Trochlear Nerve – Cranial Nerve IV/4 (Easiest Cranial Nerve)

Introduction

• Its only job is to supply one eye muscle. Therefore it has only motor fibers.
• Before it synapses in the midbrain, the neurons are UMN*. After it synapses in its nucleus in the midbrain, the fibers carried to the muscles are LMN**.

* UMN = Upper Motor Neuron

**LMN = Lower Motor Neuron

Functions in General:

• GSE (general somatic efferent AKA general somatic motor): innervates superior oblique muscle

Nucleus of Trochlear Nerve in Brainstem

• Lies at level of inferior colliculus of the midbrain.
• In anterior part of gray matter.
• Near the cerebral aqueduct.
• Nerve fiber leaving the nucleus pass posteriorly to reach posterior surface of midbrain.
• The nucleus receives corticonuclear fibers from both cerebral hemispheres.
• It receives the tectobulbar tract from the superior colliculus (to connect the trochlear nucleus with the visual cortex)
• Receives fibers from medial longitudinal fasciculus by which it is connected to the nuclei of 3^rd (oculomotor), 6^th (abducent) and 8^th (vestibulocochlear) cranial nerves

Figure 1: Cross section of midbrain at level of inferior colliculus showing the nucleus of trochlear nerve. Also showing is the decussation of both trochlear nerves and their exit from the nuclei and brainstem

Figure 2: Posterior aspect of brainstem showing the trochlear nucleus at the level of the inferior colliculus

Figure 3: Anterior aspect of brainstem showing the
appearance of the trochlear nerve anteriorly

Emerges from brainstem

• Emerges from the posterior surface of the midbrain and immediately decussates with the nerve of the opposite side.

Figure 4: Posterolateral aspect of the brainstem showing the origin of the trochlear nerve.
Red line shows the level of the section of Figure 1

Figure 5: Interior aspect of the skull showing the position of 
the cavernous sinuses

Route

• After emerging from the midbrain it continues into the middle cranial fossa.
• It passes into the edge of the tentorium cerebelli and then enters into the lateral wall of the cavernous sinus below the oculomotor nerve.

Figure 6: Coronal section of the skull showing the cavernous sinuses 
on both sides of the pituitary

Exit from skull

Nerve	Exits	Enters
Trochlear nerve (CN IV)	Superior orbital fissure	Orbit

Figure 7: Interior aspect of the skull showing the brainstem from a postero-superolateral
view and the emergence of the trochlear nerve and its exit from the skull through
the superior orbital fissure

Figure 8: Nerves passing through the superior orbital
fissure

Figure 9: Superior orbital fissure

Branches
It only supplies the Superior Oblique muscle of the eye. No other branches.

Lesions

• LMNL of the trochlear nerve causes paralysis of the contralateral eye’s superior oblique muscle.
• Trochlear nerve palsy and resulting contralateral superior oblique paralysis has varying causes and presentations. 
• The most common presentation required in medical school exams is:
• Normally the superior oblique muscle causes intorsion of the eye. Its paralysis results in extorsion of the affected eyeball.
• This leads to vertical diplopia. The diplopia increases when looking down. Why? Recall that the superior oblique has a role in downward movements of the eyeball. Therefore not only will the affected eyeball be extorted but also can't look downward properly.
• Therefore, the patient will tilt his head to compensate for the diplopia.

Figure 10

References

Drake, R., Vogl, A., & Mitchell, A. (2010). Chapter 8: Head and neck - orbit. Gray's anatomy for students (2nd ed., pp. 878-902). Philadelphia: Elsevier.
Fix, J. (2008). Chapter 11: Cranial Nerves. High-Yield Neuroanatomy (4th ed., pp. 74-87). Philadelphia: Lippincott Williams and Wilkins.

Snell, R. (2010). The cranial nerve nuclei and their central connections and distribution. Clinical neuroanatomy (7th ed., pp. 331-370). Philadelphia: Lippincott Williams and Wilkins.

Snell, R. (2007). Chapter 18: The eye and the ear. Clinical anatomy by systems (pp. 657-685). Philadelphia: Lippincott Williams & Wilkins.

Picture references

1. Done by me
2. By Gray696.png: User:mcstrother derivative work: Mcstrother (Gray696.png) [Public domain], via Wikimedia Commons / retouched from original
3. Done by me
4. "Posterolateral view of brainstem" from Netter's Atlas of Human Anatomy 5e / retouched from original
5. By Anatomist90 (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons  / retouched from original
6. Drake: Gray's Anatomy for Students 2e
7. Unknown (if you know source please message me so I can mention it)
8. "Innervation of orbit and eyeball, anterior view" Drake: Gray's Anatomy for Students 2e
9. "Formation of the lacrimal sac" by Drake: Gray's Anatomy for Students 2e
10. "Figure 11-2" by High-Yield Neuroanatomy series

Olfactory Nerve – Cranial Nerve I/1

Introduction

• Function like indicated by name: olfaction i.e. smell.
• One of the 2 cranial nerves that do not have nuclei in the brainstem and do not enter it (the other is the Optic nerve-cranial nerve 2).

Functions in General:

• SVA (special visceral afferent): it carries the sense of smell (also called olfaction)

Route

• Nerves arise from the olfactory receptors in the olfactory mucous of the upper part of the nasal cavity (above superior concha).

Figure 1: Origin of the olfactory nerve - sagittal view

• Nerve fibers pass through openings of the cribriform plate of the ethmoid bone to enter the skull.
• The nerve fibers now synapse with secondary neurons in the olfactory bulb.
•  The secondary nerve fibers then continue in olfactory tract, which runs at the base of the frontal lobe of the cerebrum.

Figure 2: Real brain specimen showing the base of the brain with the olfactory nerves

• The olfactory tract splits into two tracts called the medial and lateral olfactory striae.
• Lateral stria: axons to olfactory area of cerebral cortex. Olfactory areas of cerebral cortex collectively called rhinencephalon.
• Medial stria: carries fibers to the olfactory bulb of the opposite side.

Figure 3: Base of the brain showing the path of the olfactory nerves

The primary olfactory cortex sends nerve fibers to other brain centres to establish emotional connections to olfactory sensations.

Entry into skull

Figure 4: Closeup of the inside of the cranial cavity showing the entry of the olfactory nerve into the skull through the cribriform plate 

Nerve	Entry	Enters
Olfactory (CN I)	Openings in cribriform plate of ethmoid bone	Anterior cranial cavity

Testing for Nerve Lesion

• Close one nostril and let the patient try to identify a familiar smell (e.g. vanilla) with the other nostril. Repeat for both nostrils.
• If patient cannot identify smell then he has anosmia.
• Cause: could be an ethmoid bone fracture. 

References

Drake, R., Vogl, A., & Mitchell, A. (2010). Chapter 8: Head and neck – Cranial Nerves. Gray's anatomy for students (2nd ed., pp. 848-957). Philadelphia: Elsevier: 

Snell, R. (2007). Chapter 15: The Cranial Nerves and Trigeminal Nerve Blocks. Clinical anatomy by systems (pp. 555-575). Philadelphia: Lippincott Williams & Wilkins.

Snell, R. (2010). The cranial nerve nuclei and their central connections and distribution. Clinical neuroanatomy (7th ed., pp. 331-370). Philadelphia: Lippincott Williams and Wilkins.

Picture references

1. Derived from: By Patrick J. Lynch, medical illustrator. (labeled by was_a_bee) [CC-BY-2.5 (http://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons
2. Derived from: By Neuropathologyblog (Took Picture) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
3. Gray's Anatomy via Wikimedia Commons
4. Unknown (if you know source please message me so I can mention it)

Intro to Med School (5 Things You Need to Know At The Start) - Part 1

So you've made it to foundation year. Let me put it this way: you will be utterly lost at first. And not just directionally, the hey-I-thought-this-was-M27-not-the-cafeteria kind. I mean mentally lost. Unless you have a close upperclassman as a friend, relative or sibling, or a faculty parent who will guide you step by step, then you my friend will be slower than a ticking clock in a boring lecture to figuring things out.

I decided I didn't want loners (like yours truly) or any other student suffering the way I had. Hence, the blog and these specific posts. 


(Yes I realize I could have joined the student advisory committee and helped through the easier way instead of creating an entire blog and showering my oh-so scarce time on it. But with writing, everything is documented so students can revisit things time and time again when they feel lost and future generations also have the benefit; as opposed to a conversation or oral presentation that is said once and dissipates into thin air sooner. Also, I have full-on Sheldon-Cooper-style control over my blog.)

Before I proceed, if you have the following symptoms at any point while reading complicated words that seem foreign to you then you may be panicking pointlessly: racing heart - sweating - anxiety - restlessness.

It may seem daunting, but everything you will go through will be tremendously more exciting than it sounds. I promise. Save for tests. Tests are never fun.

Here're the basics that you need to know and might take you time to figure out on your own:

1. It's not 7 years!

When you first register in medicine, they tell you that it is 7 years. But it's not. Let me break it down to you as it really is:

• Foundation Year: The first year is called "Foundation Year". Since the University of Sharjah's Medical College follows an Australian system, you could say that Foundation year is equivalent to Grade 13 in Australia. It is nearly a continuation of high school therefore making it a semi-medicine year. The subjects are not yet basic medical sciences. 

• Here you will have a course called Introduction to Medical Education which introduces you to how you will study medicine in the following years to come. It introduces you to the PBL system, concept-maps, PBL-based presentations and report-writing in APA style.      
• Pre-clerkship phase: The second year is called "Year 1". Third year is called "Year 2". Fourth year is called "Year 3" (this is the year when most of your classmates from high school are posting Facebook pictures of their graduation projects and gowns and you're still stuck...well in year 3). These years are your basic medical sciences years.
• Clerkship phase: Fifth year is called Year 4. And last but not least sixth year is called Year 5. Year 5 is the year you graduate. In these final 2 years, you will spend your entire week (save for one day) in the hospital. That one day will be spent at the university for lectures.

Now most of you are thinking "But we were told medicine is 7 years and this only adds up to 6". While the rest of you are not heeding any attention to what they are reading. 

•      Internship: The 7th year, my lone-wolfs, is the internship year. In actuality, this year happens after you graduate. Hence, you can do it anywhere in any country your heart desires (given the place you apply to reciprocates this desire). In a lot of places, internship is counted as part of the residency program.

(But I'll leave the internship post for another time; the time when yours truly will go through it, mess up a few things, hit a few road bumps, find the signs to the road bumps after hitting the bump hard and hence transferring the newfound discoveries and knowledge to you chicklings.)

C'est moi apres the hurdles of internship

2. GPA, CGPA.....What's the difference?!

- GPA = the GPA of one semester only
- CGPA = cumulative GPA = the average of all semesters' GPA's put together, so the GPA of each semester you take will be factored in to the CGPA

3. Discounts are weird...

The discounts in the first year are easy: you got high marks in high school, so you'll get a discounted fee for foundation year.
The first year after foundation year: the CGPA you get in foundation year will get you a discount on your Year 1 fee.

BUT, but...but, in Year 1, medical students no longer deal with GPAs and CGPAs like other colleges. For the rest of your Sharjah University medical journey, you will deal with percentages. You must attain an average above 85% by the end of an academic year to qualify for a discount. Discounts vary. Above 85% gets you a 25% discount off your subsequent year fee. Achieve above 90% and you will receive a 35% discount.


4. A trip around medical campus in 80 seconds (guesstimation...)

Smurfs. That's what a lot of older students call foundation-ers. And do you know what happens to "Smurfs"? What your mom warned you of. Never talk to strangers she said. But you grew up and thought her advice didn't matter anymore. Wrong. Pranks. That's what happens to naive smurfs. Not the full-fledge must-post-this-on-youtube pranks, but direction pranks. You ask for M25 and they point you to the cafeteria. I thought students just didn't know the directions themselves when they sent me to wrong places in foundation year. Oh how naive younghood was.

Keeping in mind my map drawing skills are on the lower percentiles of the scale of map-drawing, here is your guide to campus (our medical campus, not the entire university campus. I do not feel like wasting my summer drawing a map to an entire city.). 

I suggest you print this and take it with you on your first few days. 

5. Bus stops

Bus stop labelled "Bus stop (1) benches" is the bus stop for public University City buses. These only transport to certain areas in Sharjah. So if you live in Dubai you may need to use the private buses for money.

Bus stops labelled "Medical bus stop bench" and "Pharmacy bus stop bench" are bus stops that transport you between our campus and the Women's and Men's Colleges and Fine Arts College. They are also the bus stops for buses headed to/from the medical dorms. To know which bus is which ask the driver or read the sign on the bus.

Best Medical Microbiology Books (for Med School & USMLE)

If you ever feel lost as to which book you should read for understanding a particular subject, chances are your fellow students will be of better help than teachers. Why? Simply because we have already tried several sources and will be able to give you a student-oriented view on which books works best (teachers specialize in their particular fields and they usually recommend books that are crowded with information that is redundant for students but necessary for specialists).

Note: Keep in mind what works for others may not work for you. Everyone has their own studying style. The books I recommend on my blog are what I and several of my colleagues found tremendously useful.

Forget Patrick Murray, Cedric Mims or whatever overly detailed book your microbiology teacher has recommended (unless you found it awesome in which case please write your recommendation in the comment section below so as other students may also benefit). After going through quite a few excessively elaborate books which you wouldn't need unless you were planning to be a microbiologist, here are the best ones we found:

1. Clinical Microbiology Made Ridiculously Simple:

This book is a goldmine for us! Humor, mnemonics and student-friendly explanations, it made memorizing those horrifying critter names so much easier. (It goes into too much information for USMLE purposes but the mnemonics and tables were great for quick memorizing.)

2. The Big Picture: Medical Micobiology

It has great pictures and is perfect for clinical aspects of infections. It does not talk much about the organisms. BUT I have happened to very luckily stumble upon its appendix. If you were ever lost in microbiology like I was then you've just hit the jackpot! (And by lost I mean stumbling half-blind half-drunk into a dark crowded kingdom where critters walk headstrong and proud ready to eat out your cerebral cortex cell by cell if they see the tweensiest hint of surrender. I hope that painted a picture as to how lost I was in microbiology. Pharmacology has now taken its place. I will not only make a post but host a nation-wide party if I manage to find a pharmacology summary as awesome as this one.)

It's a great summary that you will regret not reading. When you are in the middle of your exams sweating, heart-racing, going through mind-block, you will thank whoever wrote that book (Neal Chamberlain) for that summary which gives you just the right hints and points to remember the tricky infesters. 

3. USMLE Step 1 Lecture Notes: Immunology and Microbiology by Kaplan

This is also a great book to sum things up once you come to revise. I used this book in my comprehensive end-of-pre-clinical phase exam for revising. What I did was this: I read the other 2 books above and added important notes that were missing in Kaplan where due. By the time I came to revising, it was easy peasy (not the memorizing but the studying process). Whilst everyone found difficulty in the microbiology questions (which had an entire exam booklet on its own), I breezed through them (and saved time on the laborious pharmacology questions. Yes laborious. I would rather have built a castle than answered those questions.)

On its own, this book is great for the USMLE but not so much for university exams.