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147426 tn?1317265632

The Cranial Nerves and What They Are About








Vestibulocochlear (also Auditory)





A little bit on the Cranial Nerves: These are a twelve sets of nerves that sprout directly off the brain and brainstem. Thus, even though they go out into the body, (like peripheral nerves do) their origins are in the white matter of the Central Nervous System. So these 12 nerves (actually 24 nerves, one on each side) are considered white matter. A lot of the common MS sites of demyelination involve the Cranial nerves. If you have read many of my posts, you'll remember that I am always mentioning that this that or the other symptom are from lesions in the Cranial nerves.

The Cranial Nerves have an wide interaction with each other to give the brain far more information and control than the nerves do alone.  They interact within the brainstem through a series of nerve hubs called nuclei (plural of nucleus).  Through these nuclei we have greater understanding of what we see, can move our eyes in complex ways, have greater balance and coordination.

CN I - is the Olffactory Nerve -

What it does - this nerve comes directly off the brain and handles the entire job of the sense of smell.
What happens when MS attacks it - Changes in sense of smell can occur in MS, from partial or complete loss of smell to alterations in smell like smelling weird things that aren't there.

CN II - the Optic Nerve -

What it does - Yep, handles the whole process of sight and vision.  As a sensory nerve, the Optic  Nerve picks up the image in light and color and carries it backwards to the brain where the information is processed by several parts of the brain.  This forms the images we see, and adds to the information that keeps us upright in space and helps us keep our balance.  The information is also used in the cerebellum to help us with fine motor and coordination.
Lesions on this nerve from MS cause the condition Optic Neuritis, symptoms include decreased vision, pain behind the eye, pain with eye movements, loss of color saturation (vivid colors become paler or more grayed out), flashes of light, loss of parts of the field of vision

CN III - the Oculomotor (eye movement) Nerve

What it does - It's a motor nerve.  This nerve handles 4 of the six muscles that allow the eyeball to move around, instead of staring straight ahead.

It also raises one of the major muscles of the upper eyelid on the same side. Alesion will cause the eyelid on the same side to droop.

It also has the function of working to control the pupil constriction to light and to close/far vision.  Lesion can change our ability to bring objects at different distances into focus.

When there is a lesion on it it can cause double vision because the affected eye cannot move in coordination with the normal eye.

The eyelid on the same side can droop

The pupil on that side may not respond as promptly to light.

CN IV - the Trochlear Nerve

This nerve handles the muscle that allows the "opposite" eye to rotate using the supeior oblique muscle. This allows fine tuning of the eye movements other than just up/down and side-to-side. It is the only cranial nerve that crosses over to direct something on the OPPOSITE side of the face.

Lesion - double vision

CN V - the Trigeminal Nerve

This is the major sensory nerve for sensation to the face and the motor nerve for chewing. The Trigeminal Nerve also handles the sensation from the sinuses, from the outside surface of the eardrum, and from the meninges.  This nerve has three main branches. ( It's name means "three roots) V1 brings sensation from the upper part of the face and eyelid, the temple and the forehead. It may reach as far as the top of the scalp.  V2 is the mid-part of the face, cheek, nose. Also the upper teeth, gums,upper lip and inner cheek. V3 handles the sensation from the lower face, lower teeth and gums, and lower lip. It extends down to and slightly beyond the jawline.

Lesion - It is responsible for the sensations of numbness, tingling and PAIN (as in Trigeminal Neuralgia) seen in MS.  There can be an alteration of sensation noted both preceeding and following an episode of TN.  A lesion may cause inability to chew effectively or fatigue with chewing.

CN VI - the Abducens Nerve - This motor nerve handles the same side eye muscle attached to the outside eyeball and allow the eye to turn laterally to the side.

CN VII - The Facial Nerve - This nerve handles most of the muscles of the face and lips and around the eye. But it is not only a motor nerve.  It is also a sensory nerve and a visceral nerve.  (A visceral nerve causes actions in an organ of the body like causing a gland to secrete or causing the bowles to have peristalsis).   It handles part of the control of the glands of the face. It also carries sensation from the ear, the middle ear and the interior of the eardrum. It handles taste for the anterior 2/3's of the tongue on the same side. When this nerve goes wonky you get a Bell's Palsy - Drooping of the side of the face, droopy eye, etc.

CN VIII - The Auditory Nerve - This is a sensory nerve responsible for Hearing and Balance (peripheral balance) Damage to this nerve can cause hearing loss and vertigo, and tinnitus.

I'll do the last 5 nerves later.  Also some neuroanatomists claim there is a Nerve #0 which is responsible for the production of pheromones.

There are uncounted mnemonics for remembering the cranial nerves and there order.  These range from odd to silly to downright slutty.

On Occasion Our Trusty Truck Acts Funny. Very Good Vehicle Any How.

On Old Olympus' Towering Tops A Friendly Viking Grew Vines and Hops

Only On Occasion, Touching The Amorous Female Virgin Goat Vacillates A Hand

OLympic OPium OCcupies TRoubled TRiathletes After Finishing VEgas Gambling VAcations Still High

Orange orangutans often try to avoid feeding angry gorillas very ancient hotdogs.

On Old Olympus' Tufted Top A Fat Armed German Viewed An Hop

Oh Once One Takes The Anatomy Final Very Good Vacations Are Heavenly  

Only Older Octogenarians That Take A Free Viagra Get Very Aroused Here

Oh, Oh, Onward Through The Airy Facade Viewing Gorgeous Vixens Acessorizing Hedgehogs

Another set of mnemonics helps to remember the nerves' purposes and to remember the types of information these nerves carry (Sensory, Motor, or Both) is thus:

Some Say Money Matters, But My Brother Says Big Brains Matter More.

Some Say Money Matters, But My Brother Says Big Boobs Matter More.

Small Ships Make Money, But My Brother Says Big Boats Make More.

Some Say Marilyn Monroe, But My Brother Says Bridgette Bardot Mmmm Mmmmm

I had been waiting to finish this, but decided a little was better than nothing.

27 Responses
1406332 tn?1315962760
Thank you so much! Very informative.  =D I bookmarked this page already!
Avatar universal
As ususal, awesome post.  Do lesions on these nerves generally show up on a brain MRi?
I have so man of these sx's.  I am being checked for disease after disease that causes a lot of the things you explained above.  I am seeing that it really could all be from MS.

Thanks for being you
382218 tn?1341181487
agree, awesome post Quix.

Re; showing up on MRI, I have a large lesion on my brain stem that damaged my 6th cranial nerve and caused constant double vision for about a year.  I don't know if they isolated it on MRI to the actual nerve responsible, or,  just the brain stem, and concluded damage to that particular cranial nerve based on clinical observation.  Since my MRI report said nothing about cranial nerves I assume the latter.

1045086 tn?1332126422
Thanks Quix! for explaining all this and for not making us wait until you could finish it completely.  I understand better now how eyelid droop and jaw fatigue with chewing is produced by MS demyelination.  They were symptoms that had me wondering about MG a little while back.  

I look forward to your second installment on cranial nerves and expect this will become a valuable addition to the Health Page section eventually.

When I learned cranial nerves we were given the mnemonic:
On Old Olympus Thorny Top A Fin And German Viewed Some Hops (it's so stupid, it stuck with me)
Now that I see it again, I do remember the Some Say Money Matters thing.  I guess that one stayed in my memory bank just about as long as dollars stay in my piggy bank.

Sure am glad when you can be here.
987762 tn?1331027953
Hey Quix,

Once again you've pulled off a great and informative post! Is it going into the health pages?

It might take me a while to connect the dots, brain seems fried today, just not holding on to what i'm reading and understanding today lol. But trying to connect the dots with the sx just to do with my head, not including the brain confusion, and i get...

Eye pain looking up or down. ?? i also go cross eyed
Left eye, occasional opaque blob in middle vision, only lasts a few minutes at a time
Balance, fall backwards looking up

Focus - trouble staying focused, can't hold onto clarity of sight, eyes drift off target
Light sensitive - light = eye pain, pupils no longer go big in dark/low light, sometimes different pupil size but rare.
Blurred sight, not quite double more 1 and a 1/2 eg F11 looks like #111

Tingly static feeling from mid way under right eye traveling up over cheek into scalp,
Hot hot spot on scalp
Nose goes from tingles to numb
Chin tingles or tremors not sure its a new one
Left ear - just the top edge either burns or tingles
Chew fatigue
Tongue control ??? difficulty with articulation
Facial tremors ????? eye lids (still convinced my eyeballs also bounce) and possibly chin

Balance issues - fall over a lot apart from going over backwards
Vertigo - not all the time but when it hits it lasts for weeks at a time
Hearing - sound seems to switch off like the speaker has blown out but it comes back
Tinnitus - seems to happen prior to hearing switching off though not often

I'm not sure if I've got that right, or where the tremors fit in, i've just been putting it all down to lack of muscle control in the same spots day after day. I also have facial pain that comes and goes but i've had that since I was a child so dont count it because its to do with my jaw being out of align for years before they found the cause. I cant work out where the 'where I am in space' goes lol all I can say is that pain like TN isn't something i have, for that i'm grateful! :-)

634733 tn?1316625992
Awesome, so much knowledge here I am overwhelmed

Thank you

Pat x
883901 tn?1294000772
I have found this post very detailed thank-you.

I am still in the limboland, I have a couple of things being effected such as cannot taste my food at times, Tongue goes partly numb, eating also can be hard work as, my jaw gets very tired.  I have not informed my Neuro of this.

I do now suffer with Tinnitus Which is now effecting my hearing, I now have a app in Oct with the ENT.

Avatar universal
Thanks, Quix!  The first half of a health page on this.  Really good info.

One question.  If it's CNII that's responsible for ON, but CNIII that controls pupil dilation, why when I had a suspected case of ON did they keep checking for the afferent pupil effect (my spelling may be off, but they checked the pupil dilation).  Is this something that is normally seen in ON?

Thanks again for all your work on this.  It's very useful.
645390 tn?1338555377
This is an AWESOME post.  Feel like I am back in school.

Funny, I am so familiar with CN III, CN IV and CN VI from work, that I seemed to have "forgotten" about the rest.

I am thrilled with this post in a selfish way, due to the new neurologist told me that I have 4 of the nerves involved due to my MS and only one is eye related.

I really appreciate this immensely.  Looking forward to the next post...the sequel.


Both my MS neuro and now "facial pain" neuro, both said even though they cant see  lesions on my cranial nerves with the MRI, they are definitely there. Just too small to see, but not small enough to not cause damage. (I strongly agree with this!).

1140169 tn?1370185076
Well done, again!

Although by the time I finished the post, I forgot what nerve goes where, I know where to find it again (I think)

I sure hope this becomes a HP, then I know I'll be able to locate it in the future.

Thanks Quix

198419 tn?1360242356
This is outstanding! Thank you so much!
Say the word, Doc Q and I'll prep it for our HPs!

572651 tn?1530999357
A-hah!  I thought your absence was due to some knitting spree to work on your new CNS.  Instead, you were composing yet another masterpiece.  Well done and easily understood.  

As always, thanks for putting it in terms even I can understand.  Now if only I could remember all this.  I'll start studying for the exams!
1386048 tn?1281012333
incredible info and great reference!!

ditto what someone else said above about looking forward to ...the sequel!  lol!

merci beaucoup and asante sana!  (thanks and thanks)

1396846 tn?1332459510
Very very informative!!!!

Love this hope to see more soon.

147426 tn?1317265632
Steph - The pupils are a complex mix of two different actions.  They can activately (meaning under the action of muscles) both constrict to become smaller and they can dilate, becoming larger.  Each action is handled by different nerves.

There are two muscles around the pupil in the iris.  The first is a circular muscle that surrounds the pupil like a sphincter (purse-string) called the "sphincter pupillae."  This is controlled by the parasympathetic nervous system in conjunction (runs alongside) with the third cranial nerve, the Oculomotor Nerve.  When extra light hits the light-sensitive cells in the retina a signal goes to the CN III and a signal comes back via the parasympathetic system to constrict the pupil so that too much light is not let in - overwhelming the visual system.  That is why your pupils constrict in bright light.

Now, for both muscles the action in one eye also makes the same thing happen in the other eye.  So if you shine a bright light into the left eye while shielding the right eye, both pupils will constrict the same.  This is important to understand.  It happens because each nerve affecting the pupils has crossover fibers to the other eye.  When someone is unconscious and the doc checks their pupils, this is the response they are looking for - the pupillary light response which shows that the brainstem is intact.  They are checking for proper functioning of the thrid cranial nerve.

The opposite action, widening (or dilation) of the pupils is handled by the "dilator pupillae" which is part of the Sympathetic nervous system.  This muscle is the opposite of the first.  Instead of going around the pupil, the dialator fibers extend from the edge of the pupil outward to the edge of the iris, so that when they contract they literally "pull" the pupil open.  This action originates in a structure in the mid-cervical spinal cord.

Now, you asked about the afferent pupillary response or defect.  First you need to know two definitions.  There are two words used in nerve signals, "efferent" and "afferent".  One follows sensory signals to the brain - the afferent signals.  The other causes things out in the body to happen and it originates in the central nervous system.  Examples are nerves that make muscles act.  I remember which is which by remembering that efferent signals cause EFFort (like muscular effort).  Therefore the "other one, the afferent signal" does the opposite - it brings information to the game by Announcing.

In a person with MS or with optic neuritis the doctors look for something called the "Afferent Pupillary Defect."  Since the problem uses the word "afferent" we can assume that the problem will be one of sending info upward to the brain and this is exactly what they find.

In a test called the "Swinging Light test"  this is what happens.  The doctor slowly swings the penlight first into one eye.  This should cause both pupils to constrict the same amount.  Right?  Then, the light is swung to the other eye.  As it leaves the first eye and before it arrives at the other eye both pupils will start to dilate again.  So when the light arrives at the second eye you will again see constriction.  That is the normal response when everything is working correctly.

In optic neuritis the sick eye (one with ON) will not register as much light as a healthy eye.  This is because the Optic Nerve (CN II) is damaged and not able to perfectly carry all of the light signal.  So, when the light is shown in the good eye, both pupils will constrict, but the good eye will constrict more because it is getting the full effect of the light.  When the light is moved to the affected eye (the one with ON) not so much light is signalled and the constriction is less marked and the eye may appear to "paradoxically" dilate.  This is called the Marcus-Gunn pupil and is an afferent pupillary problem.

It's not that the pupil in the bad eye actually dilates actively, it's just that it isn't stimulated to constrict as much, so it relaxes to the amount of constriction that fits with the reduced light it is receiving.  

So in this case the problem of the pupil which is not constricting enough is due to the lesion on the Optic Nerve, CN II, and not to the Oculomotor Nerve.

So, when you had optic neuritis and had the problem with the pupil, it was all a part of damage to the same cranial nerve, the Optic Nerve (II).

152264 tn?1280354657
Thanks for great info, Quix. I've got symptoms for numbers 5 and 8 (confirmed with very abnormal AEPs for #8). Bilateral!

Anyway, a question--are those nuclei IN the brainstem, or just beside it? For some reason I always assumed the "nuclei" I've seen mentioned in relation to the vestibular nerve, e.g., are at the intersection of the brainstem and nerve, but then I have not really studied this.

And I did have that very weird crossover thing where left-side BPPV triggered what was almost surely a stapedial spasm in the right ear--which I think had to be a crossed wire in the brainstem itself?
147426 tn?1317265632
The nuclei are within the brainstem itself.
Avatar universal
Thank you, Quix!!  That was a perfect explanation.  I love understanding these details and having difficulty finding this information.  Once again, I'd suggest this be part of a HP on ON.  This is really great info and I so appreciate your time in giving such a thorough answer!
1207048 tn?1282174304
Thank you Quix!! This is a wonderful post!

Do you mind if I ask for your 2 cents? My neuro (an neuro-ophthalmologist) is pretty closed lipped during the exam and I find out most my info as he dictates to the PA at the end of the exam.

I had been telling my neuro about 2 eye issues:
1- about 3 weeks ago my husband noticed my pupils were huge, basically fully dilated, while in a well-lit room. This lasted over an hour, and was still there when I went to sleep, but was gone in the morning. I had complained of a headache, maybe from the extra light my pupils were letting in?
Then last Friday afternoon I was outside in the bright sunny day for a few minutes. When I went back inside, where it is much dimmer (we get morning sun in the house), I thought I was going blind! Everything was very blurry where I could only make out shapes. It was very dark and there was a blue tint to everything (I have blue eyes, if that means anything.) It lasted 5 minutes and then suddenly snapped back to normal. When I told my husband he thought it was my pupils not dilating when I went back inside.

And 2) For 3 days last week I would see flashes in my peripheral vision if I looked to the side, like camera flashes going off on either side of me. This went away after 3 days.

My neuro did repeat OCT and VER tests on me. My OCT was unchanged from April, showing slight optic nerve thinning on my left, but still within normal limits. The VEP in April showed an 8 millisecond delay in my left eye. The one on Monday showed a 10 millisecond delay.

He did not do the flashlight test, but he did put his hands up to cover one eye and switched back and forth, is that kind of the same test or something different?

My fundus exam has always been normal. When he was dictating to the PA, the PA mentioned something that sounded like "phototopis". I've tried looking online, and can't find anything. My neuro also said "Acute optic neuritis" and he has never said those words before, though I have seen him for eye issues the last few visits. My eyes just don't seem right, but they also did a regular "read the chart" eye exam on Monday and it was 20/20!

I'm sorry this is so long! I'm wondering, would what I have experienced the last few weeks be why my neuro said optic neuritis? I had gone to see him because the vertigo started back up last thursday, luckily it is mostly a low-level dizziness with the full vertigo flaring up 4-6 times a day & lasting 1-3 hours, so my neuro did not suggest steroids (whew! I'm not looking forward to doing those again!) but he did say he is 80% sure I have MS, up from 70% at my last visit. And he has suggested I start Copaxone, which I will be.

Thank you for your thoughts on these questions! And, thank you for the wonderful post!
147426 tn?1317265632
I misspoke in the second paragraph about the Swinging Light Test.

"In a test called the "Swinging Light test"  this is what happens.  The doctor slowly swings the penlight first into one eye.  This should cause both pupils to constrict the same amount.  Right?  Then, the light is swung to the other eye.  As it leaves the first eye and before it arrives at the other eye both pupils will start to dilate again.  So when the light arrives at the second eye you will again see constriction.  That is the normal response when everything is working correctly.

In optic neuritis the sick eye (one with ON) will not register as much light as a healthy eye.  This is because the Optic Nerve (CN II) is damaged and not able to perfectly carry all of the light signal.  So, when the light is shown in the good eye, both pupils will constrict, "maximally.  When the light is moved to the affected eye (the one with ON) not so much light is signalled and the constriction is less marked and the eye may appear to "paradoxically" dilate.  This is called the Marcus-Gunn pupil and is an afferent pupillary problem."

Jess - the perception of flashing lights is called photopsis or photopsia.  Maybe that is what you heard.

The test where they cover one eye at a time might not use enough light (if they are depending on regular room illumination) to detect a Marcus Gunn pupil.  I always used the cover test to detect a lazy eye.  So it could be used for that I suppose.  Not sure what he was doing.

A weird thing about optic neuritis is that, though it makes things seem blurry, it often does not affect the visual acuity.  So, the sight might be normal on the Snellen Chart.  Also, if there is blurriness it is not treatable with new glasses.

It sounds like the neuro sees all this as evidence of optic neuritis.

Wish I could say more.

649926 tn?1297657780

   Great Stuff!! I honestly only read a tiny bit because I have eye pain and am waiting for my IV steroids but I know that I am going to be coming back to this when the pain/vision gets better.

  Maybe you will have part #2 done by then ? No pressure the first half could last for weeks of reading.

Thanks again  Erin :)

1207048 tn?1282174304

Thank you for taking the time to answer my additional questions! I really appreciate it :-)

At this point I guess it's not important what brought him to the point of offering Copaxone...I'm just glad he did!

I hope you are feeling well today!
198419 tn?1360242356
I have given this Cranial Nerve discussion (via link) prepared by Doc Quixotic to many people over the years as background info, and it's a mainstay educational piece on our forum.

While looking up additional info for a member who had a question about cranial nerves, I gave her this old discussion link, but also found this other "jewel" very useful and complimentary to our initial discussion!

Please check it out - photos during exam as well!


-Shell .
4461995 tn?1357955108
THANK YOU!!!!    VERY helpful.  I now know that the horrid pain in my ear was trigeminal neuralia!  Not an illusion.    =^-^=
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