nerve. The accessory nerve is going to feed some
2:09
motor fibers into the vagus nerve the level of the inferior ganglion, and then go together with the
2:16
pharyngeal branch to form the pharyngeal plexus, and innervate the muscles of the soft palate
2:21
except the tensor veli palatini, and muscles of the pharynx, except the stylopharyngeus. And, in
2:28
some variations the nerve might also go together with the recurrent laryngeal nerve, to help
2:33
innervate the intrinsic muscles of the larynx. The external branch is easy, it’s going to
2:40
descend along the neck, grab some fibers from the anterior ramus of C2, C3 and C4 from the
2:47
cervical plexus, and then go innervate the trapezius and the Sternocleidomastoideus.
2:53
So that is this scheme. It’s just schematic but knowing this will give you the essential
2:58
knowledge of this nerve. Let’s now talk about it in a little more detail.
Nuclei of Accessory Nerve
3:03
Now, here you see the spinal cord, the brainstem, cerebellum, and the cerebrum.
3:08
In the brainstem, you’ll find the nucleus ambiguous, specifically located in the
3:13
medulla oblongata. This nucleus is going to give off the cranial root of the accessory nerve.
3:20
Now, if we take a section of the spinal cord, we’ll see this. This is the grey matter and
3:25
the white matter of the spinal cord, and we see the spinal Nere. Within the grey matter of the
3:31
spinal cord, we can find the anterior horn, giving off motor fibers. And a Postrior horn,
3:38
which receives sensory fibers. In the white matter, we can see the posterior funiculus,
3:44
anterior funiculus and the lateral funiculus. On the lateral aspect of the anterior horn,
3:50
in the lateral funiculus, you’ll find a special nucleus here. And this special nucleus is
3:57
specifically located in the six upper segments of the cervical spinal cord. Some sources say five
4:03
upper, some sources say the upper 4. There are variations, but until C6 is the generally accepted
4:09
region to where this nucleus is located. This nucleus is called the spinal accessory nucleus.
4:16
And it’s going to give off fibers that go in between the motor root and the sensory root
4:21
of the spinal nerve, and it’s going to ascend. So. Here we see the nucleus, located in C1, C2,
4:28
C3, C4, C5 and C6. Again keep in mind there are variations. But essentially. They’re all
Course of Accessory Nerve
4:35
going to give off a branch that’s going to ascend. And all of these branches are going
4:40
to go together through the foramen magnum, and then join the cranial aspect of this nerve. So
4:47
now we got the cranial root and the spinal root going together within the cranium. This
4:52
is how the nuclei of the accessory nerve is distributed along the central nervous system.
4:57
So, this is a little more realistic visualization of how this looks like.
5:02
What happens is, the spinal root is going to ascend as you see here,
5:06
and go through foramen magnum. The spinal root is then going to fuse with the cranial root,
5:12
and form the trunk of the accessory nerve. And the trunk of the accessory nerve is going to exit the
5:18
cranial cavity through the jugular foramen. Now that the accessory nerve has left the
5:24
cranium through the jugular foramen, it’s going to divide into the external branch,
5:29
and the internal branch. The internal branch is going to join the vagus nerve at the region of
Internal Branch
5:35
the inferior ganglion, and provide with extra motor fibers to the pharyngeal branch of the
5:39
vagus nerve to help form the pharyngeal plexus to supply muscles of the pharynx
5:44
and palate. And it might also give some branches to the recurrent laryngeal nerve,
5:49
to innervate most intrinsic muscles of the larynx. So that is that one. The internal branch is going
External Branch
5:57
to go down. On it’s way, it’s going to pick up fibers from the cervical plexus. Specifically it’s
6:03
going to pick up branches from the anterior ramus of the spinal nerve coming from C2,
6:09
C3 and C4 spinal cord segments. And then it’s going to go innervate the Sternocleidomastoideus,
6:16
which help turn the head to the contralateral side and flexion of the neck to the ipsilateral side.
6:22
And it’s going to innervate the Trapezius, which helps with extension of the neck and retraction,
6:28
elevation, and rotation of the scapula. Alright. So that was everything I had for the
Recap
6:34
accessory nerve. Here again you see the scheme for the accessory nerve. It’s just schematic,
6:39
not exactly anatomically correct but I hope it’ll help you gain a good understanding of this nerve.
6:44
So, we now covered the accessory nerve. The next video is going to be about the twelfth
6:49
and last cranial nerve, the hypoglossal nerve. Thank you so much for watching another one of my
6:54
videos. If you enjoyed, learned something from it, please remember to like, comment your favourite
6:58
moment, subscribe. Turn on those notifications. If you are looking for other ways to support,
7:02
go ahead and check out the link in the description box. Have fun ya’ll. Peace.