Think of the cranial nerves as your friend.
No, not Pam from HR who sometimes gifts you cupcakes simply because they were leftovers from her six-year-old's Minecraft themed birthday party, but the friend who tells you that "ooh, you're looking a bit rough today, buddy" or "ooh, that looks tasty", only to then completely demolish the—rather delicious—apple pie that you were in the midst of eating.
Anyway, in order for you to be able to grasp what each one of the twelve (yes, there's a cranial nerve zero and talk of even more, but let's just stick with the OGs), it isn't enough to simply be acquainted with, for instance, what their fibre type is (is it sensory, motor or both?) You must be able to understand why that is so. I won't go into full evolutionary detail behind why the cranial nerves exist as they do, but it's imperative that you at least have some kind of basic knowledge on its embryology.
I'm not a zoologist (hell, I'm not even a neuroscientist ... yet) so I can only relay to you what my professor (who actually happens to have his roots in zoology before he delved into the world of neuro) had taught me. It'll be a very condensed version of what he said, mind you, as I recall most of this from the top of my head but as a Neuro student, I feel obliged to share such knowledge with you regardless of the level of detail.
Anyway, let's get straight into it. Below we have a schematic of the human embryo where roughly by four weeks you see these four swellings around the area that will eventually become the throat (shown as A in Image 1).
 |
Image 1. Gill arches in a human embryo will eventually form the jaw, larynx, ears
and parts of the throat.
|
The cells within each of the gill arches migrate up and around your face, forming muscles, bone, whatever's programmed within the cell to form, so take note of the following:
First Arch — tissues will form the upper and lower jaw and is innervated by CN V (hopefully you can slowly start to see how all of this connects).
Second Arch — tissues from the neck will migrate up and all over your face, forming most of the muscles which control facial expression; innervated by CN VII.
Third Arch — upper larynx and upper neck, innervated by CN IX i.e. glossopharyngeal for swallowing!
Fourth Arch — parts of our larynx, basically lower throat, innervated by CN X.
Fifth Arch — we humans don't have this, but our fishy ancestors did. It sort of just degenerated when it came to us.
Sixth Arch — yes, this exists but I wouldn't worry too much about it. It's basically a recurrent laryngeal nerve (branch of the Vagus).
Still confused? Think of it this way: take the second arch, for example, it's what forms the muscles that control facial expression (i.e. strapedius) and are innervated by the facial nerve (CN VII). However, there are tiny muscles of the ear that are also controlled by the same nerve. Why is this so? I mean, it's your ear ... and muscles responsible for facial expression, how on Earth do these structures even relate? The simple answer is that they're both derivatives of the same arch i.e. the second arch. We can then take this a step further and examine why CN VII has both sensory and motor components, but I'm not fully equipped to explain that in the greatest of detail so I hope the below diagram can give you an idea:
 |
| Image 2. Each gill is supplied by blood vessels, a bar of cartilage and a nerve which aid in fishy (how scientific of me) function. |
It's not entirely wrong to say that there's a shark within us all. I mean, if you take a look at fish embryos, you'll see that they, too, have these swellings and indentations. If you haven't already identified the pattern, just look at the cranial nerves of humans and then back to the cranial nerves of fish. CN V, VII, IX and X each have motor and sensory components. If you look at Image 2, it isn't hard to see why that's so. Towards the caudal end, you can see the gill arches (labelled 1, 2, 3...) that are comprised of somatic motor and visceral motor fibres with some visceral/somatic sensory fibres coming in beside them as well. It's a very similar state of affairs; affairs that I'll allow you to ruminate on.
Okay, so having the basic embryology in mind, we can finally get down to the nitty-gritty of actually remembering the cranial nerves and everything that each one of the twelve encompasses. I'll share some mnemonics that I used (and still use) to remember these because yes, it's overwhelming, especially if it's your first time studying them, but I hope that my tips will at least alleviate all the stress. I was in the exact same boat as you!
Oh, Oh, Oh They Travelled And Found Voldemort Guarding Very Ancient Hallows
(Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Accessory and Hypoglossal)
Some Say Money Matters But My Brother Says Big Brains Matter Most
(S = Sensory, M = Motor, B = Both)
The above mnemonics have seriously saved me and my academic career. I don't remember where exactly I found these but I'll link it once I do. Alas, this is only the beginning of what you (well, I) had to know about the cranial nerves as it wasn't enough to simply know the name (Oh = Olfactory) and its corresponding fibre type (Some = Sensory).
(Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Accessory and Hypoglossal)
Some Say Money Matters But My Brother Says Big Brains Matter Most
(S = Sensory, M = Motor, B = Both)
The above mnemonics have seriously saved me and my academic career. I don't remember where exactly I found these but I'll link it once I do. Alas, this is only the beginning of what you (well, I) had to know about the cranial nerves as it wasn't enough to simply know the name (Oh = Olfactory) and its corresponding fibre type (Some = Sensory).
Click the above title to be redirected to a Google Drive with a summary of the cranial nerves that I've typed up. However, I must admit that being able to physically touch and see a human brain significantly helps with the identification process. Some, if not most, of you won't have access to such resources so this or this should hopefully suffice.
To add to the table I've linked above, I have yet another useful mnemonic I used in my studies to help remember which extraocular muscle is innervated by which cranial nerve. It's simple:
To add to the table I've linked above, I have yet another useful mnemonic I used in my studies to help remember which extraocular muscle is innervated by which cranial nerve. It's simple:
LR6SO4
Where LR = Lateral Rectus and the subscript, 6, refers to CN 6 and
SO = Superior Oblique and the subscript, 4, refers to CN 4.
Everything else (Inferior Oblique as well as the Medial, Superior and Inferior Recti) are innervated by CN 3.
CN III: OCULOMOTOR
1. Pupillary Sphincter and Ciliary Muscle rendered non-functional —
mydriasis: lens cannot be focussed for near vision and does not constrict to light.
3. Ipsilateral eye deviates laterally as the Lateral Rectus is unopposed (remember, CN III innervates the Medial Rectus).
Further results in a lateral strabismus which in turn produces diplopia.
4. Vertical eye movements are also impaired given that Inferior Oblique, Superior and Inferior Recti are affected.
CN IV: TROCHLEAR
1. Superior Oblique, which CN IV innervates, is responsible for moving eyes downwards and laterally, if this is compromised by a lesion, will result in diplopia.
CN V: TRIGEMINAL
1. Trigeminal Neuralgia ensues — characterised by excruciating pain.
(NB: the more caudal a lesion is, the larger the area is spared around the mouth.)
CN VI: ABDUCENS
1. Medial Strabismus — unopposed Medial Rectus (similar principle as CN III but different muscles involved).
Ipsilateral eye cannot abduct (hence, abducens), contralateral eye can't adduct past the midline and there is a lateral gaze paralysis.
CN VII: FACIAL
1. Facial Paralysis.
2. Non-functional lacrimal and salivary glands, however, in the event of a facial injury, the nerve may grow back (considering they're peripheral nerves) but will often get lost and travel to the wrong target. For instance, fibres that should be going to salivary glands instead go to lacrimal glands i.e. you cry when smelling, let's say, a delicious curry.
CN VIII: VESTIBULOCOCHLEAR
1. I mean, this is self-explanatory.
CN IX: GLOSSOPHARYNGEAL
1. Gag-reflex affected.
2. Glossopharyngeal Neuralgia.
3. Difficulty swallowing.
CN X: VAGUS
1. Swallowing and phonation affected.
CN XI: ACCESSORY
1. Weakness of sternocleidomastoid and trapezius muscles.
CN XII: HYPOGLOSSAL
1. Weakness of ipsilateral side of the tongue.
2. Bilaterally, ability to speak and eat will be affected.
1. Weakness of ipsilateral side of the tongue.
2. Bilaterally, ability to speak and eat will be affected.
As with everything in Neuroscience, the brain is a highly complex entity in that it's hard to pinpoint what exactly the function is of any part of the brain. For instance, the prefrontal lobe is generally associated with higher order thinking and the basal ganglia is generally associated with motor control. Lesions to the brain typically do not target specific areas as they encompass a larger area of cortex, so who knows what exactly is going on?
Anyway, I hope you enjoyed the first instalment to my "Navigating Neuro" series. If you have any queries or want me to reiterate some of my points, please just ask. Any suggestions would also be very much appreciated. However, I would like to make clear that these are only the basics as I don't plan on sharing anything beyond my scope of knowledge, nor do I plan on further complicating such an already complex structure. What I'm sharing should always be supplemented with additional research (whether it be via textbooks, journal articles or learning websites) but I can only hope that this series will at least help you with gaining some sort of clarity on a rather elaborate, rather perplexing, albeit incredibly interesting, branch of science.
Wishing you luck,
No comments