What's up? Time Talks Med here. Let's continue our cranial nerve series. Cranial nerves are 12 pair of nerves that exit the brain and the brainstem. And in this segment we will talk detailed about the 11th cranial nerve, the accessory nerve.
And we will do that by first drawing a quick scheme of the accessory nerve pathway. We will talk about the distribution of nuclei and the course of this nerve, and then go through the two main branches that this nerve give off. Awesome! So The accessory nerve is purely a motor nerve. It provides motor function to the sternocleidomastoid and the trapezius.
And it goes together with the vagus nerve to innervate the muscles of the pharynx, larynx and the soft palate. So it's accessory to the vagus nerve. Now, this nerve has two main nuclei.
Just like the glossopharyngeal and the vagus nerve, this nerve has motor fibers coming from the nucleus ambiguous. located in the medulla oblongata. And we have the spinal accessory nuclei. These are nuclei that lie in the spinal cord, in the six upper segments of the spinal cord, lateral to the anterior horn of the gray matter. The nucleus ambiguus give rise to the cranial root of the accessory nerve, while the spinal accessory nucleus give rise to the spinal root of the accessory nerve.
The cranial root is going to go out from the brainstem through the retrooliver groove, While the spinal root is going to go up, enter the cranium through the foramen magnum, and fuse with the cranial root of the accessory nerve, which then will form the trunk of the accessory nerve. The trunk of this nerve is then going to go through the jugular foramen, and then split into the internal branch and an external branch. If you remember from the previous video when we talked about the vagus nerve, remember the vagus nerve has a superior ganglion of the vagus nerve, and the inferior ganglion of the vagus nerve. The accessory nerve is going to feed some motor fibers into the vagus nerve at the level of the inferior ganglion, and then go together with the pharyngeal branch to form the pharyngeal plexus, and further innervate the muscles of the soft palate, except the tensor veli palatini, and muscles of the pharynx, except the stylopharyngeus. And, in some variations, the nerve might also go together with the recurrent laryngeal nerve.
to help innervate the intrinsic muscles of the larynx. The external branch is easy. It's going to descend along the neck, grab some fibers from the anterior ramus of C2, C3, and C4 from the cervical plexus, and then go innervate the trapezius and the sternocleidomastoidus.
So that is this scheme. It's just schematic, but knowing this will give you the essential knowledge of this nerve. Let's now talk about it in a little more detail. Now, here we see the spinal cord, the brainstem, cerebellum, and cerebrum. In the brainstem, you will find the nucleus ambiguous, specifically located in the medulla oblongata.
This nucleus is going to give off the cranial root of the accessory nerve. Now if we take a section of the spinal cord, we will see this. This is the grey matter and the white matter of the spinal cord.
And we see the spinal nerve. In the grey matter of the spinal cord, we can find the anterior horn, giving off motor fibers, and a posterior horn, which receives sensory fibers. In the white matter, we can see the posterior funiculus, anterior funiculus, and lateral funiculus.
On the lateral aspect of the anterior horn, in the lateral funiculus, you will find a special nucleus here. And this special nucleus is specifically located in the six upper segments of the cervical spinal cord. Some sources say 5 upper, some sources say the upper 4. There are variations, but until C6 is the generally accepted region to where this nucleus is located.
This nucleus is called the spinal accessory nucleus, and it's going to give off fibers that go in between the motor root and the sensory root of the spinal nerve, and it's going to ascend. So here we see the nucleus, located in C1, C2, C3, C4, C5, and C6. Again, keep in mind there are variations, but essentially, they're all going to give off a branch that's going to ascend.
And all of these branches are going to go together through the foramen magnum, and then join the cranial aspect of this nerve. So now we got a cranial root and a spinal root going together within the cranium. This is how the nuclei of the accessory nerve is distributed along the central nervous system. Now this is a little more realistic visualization of how this looks like.
Now what happens is, the spinal root is going to ascend, as you see here, and go through the foramen magnum. The spinal root is then going to fuse with the cranial root and form the trunk of the accessory nerve. And the trunk of the accessory nerve is going to exit the cranial cavity through the jugular foramen. Now that the accessory nerve has left the cranium through the jugular foramen, it's going to divide into an external branch and an internal branch.
The internal branch is going to join the vagus nerve at the region of the inferior ganglion, and provide with extra motor fibers to the pharyngeal branch of the vagus nerve to help form the pharyngeal plexus to supply muscles of the pharynx and palate. And it might also give some branches to the recurrent laryngeal nerve to innervate most intrinsic muscles of the larynx. So that is that one.
The internal branch is going to go down, and on its way, it's going to pick up fibers from the cervical plexus. Specifically, it's going to pick up branches from the anterior ramus of the spinal nerve coming from C2, C3, and C4 spinal cord segments. And then it's going to go innervate the sternocleidomastoidus, which helps turn the head to the contralateral side, and flexion of the neck to the ipsilateral side. And it's going to innervate the trapezius, which helps with extension of the neck and retraction of the elevation and rotation of the scapula. Alright, so that was everything I had for the accessory nerve.
Here again you see the scheme for the accessory nerve. It's just schematic, not exactly anatomically correct, but I hope it will help you gain a good understanding of this nerve. So we now covered the accessory nerve. The next video is going to be about the 12th and last cranial nerve, the hypoglossal nerve.
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