Course, branches and nuclei of the accessory nerve.
Hello everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where this time we're going to be talking about the accessory nerve. What we're going to be doing on this tutorial is essentially exploring this image that you see now on the screen. This image here is one of the main focus. You can see the accessory nerve here highlighted in green as well as some elements of the skull here – you can see the foramen ovale. All of these structures you can see that they're cut. You see also here some of the cervical vertebrae cut. The spinal cord in all its glory here. You see some muscles cut that we're going to be talking about them later on on this tutorial – just to give you an idea of what we're looking at. And we're also looking at all of these from an anterior view, so keep that in mind. So again just as a reminder, the nerve that we're looking at here that is highlighted is the accessory nerve.
As you know, there are twelve cranial nerves that arise from the brain and the brainstem, and the accessory nerve is the eleventh cranial nerve. Sometimes, we also mention it as CN XI which is the Roman numerals for eleven – so CN cranial nerve 11. The accessory nerve consists of both efferent motoric and afferent proprioceptive fibers. And what does that mean? This is what we're going to be talking about next here on our image here that you see again the nerve highlighted here in green but right now we're looking at a basal view of the brain where you see other cranial nerves as you can see in yellow and the accessory nerve in green. So let's briefly talk about the efferent fibers.
The term efferent refers to the fact that these nerve fibers carry signals or nerve impulses from the central nervous system towards your muscles, and also other organs to then effect movement. These fibers are known as efferent motoric fibers because they effect movement. So essentially what happens is what you see here. So to put it very simply, a signal comes from the central nervous system or CNS or brain and is then carried to the muscle or organ which results in an action or movement happening. One thing important to mention is that there are two types of efferent motor fibers. One type is known as general somatic efferent fibers or GSE which are found in the nerves that supply skeletal muscle, and the second type is known as general visceral efferent fibers (GVE) and as the name indicates – visceral – these fibers are then found in the nerves that supply cardiac muscle, smooth muscle and glands – so muscle that we find in organs. The word visceral means organs.
Now let's go to this image here where we have a close up of the accessory nerve – so this is a lateral view – showing many had cuts here but you can clearly the accessory nerve highlighted in green in all its glory and then you have here the foramen ovale where other nerves are passing through and you see here the skull cut, a bit of the brainstem and some of the muscles, some of the pharyngeal muscles, just to give you an idea of what we're looking at right now because we're going to be talking about the afferent fibers.
The term afferent refers to the fact that these types of nerve fibers carry signals or nerve impulses from sensory stimuli at the periphery towards then the central nervous system. So essentially what we're saying is that in these fibers the signal will be carried in the opposite direction as we've seen in the efferent fibers – so that is to say that from the periphery or limbs all the way to the central nervous system or to your brain. The stimulus in this case can be any one of the basic senses such as taste, touch, temperature, sound and visual stimuli, etcetera. One point that I would like to add about afferent fibers just briefly is that they can be divided into four different types: general somatic afferent fibers, general visceral afferent fibers, special somatic and special visceral – all of these we will then discuss in a lot more detail on separate tutorials but I just wanted to have them here as reminders.
Now let's go back here to talking about the accessory nerve as I already mentioned is the eleventh cranial nerve. Its fibers will be innervating a lot of structures including very important muscles – the sternocleidomastoid muscle. This one that if you remember from the first slides one of the muscles that we see on this image here cut – the sternocleidomastoid muscle – and now it's highlighted in green as you can see. So this is a superficial muscle of the neck that you can clearly see and feel. The other one – a very important muscle of the shoulder girdle and your back – the trapezius muscle that you also see here highlighted in green. So these are two muscles that will be innervated by the accessory nerve.
Going back to here this image of the accessory nerve highlighted in green, I wanted to mention that it's traditionally divided into two parts. There is a cranial part of the nerve and also a spinal part – both of them we're going to be discussing on the next slides. We're going to start talking about the very first one here on the list. This structure that you see highlighted in green – the cranial root of the accessory nerve – right here. The cranial part of the accessory nerve, it arises from the lateral part of or the lateral aspect of the medulla oblongata as you can see here. This part specifically arises from the nucleus ambiguus. Now it's important to mention that this part of the nerve – the accessory nerve – is rarely mentioned when discussing the eleventh cranial nerve because it is the smaller part of the nerve and the fibers from it go on to then join fibers of the vagus nerve.
If you remember from our list, the next part is going to be then the spinal part of the accessory nerve and right now on the image, we have then the spinal root of the accessory nerve highlighted in green as you can see right here. The spinal part of the accessory nerve is also known as the spinal accessory nerve and it arises from the upper part of the cervical spinal cord from the spinal nerve roots, C1 to C5 or to C6 as well. This part of the accessory nerve is the one usually discussed in literature when talking about the nerve – the accessory nerve.
Now that we have a better understanding of the meaning of efferent and afferent fibers, let's have a look at the efferent pathway of the accessory nerve from initiation of impulses in the brain to their termination in the periphery. Now let's start with this structure here that you see now highlighted in green, this is known as the nucleus ambiguus. Here is where the afferent motoric fibers of the accessory nerve arise from. And as you can see here, the nucleus ambiguus is located at the tuberculum gracile of the brainstem. The fibers that arise from the nucleus ambiguus comprise then the cranial part of the accessory nerve as we've seen on the previous slides when we talked about the cranial part of the accessory nerve.
On the next image what we're going to be doing is then to go to the dorsal view of the brainstem. You can see here then the cerebellum cut to show you this highlighted structure which is the gracile fasciculus. More fibers of the accessory nerve will emerge from the gracile fasciculus seen here from this posterior view of the brainstem. These fibers coming from the gracile fasciculus join to form the spinal root of the accessory nerve which first ascends to then enter into the skull through the foramen magnum. Remember, we also talked about this on previous slides when we discussed the spinal root of the accessory nerve.
Continue on with the pathway here, we see this highlighted structure which I also talked about before. This is a passage and this is known as the jugular foramen. The fibers of the cranial part and the spinal part of the accessory nerve exit the skull through this passage here – the jugular foramen. You can see them here. Although the fibers from the cranial part and the spinal part of the nerve exit the skull together, their fibers do not mix – don't forget this, this very important. Fibers from then the cranial root of the accessory nerve which as we have seen arise from the nucleus ambiguus and then synapse on this structure that you see now highlighted in green on the screen – the superior ganglion of the vagus nerve also known as cranial nerve number ten. Now after exiting the skull, the fibers go on to then run within the vagus nerve as you can see here now highlighted in green on the same image. Now once these fibers mixed with the vagus nerve, they end up in the pharyngeal and laryngeal branches of the vagus nerve innervating the muscles of the larynx with the exception of the cricothyroid muscle.
Continuing on with our pathway here, you can see this structure here highlighted. This is then the spinal part of the accessory nerve which fibers as we saw arise from the gracile fasciculus and go on to form the main part of the accessory nerve which provides then motor innervation to those muscles that we talked about before – the sternocleidomastoid muscle and the trapezius muscle.
Now some of the fibers arising from the gracile fasciculus also go into the first to fourth cervical spinal nerves C1 to C4. Right now on the image you see here highlighted if you can see here – this is the spinal nerve C1. Then below we can find then spinal nerve number two or C2 to be more specific - we're talking about cervical spinal nerves. Then we see here a little bit further down C3 or spinal nerve C3 and finally all the way down here on our image, we find then C4.
We are down now with the afferent pathway of the accessory nerve and I would like to quickly mention the most relevant clinical notes related to this nerve. An upper neuron or motor neuron lesion of the accessory nerve results in weakness in the muscles it will be innervating like we talked about before. You probably remember some of the muscles and one of them is now highlighted on the screen –the sternocleidomastoid muscle. Another one that will be affected is this one that you see now from the back – this is the trapezius muscle. Now such lesion causes weakness of the contralateral or the opposite side of the trapezius muscle.
On the sternocleidomastoid muscle as you can see, a lesion of one accessory nerve then leads to weakness on the sternocleidomastoid found on the same side. When muscle weakness is seen only in the sternocleidomastoid muscles, this can be an indication of an upper cervical accessory root lesion or a brainstem tegmentum lesion. This brings up to the end of this tutorial where I'm just showing you right now an image of the dorsal view of the accessory nerve highlighted in green as it goes to then innervate the trapezius muscle on the back.
Now just to recap a summary before we complete this tutorial. We have looked at the definition of afferent or efferent versus afferent fibers. We have also discussed the two parts of the accessory nerve – the cranial part and the spinal part – and we have looked at the efferent pathway of the accessory nerve.
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