Cranial Nerves Basics

Author: Dr Peter de Souza
Last modified: 17 December 2020


Okay so this is a tutorial on the cranial nerves. Cranial nerves are nerves that emerge directly from the brain, and you've got 12 pairs of cranial nerves, and they're a part of the peripheral nervous system. All 12 pairs are a part of the peripheral nervous system, except cranial nerve number 2, which actually emerges from, which is kind of like an extension of the brain, so it's not technically a peripheral nerve.

In this tutorial we're going to go through the 12 cranial nerves, we're going to look at where they originate from in the brain, and we'll talk a little bit about their function.
The cranial nerves have lots of different functions. You've got somatic and visceral components, they've got motor components and there’s some nerves which have special sensory components, so components to do with the special senses, like smell, vision, hearing, balance and taste.
If I just rotate the model of the brain around, you can see, so we're looking at the base of the brain, and you can see all these nerves emerging from the brain. The first two actually emerge from the telencephalon and diencephalon, but the rest of the nerves, nerves 3 to 12, emerge from the brainstem. That’s the midbrain, the pons and the medulla.
The nerves are sort of numbered, from 1 to 12, from kind of top to bottom, if you like. I'm just going to run through a list of nerves.
You’ve got the olfactory nerve, number 1, optic, oculomotor, trochlear, trigeminal, abducent, facial, vestibulocochlear, glossopharyngeal, vagus, accessory, and hypoglossal nerves. They’ve got kind of strange names, but there is a mnemonic for remembering these.


I'll give you the clean version, which is "Oh, oh, oh, to touch and feel very good velvet, ah, heaven", so the dirty version is what probably most people are taught, and it involves touching and feeling other things.
You can use your imagination there.There’s also a mnemonic for remembering the functions of the cranial nerves.
This is some "Some say marry money, but my brother says big boobs matter more", so "S" is sensory, "M" is motor, "B" is both - so sensory and motor. Keep these mnemonics in mind when we go through the nerves.

Olfactory Nerve

We’ll start with cranial nerve number 1, which is the olfactory nerve, and this nerve is responsible for olfaction, so the sense of smell. There are receptors in the nasal cavity which pick up the sense of smell, and the nerves pass through the nasal cavity, up through the cribriform plate of the ethmoid bone, and synapse on this, the olfactory bulb.
You can see this projection along the base of the brain, so this is called the olfactory bulb, and the olfactory nerves, synapse on to this olfactory bulb.
I've just switched over to this model, and you can see these little nerves, coming up to synapse on to the olfactory bulb.
if we just take a quick look at the cribriform plate, we're looking at a side view of the skull here, so we're going to rotate the skull round and look at the interior of the skull, so we're looking from a superior view at the inside of the skull.This is the anterior part, and the cribriform plate lies here, so it's not shown clearly here, but the olfactory bulb runs along the cribriform plate and there are little holes in the cribriform plate, where the olfactory nerves pass through.
If I just rotate the model round, you can see these olfactory nerves passing up through the foramina, to synapse on to the olfactory bulb.

Optic Nerve

Next we've got the optic nerve, and you can see these two nerves on either side, these big nerves coming here, and you can see that they cross at this area called the optic chiasm, so these nerves, the optic nerve, nerve number 2 is responsible for vision, so it carries sensory information from the retina to the brain.

Oculomotor Nerve

The third nerve is the oculomotor nerve. Now we're coming, this is the first nerve that originates from the brainstem. You can see its origin here, in the interpeduncular fossa, so you can see these two sort of pillars, these are called cerebral peduncles, and these are part of the midbrain.
The oculomotor nerve originates anteriorly on the midbrain, in the floor of the interpeduncular fossa. You can see the oculomotor nerve originating here. And this nerve is responsible for innervating muscles that move the eye, so extraocular muscles, and it also innervates the pupillary sphincter.

Trochlear Nerve

Nerve number 4 is called the trochlear nerve. If we just zoom in a bit to the brainstem, and I'll just remove the cerebellum, and I'll remove the cerebral hemispheres, so it just makes things a little bit more clear.
We’ve just seen the oculomotor nerve here, originating in the interpeduncular fossa, and if we follow the, if we rotate the model round to look at the dorsal surface of the midbrain, we can see the trochlear nerves originating.
The trochlear nerve is another nerve which is involved with movements of the eye, so it only innervates one eye muscle, called the superior oblique muscles. It originates on the dorsal midbrain, and it winds round the sides of the cerebral peduncles, so the lower margins of the cerebral peduncle.

Trigeminal Nerve

Next we've got the trigeminal nerve, so this, so you can see this big fat nerve coming off the side of the pons on the middle cerebellar peduncle, so this is the trigeminal nerve, so you can see it here, originating laterally on the pons, on the middle cerebellar peduncle.
The middle cerebellar peduncle is the bit which connects the pons to the cerebellum. I'll just rotate round so you can see that more clearly, so you can see the big trigeminal nerve coming off the side of the pons.
This nerve is responsible for sensation to the face, and if you just follow the nerve along, you can see this big bulge, so this is the trigeminal ganglion, so this is a group of cell bodies of the sensory neurones, and then it splits into three branches, so if I zoom in a bit, you can see these three nerves coming off.
You’ve got three branches of the trigeminal nerve; you've got the ophthalmicmaxillary and mandibular nerves, so that's just worth remembering. As well as providing sensation from the face, the trigeminal nerve is responsible for innervating the muscles of mastication.

Abducent Nerve

if we just rotate the model around again, so we're looking anteriorly at the brainstem, just follow the brainstem down, we can see this this nerve here originating at the border between the pons and the medulla, so this nerve is called the abducent nerve, and this is another muscle, sorry nerve, involved in innervating the extra-ocular muscles of the eye.
It innervates one muscle of the eye called the lateral rectus muscles.
We’ve now talked about three muscles, sorry three nerves responsible for innervating extra-ocular muscles - oculomotor nerve, trochlear nerve and now the abducent nerve. This originates at the border between the pons and the medulla.

Facial Nerve

If we just rotate the model again, looking laterally, if we follow the brainstem laterally, we've got two nerves here, and this part of the brainstem is called the cerebellopontine angle.
Just above the olive of the medulla, you've got this nerve here, called the facial nerve.
This originates at the cerebellopontine angle above the olive, and it's responsible for muscles of facial expression, and it receives taste from the anterior two thirds of the tongue, and it also has secretomotor function to the salivary glands and the lacrimal glands.

Vestibulocochlear Nerve

Just lateral to this nerve, you've got the vestibulocochlear nerve, which also originates in the cerebellopontine angle, and this nerve is responsible for hearing and balance. It has two branches, the vestibular part is responsible for balance, and the cochlear part is responsible for hearing.

Glossopharyngeal Nerve

And then just below it, we've got this nerve here, which is called the glossopharyngeal nerve, and this originates just dorsal to the olive, so you can see its position here just behind the olive, on the lateral margin of the medulla.
This nerve is responsible for taste from the posterior third of the tongue, so remember the facial nerve was responsible for taste from the anterior two thirds, the glossopharyngeal nerve is responsible for taste from the posterior third, so it's responsible for taste, and it's got this secretomotor function to the parotid gland, and it is also responsible for innervating the muscles in the throat, and carrying sensation from various areas in the throat.

Vagus Nerve

That’s nerve number 9, the glossopharyngeal nerve, and nerve number 10 originates just below it, and this is the vagus nerve, so this originates in the postero-lateral sulcus of the medulla, so just behind the olive, and just below the, just below where the glossopharyngeal nerve originates.
The vagus nerve innervates lots of laryngeal and pharyngeal muscle, and it innervates almost all of the abdominal and thoracic viscera.

Accessory Nerve

Next we've got the accessory nerve, so this is nerve number 11, and this nerve has cranial and spinal roots.
It’s got, it's this nerve here, it isn't shown sort of, it's not best illustrated by this model, but you can see it’s got a lower origin and an upper origin, so it’s got a cranial and spinal root.
This nerve is responsible for innervating the sternocleidomastoid and the trapezius muscles.

Hypoglossal Nerve

And finally we've got the 12th nerve, so this is the hypoglossal nerve, and you can see it here, so this sits between the pyramids of the medulla, so these are the medullary pyramids, and it sits between these pyramids and the olive, so you can see that here, the hypoglossal nerve. And this nerve is responsible for innervating the muscles of the tongue.
Those are the 12 cranial nerves, you've mnemonics for remembering them and you've seen sort of where they originate from on the brainstem and the brain.