4 cranial nerve

8/2/2019 4 Cranial Nerve

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Cranial nerves

Today we will talk about "cranial nerves" there is a table in your textbookabout these cranial nerves, you have to read your textbook.

As a dentist you have to know 5 things about the cranial nerves, these arethe main objectives: I want you to distinguish these nerves on the brain model in the lab ok.Because you're going to be asked about them in the practical exam. Whenyou hold the brain, you should know that this cranial nerve is number 1 or2.

You have to identify where each nerve emerges from the brain, for

example when I ask you the nerve that arises from midbrain isthecranial nerve that arises from the pons in the brain isyou have to knowwhere each one came from. We will speak about each one in more details.

You have to know the skull foramina through which each nerve passes.For example the trigeminal nerve, where does it come from?3 foramina:1.sup.orbital fissure2.rhotundum3.ovali.Another example CN (cranial nerve number 9) the glossopharyngeal:from the jugular foramen.

You have to know the main function of each nerve; the main one, not incomplete details. For example the hypoglossal CN XII is motor to thetongue.

The last thing I want you to know is the complete details aboutCN V=the trigeminal nerve, and CN VII=the facial nerve.

these are the main objectives you need to know about the cranial nervesand those are the main parts I will be asking you about, whether in the labor in the theory exam.

Starting with: why do we call them cranial nerves? Because they aredirectly arising from the brain not from the spinal cord.

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26th of March


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there are 12 PAIRS of nerves this means there are 12 on the right sideand 12 on the left arising directly from the brain, passing through thecranial foramina in the skull and hence their name.

we consider them as part of the peripheral system.

Cranial nerves are numbered in order as they arise from the brain, from

anterior to posterior with Latin numbers. The first one that appearsanteriorly is called cranial nerve I then posterior to it is IIetc "in Grant'sAtlas picture 9.1"

The 1st one is the olfactory nerve CN I which is the smelling nerve andit is the most anterior.

The 2nd is the optic nerve CN II.

The 3rd is the occulomotor nerve CN III: occulo=referring to theeyeball and motor=movement so it is related to the movement of the eye.

The 4th is the trochlear nerve CN IV: which is the most slender one(.)and the smallest. It also provides the muscles of the eyeball

The 5th is the trigeminal nerve CN V: the largest one, once it arises itdivides into 3 main branches: ophthalmic, maxillary, mandibular.

The 6th is the abducent nerve CN VI: from "abduction": for theabduction of the eye. When you move your eye laterally ) )

The 7th is the facial nerve CN VII: providing the muscles of the face

giving all the expressions when you smile and when you're sad.

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As you know we have 2 parts of the nervous system :

1st the central nervous system which is: the brain and the spinal cord

2nd the peripheral nervous system which is: the peripheral nerves distributed throughout

the body and the cranial nerves are part of them.

The smallest is the 4th

The largest is the 5th


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The 8th is the vestibulocochlear nerve CN VIII: responsible for thebalance and hearing.

The 9th is the glossopharyngeal nerve CN IX: arises from the lateral partof medulla oblongata. Glosso: referring to tongue, Pharyngeal: to

pharynx. The function is providing the sensation to the posterior third ofthe tongue and to the pharynx.

The 10th is the vagus nerve CN X: passing in along way through yourbody to the thorax and abdomen it's mainly providing parasympatheticinnervations; this is in the autonomic nervous system.

The 11th is the accessory nerve CN XI; it has 2 roots: spinal &cranialroots. The spinal root provides the trapezius and SCM muscles.

The 12th nerve is the hypoglossal nerve CN XII: it has many rootlets just

beside the pyramid of medulla oblongata for movement of the tongue."This nerve works very well in people who talk too much"

The Olfactory nerveAn entirely sensory nerve that carries the special sensation: smelling.

The olfactory axons emerge from the olfactory epithelium

in the upper part of the nasal cavity.

This olfactory epithelium has smell receptors, the axons of thesereceptors extend from the olfactory epithelium in the upper part of thenasal cavity to pass through the cribriform plate of ethmoid which is

perforated.

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**Both CN VII & VIII

are close to each other and pass through the same foramin:internal auditory meatus

From the level of superior conchae and above we have a special

lining epithelium where the respiratory epithelium changes to

olfactory epithelium.


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"We have magnified the picture of cribriform plate of ethmoid from thesagittal section (in slide#6) you have about 20 perforations in each side inthe cribriform plate of ethmoid so the total is 40."

So the axons extend from smell receptors in the olfactory epitheliumpassing through these perforations into the inside of the cranial cavity.

In the cranial cavity they aggregate together and synapse with otherneurons; other nerves' cell bodies where a dilatation forms, we call it theolfactory bulb. This bulb is "sleeping" just above the cribriform plate ofethmoid, it's the region where the 1st order axons synapse with the 2nd

order neurons .

So at first the olfactory neurons emerge from the nasal cavity, through thecribriform plate of ethmoid to the olfactory bulb, where they synapse withthe 2nd order neurons.

Now the axons of these neurons (2nd order neurons) of the olfactorybulb extend in a tract, we call it the olfactory tract, all the way inside thebrain to the temporal lobe.

In the temporal lobe there is the primary olfactory area, which is the

smell area.

What I want you to know is that the olfactory bulb represents thesynapse area, this means it has the nerves' cell bodies and the tract has theaxons of these neurons(the 2nd order neurons) that extend from the bulb tothe temporal lobe inside the brain.

So when conducting the smelling sense, the receptors of the olfactoryepithelium in the upper part of the nasal cavity carry the sensation via

their axons into the cribriform plate ethmoid toward the bulb where theysynapse with the 2nd order neurons and the 2nd order neurons carry it fromthe bulb (cell bodies) in a tract (axons) toward the primary olfactory(smell) area in the temporal lobe of the brain.

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This is how I picture it

The optic nerve

It's entirely sensory; conducting the vision sense

The axons extend from an inner layer of the eyeball called the retina,where you have the vision receptors rods and cones.

These axons extend to form the optic nerves that pass through opticforamina or canals in the skull. Once they get inside the skull in themiddle cranial fossa, they make the chiasma (crossover). [Chiasma meansx shape and crossover].

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Study aid: "this wasn't mentioned by Dr.Alloh"There is a way to memorize which is sensory and which is motor

and which is both.

I II III IV V VI VII VIII IX X XI XII

"Samer Send Massive Mail 2 Malek 2 Sami & 22More Men"

S=sensory, M=motor, 2=both.

For example: Sami means the CN I is sensory, 2 means CN V is both

sensory & motor, Malek means CN VI is motor


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From the chiasma, other axons extend to form the optic tracts.

then the optic tracts go to the thalamus, which is a major station forsensory pathways, and they synapse there in the thalamus and then the 2 nd

order neurons go to the centre of vision in the occipital lobe posteriorly inthe brain sulcus, mainly in the calcarine sulcus(not sure about the spelling)

Retina of the eyeball optic canal to skull chiasma optic tractsthalamus(synapse) by 2nd order neurons to occipital lobe.

Now, what happens in the crossover?

We expect that the right neurons go to the left side and the left ones go tothe right side. But what's happening actually is that the medial half of theneurons is the only part crossing over in the optic chiasma while thelateral half is not, it stays in the same side of the brain.

For example the right optic nerve of the right eyeball has a medial and alateral half, the medial crosses over to the left side while the lateral staysin the right.

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The optic tracts are different

from opticnerves, why? Becausethere was a crossover (after the

chiasma we call them tracts)

Optic disk


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We have 2 fields of vision for each eye :

1-the nasal field medially: (referring to the nasal bone) the things you seein the medial side

2-the temporal field laterally: (referring to the temporal bone) the thingsyou see in the lateral side.

The temporal field (the lateral things you see) goes to the medial half ofthe optic nerve that crosses over to the opposite side.The nasal field (the medial things you see) goes to the lateral half ofneurons that don't crossover so it stays on the same side.

For example:

As you see in the previous picture the temporal field of the right eye goesto the medial half and then it crosses over to the opposite side(the left).The nasal field of the same (right) eye goes to the lateral half and becausethe lateral neurons don't crossover it stays on the same side (the right).Clearrr??

1st conditionPapilledemaThe optic foramen is covered by 3 layers of meninges: dura, arachnoid& piamatter.The optic nerve, as it's going to the eyeball through the optic foramen,

pulls these layers along with it until it reaches the eyeball, so as it ispassing through, it carries a meningeal sheath: dura, arachnoid,subarachnoid space containing CSF (cerebrospinal fluid), & pia mater.So the optic nerve is covered by the meningeal layer and the CSF is goingwith the optic nerve all the way to the eyeball.

now when there is an increased intracranial pressure (increased CSFpressure) this pressure increases on the optic nerve and reaches the eyeballin the optic disc "I marked it on the previous picture".The optic disc is where the optic nerve enters the eyeball. When theintracranial pressure or CSF pressure increases in the optic disk region,what happens? There will be swelling. This edematous swelling means

there is papilledema. Papilla referring to optic disc.

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Clinica

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So when you put the ophthalmoscope in the eye you will see it as abulging or swelling around the optic disc this is because of the pressure ofthe CSF on the retina of eyeball this is an indication of an increasedintracranial pressure.

2nd conditionVisual Field Defects:

It is when you have difficulties in your vision, resulting from having alesion or damage or injury to the vision pathways: optic nerve, opticchiasma, or optic tract.

unilateral blindness: is when you have loss of vision in one eye.

hemianopsia: hemi = one half of the eyeball, a = without , nopsia =vision so it means loss of vision of one field of the eyeball; either thetemporal or the nasal.

the type of defect depends on where the lesion occurs, if it occurs in:

the optic nerve unilateral blindness occurs.For example if the right optic nerve is damaged, complete loss of vision inthe right eye will occur. So when you diagnose the patient, this is how he

sees with normal eyes (referring to upper circles in slide#18)he can seewith the right and left. But if he can't see in one of the eyes for examplethe right one, this indicates damage to the right optic nerve. This is calledunilateral blindness whether it's right or left side. okkkkkkay!!!

the optic chiasma bitemporal hemianopsia.The damage this time occurs at the level of the optic chiasma. Nowwhich half crosses over in the chiasma?? The medial half! This halfusually takes which field?? The temporal field, so in this case the vision

will be lost in the temporal field of both eyes.

When you have a cut in the optic chiasma, which half is going to becut? The medial one of both eyes: so the medial half can't crossover

because it was cut and lost the connection with the brain. Which field dowe have in the medial half? The temporal field: so we lose the temporalfield of both eyes, and the patient can't see the lateral things; only themedial ones. This is called bitemporal hemianopsia; bitemporal meansloss of vision in both of the temporal fields, hemianopsia means half ofthe vision in one eyeball.

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The optic tract Contra-lateral Homonymous Hemianopsia (THEMOST COMMON INJURY)

The tract, whether the right or the left one, carries the nasal fieldlaterally to the same side, and the temporal field medially to the oppositeside. By damage of the tract you will have a distorted image of a completehalf (right or left fields).

For example the right tract got damaged in the right eye, the lateral iscarrying the nasal field (which is the left field of the right eye) "picture inslide #14", you also have a cut in the medial half of the left side which iscarrying the temporal field of the left eye (which is the left field of the left

eye) so the whole left side (field) has loss of vision.so the cut in the right optic tract results in a left homonymoushemianopsia, homonymous means in the same side; left or right. So whenthe patient has lost the vision in the left side of the right eye (nasal field)and the left side of the left eye (temporal field) that means there is adamage in the right tract. And if he has loss of vision in the right fields,this means he has loss of vision in the left tract.

"that was how Dr.Alloh had explained it but I will summarize it forsimplicity in the next paragraph. Please follow the picture in slide#14"

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When the right tract is injured the left fields of vision are lost.

When the left tract is injured the right fields of vision are lost.

For example the right tract(which is neurons after chiasma) got damaged, so wehave a cut in the lateral half coming from the same side (right) and a cut in the

medial half coming from the opposite side (left). Now what are the fields thatgot damaged? The nasal field of the right eye (considered the left field of the

right eye) & the temporal field of the left eye (the left field of the left eye)respectively. So when you have damage in the right tract, your left fields of

both right and left eyes are lost.


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So when I ask you the unilateral blindness results from a damage in the optic nerve.the bitemporal hemianopsia results from a damage in the optic chiasma.

the homonymous hemianopsia results from a damage in the optic tract.

these are the important clinical conditions, you have to memorize them:the papilledema which is an indication of intracranial pressure.the visual defects regarding the optic pathways.

The Oculomotor Nerve

CN III : from its name it is motor, carrying motor innervations and italso carries parasympathetic innervations.

It emerges from the anterior aspect of the midbrain (midbrain is part ofbrain stem)

Once it arises it divides into 2 divisions: superior & inferior.Both branches enter the orbit through superior orbital fissure and they will

be distributed to the muscles of the eyeball.

We have several muscles in the eyeball mainly the extrinsic ones; CNIII innervates all the muscles that move the eyeball except for 2:

superior oblique : (that moves the eye inferior and lateral) innervatedby trochlear nerve CN IV.(SO4)lateral rectus : (this is the muscle of cheating) innervated by theabducent nerve CN VI.(LR6)

we refer to them in the equation as SO4 & LR6:SO4 = superior oblique innervated by the fourth cranial nerve.LR6 = lateral rectus innervated by the sixth cranial nerve.

And all of the remaining muscles of the eyeball are innervated by theoculomotor nerve.

Introduction to the eye muscles:

When you look at the eye inside the orbit, you will see muscles attachedto it to move it:

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Superior rectus: appears straight; its function is lifting the eyesuperiorly. It's when you look upward.

Inferior rectus : opposite to superior rectus; moves the eye inferiorly.

Medial rectus : moves the eye medially.Lateral rectus: moves it laterally.Superior oblique: inserted in an oblique way in the posterior half of theeyeball superiorly; it pulls the eyeball from its superior aspect and movesit inferiorly.Inferior oblique: inserted in the posterior half of the eyeball inferiorly; it

pulls the eyeball from its inferior aspect it & moves it superiorly.

The last 2 muscles are inserted in the posterior half of the eyeball.They are inserted in an oblique way from medial to lateral (not straight) sosuperior oblique pulls the eyeball infero-laterally and inferior oblique

pulls the eyeball supero-laterally because they are going medially.

these are the 6 extrinsic muscles of the eye, all innervated by theoculomotor nerve except for SO4 & LR6.

Also I want you to add here, the oculomotor nerve provides autonomicmotor innervations to thesphincter pupilli muscle (intrinsic muscle) whichshrinks your pupil.

if you were in a place with strong light and then you go to a dark placeyou can't see a thing, you stay for a while until your pupil gets dilated andthe vision gets better; this is the dilator pupilli.or vice versa, when you go from a dark place into a lit () place,

your pupil gets smaller. The muscle constricting the pupil is sphincterpuplli .

these are the muscles diagnosed by the doctors, when they put light inyour eyes. Why do they put the light? they want to test the function of thedilator pupilli muscle; if the pupil opens or not. So this is a vital sign; andwhy is it a vital sign? Because it is controlled by the autonomic nervoussystem.

Nervous system is 2 parts :

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somatic: voluntary system, for example movement the skeletal muscles.

autonomic: involuntary movement, which issympatheticparasympathetic

Sympathetic: gives you sympathy in dangerous situations( )hence its name. When you are in a dark room and you want to see more,the sympathetic innervations work on the dilator pupilli muscle to dilatethe pupil so that more light gets into the eye resulting in better vision

Parasympathetic: it happens during rest, so when you're relaxed in afully lit room the parasympathetic works through the oculomotor nerve tothe sphincter pupilli muscle providing constriction of the pupil.

So the oculomotor nerve provides:motor innervations for all the extrinsic muscles (outside the eyeball)except for SO4 & LR6. autonomic (parasympathetic) innervations for one intrinsic muscle(inside the eyebll): the sphincter pupilli

Trochlear nerve

CN IV: Emerges from the posterior aspect of midbrain.

enters through superior orbital fissure to provide innervations for SO4 .

the smallest cranial nerve

the main function: motor innervations to the SO4(superior obliquemuscle)which moves the eyeball inferio- laterally (we will speak aboutthis muscle in more details when we speak about the eyeball and theorbit).

Now before we talk about the trigeminal nerve CN V we will talk aboutthe abducent nerve CN VI.

Abducent nerve

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CN VI: a motor nerve for lateral rectus which moves the eyeballlaterally (LR6).

Emerges from the groove or the junction between the pons and medulla

oblongata.

passes through superior orbital fissure.

Trigeminal nerve

CN V: the largest nerve. arises from the pons itself.

Is a xixed nerve having: a large sensory root: mainly going to: the face and to the teeth. a small motor root going to:the muscles of mastication which are 4 muscles

another 4 muscles:mylohyoid : the floor of the mouth.

anterior belly of digastric . Tensor of the tympanic membrane: tensor tympany.

Tensor of the soft palate tensor villi palatini

So CN V provides motor root for 8 muscles.

Once it arises it divides into 3 parts:Ophthalmic V1: {we talked about its story previously in the facelecture}passes to the orbit through superior orbital fissure.it is entirely sensory.Provides sensations to:the tip of the nose,the region of the orbitthe foreheadthe anterior half of the scalp

when the ophthalmic reaches the orbit at that level it gives 3 branches:frontal: to the frontal bone, divides into 2 branches.

nasociliary: divides into several branches but terminates into 2 in theface

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-a small anterior division: gives 3 motor branches and 1 sensory-a large posterior division: gives 3 sensory branches and 1 motor.

2 branches of the main trunk:

1-a sensory nerve to the meninges: called meningeal branch; gets back toforamen spinosum or sometimes foramen ovali. Once it passes throughspinosum, it is called nevous spinosus which is a nerve to the meninges, asensory one, coming from the main trunk of V3.

2-the 2nd branch of the main trunk is a motor nerve to: medial pterygoid &the 2 tensors: tensor tympani & tensor palatini.We said that a small anterior division of the main trunk gives 3 motor

branches and 1 sensory (the total= 4): 3 motor nerves: going to the remaining mastication muscles:lateral pterygoid & masseter & temporalis

1 sensory to the cheek : called the buccal nerve; when you pinchsomeone from his cheek, this is carried by the sensory buccal nerve ofmandibular (anterior division)

A large posterior division: gives 3 sensory branches and 1 motor (4).

3 sensory:1-lingual nerve: the most anterior one; gives sensation to the anterior 2/3

of the tongue. Once it leaves the posterior division, it goes deep to thetongue.

2-inferior alveolar nerve (inferior dental nerve): enters the mandible to theteeth and gives sensation to the lower teeth.3-auriculotemporal nerve: sensation to the auricle and temporal region.

Those are the main 3 sensory nerves of posterior division ofmandibular.

1 motor:Nerve to mylohyoid: a small branch goes to mylohyoid (the floor of the

mouth) and anterior belly of digastric.

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"So the sensation of anterior 2/3 is from the lingual and post 1/3 is from theglossopharyngeal CN IX and motor innervations from hypoglossal CN XII"


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Ophthalmic: provides sensation to: Maxillary to:the tip of the nose, the maxillary region

the region of the orbit zygomatic regionthe foreheadthe anterior half of the scalb.Mandibular to:lower cheek

mandibular area

You have to read every detail in your book about the trigeminal nerveNext lecture we will finish the remaining cranial nervesBest wishes for everyone.The END

Forgive me for any mistakes.

Done by: Esra'a Daraghmeh

a big fat thank you lakoll elly befar3'o elmo7adarat 'cause I just found outthat it's really not as easy as it seems not to mention the back pain or theeyes going out of the skull(anterolaterally) and I don't wanna start with thelousy records.

Bas kolloh behoon la dof3etna el7elweh.

True words from the heart to my lovely friends who gave the days theirsweetness. If I ever regret anything it is that I didn't get to know you

before.To my friends :Alaa, leena, buthaina, 3areen, 3obaida, rawan, hiba,bashayer, Ayat, Deema, Shatha, Ameera, Noora, Manal, Aya, 3abeer,Leena.k, do7a, Wafaa, Shereen, Huda, w lakol eldof3a

And special salam la sadee8et el3omr Sabreen.

Good luck in mid second & final and don't worry there will always beanother semester.

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4 cranial nerve

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