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Main nerves of the head and neck

Major nerves of the head and neck.

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Transcript

Hey everyone! This is Nicole from Kenhub, and in this tutorial, we'll be looking at the main nerves that supply the head and the neck.

I’m just going to begin by showing you this illustration, and here we can see some of the main nerves of the nervous system. But in this tutorial, we’re going to be focusing specifically on the major nerves that provide sensory and motor innervation to the head and the neck. We’re also going to introduce you to the twelve cranial nerves and some of their branches and then we’ll talk a little bit about the cervical plexus giving a few examples of the structures these nerves supply and what kinds of innervation they provide. And because this is a basics tutorial, we won’t be overwhelming you with too much material today. Instead, we’re just going to be looking at a few important basic points about each of them that are useful for you to know. So, without further ado, let’s jump in to take a look at the cranial nerves.

So, in this slide, we can see the brain from an inferior view and in this image, we can see the twelve cranial nerves emerging from the brain. Now, these nerves are called the cranial nerves due to the fact that they emerge directly from the cranium rather than the spinal cord. Each of these nerves has a name as well as a number from one to twelve and they’re arranged roughly in a numerical order.

We’re going to start, of course, with the first cranial nerve or CN I which is the olfactory nerve, now highlighted in green, and its main role is to govern the sense of smell. It also consists of both afferent and efferent sensory fibers. Let’s move on now to have a look at a medial view through the skull just here. And here we can see a better view of the position of the olfactory nerve. And as you can see, the olfactory nerve fibers exit the skull through the cribriform plate of the ethmoid bone, and they terminate in the roof of the nasal cavity. Now, because the olfactory nerve governs the sense of smell, damage to this nerve or the olfactory mucosa can, as you’d expect, lead to a localized or, in some cases, a total sense of loss of smell.

Also seen here from the base of the brain is the second cranial nerve known as the optic nerve. And the optic nerves arise from the convergence of the axons of the cells of the retinal ganglion at the back of each eye as we can see just here. And they go on to form a partial decussation at the optic chiasm which is just here. After the optic chiasm, the fibers of the optic nerve then go on to form the optic tract which contains both collateral nasal retinal fibers and ipsilateral temporal fibers. And this is important to know as this mixture of fibers in each optic nerve ensures bilateral vision.

Of course, as you may have guessed, the optic nerve governs both ocular and sensory functions and damage to the nerve fibers of the optic tract can therefore result in altered sight in one or both eyes, and in extreme cases, lesions of the optic nerve can also lead to blindness.

The third cranial nerve, the oculomotor nerve, can be seen in green, and this nerve is responsible for innervating the extraocular muscles of the eye along with the eye, ciliary muscle, and sphincter pupillae muscle with the exception of the superior oblique muscle and the lateral rectus muscle which are innervated by different nerves. The oculomotor nerve arises from the left and right side of the pons with its fibers coming from both the nucleus of the oculomotor nerve and from the accessory nucleus of the oculomotor nerve. After that, the nerve then exits the skull through the superior orbital fissure which is just here.

The trochlear nerve is the fourth cranial nerve – and that’s highlighted just here – and it arises from the lateral superior margin of the pons on the left and the right side and it exits the skull through the superior orbital fissure which is just here just like the oculomotor nerve. The trochlear nerve innervates the superior oblique muscle of the eye just here, therefore, facilitating the abduction, depression and internal rotation of the eyeball.

Next, we’re going to move on to the fifth cranial nerve which is the trigeminal nerve, and this is the one that students usually consider to be the most complicated cranial nerve and some will give it a little extra attention. If we take a closer look at the trigeminal nerve just here and also let’s just add in this an overlay of a face so you get a better idea of how it all works together. So, the trigeminal nerve gives rise to various branches that provide innervation to the face, the forehead, the lateral part of the head, the upper part of the neck as well as some of the tissues within the head and the neck.

This nerve has three major divisions – the ophthalmic nerve which is the first division of the trigeminal nerve, the maxillary nerve which is the second division of the trigeminal nerve, and the mandibular nerve which is the third division of the trigeminal nerve. The best way to discuss the fifth cranial nerve is by looking at its three divisions one at time, so let’s go through that now.

We’re going to start off with the first division of the trigeminal nerve which is the ophthalmic nerve, and the ophthalmic nerve exits the skull through the superior orbital fissure just here and provides innervation to the forehead, the nose, the nasal cartilage, and the ophthalmic region of the face via its branches. Now, one of these branches is known as the supraorbital nerve and we can see it a little bit better if we zoom in a bit just here. And the supraorbital nerve supplies the skin of the forehead as well as the scalp, frontal sinuses, and the conjunctiva of the eye. Another branch is the supratrochlear nerve in green, and before exiting the orbit, the supratrochlear nerve innervates the frontal sinus after which it goes on to provide cutaneous innervation to the scalp.

The second division of the trigeminal nerve, the maxillary nerve, exits the skull through the foramen rotundum at the base of the skull and gives rise to several branches. And these branches innervate the area of the face from below the eyes to the upper jaw. One such branch can be seen here which is the infraorbital nerve and this nerve exits the skull through the inferior orbital foramen to supply the face. Now, let’s note that the maxillary nerve also gives rise to other branches that are not covered in this tutorial, but we do cover them in more detail in our tutorial on the maxillary nerve, so make sure to check that out if you’re curious.

The third major division of the trigeminal nerve is the mandibular nerve, and this branch innervates the lower face, the lower jaw, the lower dentition, and the floor of the oral cavity. And it exits the skull through the foramen ovale at the base of the skull. This nerve also gives off its own branches. For example, this nerve here known as the buccal nerve, and this branch innervates the skin over the buccinator muscle as well as the mucous membrane that lines the inner cheek.

Another branch of the mandibular nerve is the mental nerve, and this nerve actually arises from the inferior alveolar nerve which is the largest branch of the mandibular nerve. The mental nerve innervates the lower dentition, the gingiva and exits through the mental foramen to supply the mucous membrane and the skin of the chin.

That’s all we’re going to be covering on the trigeminal nerve in this tutorial. Briefly, we’ve given you an awareness of its structure and function that you can carry through two more detailed tutorials.

Now, let’s move on to the sixth cranial nerve which is known as the abducens nerve. Now this nerve whose fibers arise from the facial colliculus of the brainstem exits the cranial vault via the superior orbital fissure to supply the motor innervation to the lateral rectus muscle of the eye which is just here.

Let’s move on to talk about the seventh cranial nerve named the facial nerve. Now, this nerve supplies the muscles of the facial expression, so it’s basically one of the main nerves that allows you to make all sorts of expressions from smiling to frowning or wrinkling your forehead, just like our friend on the right. The facial nerve also provides taste sensation to the anterior two-thirds of the tongue and it gives rise to numerous branches – one of which is this nerve that we can see here known as the posterior auricular nerve which innervates some of the muscles of the outer ear.

Next, we’ll take a look at the eighth cranial nerve which is known as the vestibulocochlear nerve. This nerve’s primary function is to provide sensory innervation to the inner ear. Now, let’s flip around our image of the brain a little bit so that we can see it from a lateral view, and from this view, we can see that the fibers of the nerve exit the skull through the internal acoustic meatus.

One more thing I want to say about this nerve is that because this nerve is comprised of two different fibers, they terminate in different areas. The cochlear fibers of this nerve terminate in the cochlea, whereas the vestibular fibers of the nerve terminate in the ampullae of the vestibulum as you can see just here.

Let’s now talk about the glossopharyngeal nerve which is also known as the ninth cranial nerve. The glossopharyngeal nerve exits the skull through the jugular foramen and provides motor innervation to the stylopharyngeus muscles and the parotid gland as well as sensory innervation to the tonsils, the pharynx, and the middle ear and taste sensation to the posterior third of the tongue.

Alright so so far we’ve looked at nine of the twelve cranial nerves that provide motor and sensory innervation to the structures of the head. Now, let’s have a look at the tenth cranial nerve – the vagus nerve. Now, this nerve is a little bit different to the other cranial nerves we have looked at because it supplies structures beyond the head and neck regions. For example, it exits the skull via the jugular foramen as well as supplying the external ear and the neck region. It gives off branches that innervate several structures including the palate, the pharynx, the larynx, the trachea, the bronchi, the heart, and even as far down as the gastrointestinal tract as you can see.

One branch of the vagus nerve is known as the recurrent laryngeal nerve and this nerve branch functions to supply most of the intrinsic muscles of the larynx.

Continuing with the cranial nerve, the next nerve we’re going to look at is the accessory nerve which is the eleventh cranial nerve and the accessory nerve also exits the skull via the jugular foramen and goes on to provide motor innervation to the sternocleidomastoid and trapezius muscles.

And, finally, we’ve now reached our twelfth cranial nerve which is otherwise known as the hypoglossal nerve. And the hypoglossal nerve provides motor innervation to the muscles of the tongue except for the palatoglossus muscle. The hypoglossal nerve exits the skull via the hypoglossal canal and also gives off fibers, C1 to C3, which innervate the infrahyoid muscles of the neck.

Alright, now that we’ve briefly covered each of the twelve cranial nerves, let’s take a look at some of the major nerves arising from the cervical plexus.

Now, the cervical plexus is a cluster of nerves formed by the ventral rami of cervical nerves C1 to C5 and it gives off four sensory branches which supply the skin of the anterolateral neck, the superior thorax, and an area of the scalp between the auricle and the external occipital protuberance. It also gives off motor branches that supply the muscles in these regions as well.

Let’s take a look at some of the important nerves that are given off here.

So, the first nerve we’re going to look at is the great auricular nerve, and the great auricular nerve is one of the four sensory branches that arise from the cervical plexus. Now, this nerve is formed by the loop of the ventral rami of nerve C2 and C3. The great auricular nerve innervates the skin over the parotid gland, the posterior aspect of the auricle or the outer ear, as well as the area of skin between the angle of the mandible and the mastoid process.

The next nerve we’ll be looking at is the transverse cervical nerve which also arises from the ventral rami of nerve C2 to C3 and is the third sensory branch that arises from the cervical plexus. Now as you can see, this nerve curves around the middle of the posterior border of the sternocleidomastoid muscle crossing it and going on to supply the skin that covers the anterior triangle of the neck.

Next here, we can see the cervical ansa or ansa cervicalis which is formed by the motor branches of the cervical plexus, and it arises from the ventral rami of nerve C1 to C3. The cervical ansa is a loop of nerves that has five motor branches and these branches supply the infrahyoid muscles in the anterior cervical triangle which are very important muscles for the process of swallowing and of speech.

Finally, another very important motor branch of the cervical plexus and the last nerve we’ll look at in this tutorial, is the phrenic nerve. And this nerve is formed mainly by the fibers from the cervical nerve C4 but it also receives some contributions from the cervical nerve C3 and C5. And this nerve contains not only motor fibers but also contains sensory and sympathetic fibers. As such, the phrenic nerve provides motor innervation to the diaphragm and carries sensory information from the central part of the diaphragm.

The phrenic nerve also innervates the mediastinal pleura and the pericardium of the heart. Because the phrenic nerve provides motor innervation to the diaphragm, damage to this nerve can result in paralysis of the diaphragm, therefore, resulting in breathing regulation disorders. And the phrenic nerve also happens to be one of the primary nerves involved in hiccups. Cases of persistent hiccups that are not responding to conservative and pharmaceutical treatment may be treated by inducing a temporary paralysis of the diaphragm known as a phrenic nerve block.

And that’s all of the major nerves we’re going to be looking at in this tutorial. Thanks for sticking with me this far. Before we recap though, let’s take a look at some useful clinical notes.

And as we’ve seen, the cervical plexus is formed from the ventral rami of spinal nerves C1 to C4. In neck surgery, a procedure known as cervical plexus block is used to anesthetize the areas over the neck, the occipital region, the upper pectoral region, and the shoulder region. Cervical plexus block can be achieved by injecting an anesthetic agent at several points along the posterior border of the sternocleidomastoid muscle, and this is done because the cervical plexus is situated deep to this muscle.

Now, it’s important to remember that the cervical plexus block procedure is not performed on patients with pulmonary or cardiac disease. Cervical nerve block also results in the paralysis of the phrenic nerve which as we saw innervates the diaphragm, the mediastinal pleura and the pericardium of the heart.

So that brings us to the end of this tutorial on the main nerves of the neck. Just to recap, we briefly looked at the twelve cranial nerves which included the olfactory nerve, the optic nerve, the oculomotor nerve, the trochlear nerve, the trigeminal nerve, the abducens nerve, the facial nerve, the vestibulocochlear nerve, the glossopharyngeal nerve, the vagus nerve, the accessory nerve, and the hypoglossal nerve. And we briefly looked at the areas that they supply in the head and neck regions. Finally, we looked at the cervical plexus and looked at the four nerves which arise from it which are namely the great auricular nerve, the transverse cervical nerve, the ansa cervicalis, and the phrenic nerve.

And that concludes our tutorial for today. You can find more detailed tutorials on each of these nerves covered in this video at Kenhub, so be sure to check it out. And thanks again for watching and see you next time!

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