Cranial Nerves

Question 1

What are the most important nerves in dentistry (dental-related nerves)? And why?

Answer 1

1. Trigeminal nerve (CN V)
2. Facia nerve (CN VII)

-As we deal with these nerves daily in the clinic either by anaesthetising them (pain control) to facilitate comfortable dental treatment or by avoiding damaging them during surgical procedures (avoid facial weaknesses and palsies

Question 2

There are 12 cranial nerves, explain how were they named and numbered?

Answer 2

• Named based on their functions
• Numbered based on where they arise from in the brain

Question 3

What are the parts of Brainstem?

Answer 3

1. Midbrain
2. Pons
3. Medulla Oblangata

Question 4

Cranial Nerve Nuclei (7 types of information transmitted by cranial nerves)?

Answer 4

Afferent (Sensory):
Transmit sensory info from sensory receptors of body towards CNS “Brain”

1. GSS (General Somatic Sensory): receive sensory information from skin, skeletal muscles and joints
2. GVS (General Visceral Sensory): receive sensory information from the viscera (organs)
3. SSS (Special Somatic Sensory): receive sensory information (senses) derived from Ectoderm (ectodermal retina “sight”, cochlear “sound” and vestibular apparatus “balance”)
4. SVS (Special Visceral Sensory): receive sensory information (senses) derived from Endoderm (endodermal nose “smell” and tongue “taste”)

Efferent (Motor):
Transmit motor information from CNS “Brain” towards muscles and glands “Body”

1. GSM (General Somatic Motor): provide motor innervation to skeletal muscles
2. GVM (General Visceral Motor): provide secreto-motor function to smooth muscle “gut” & glands
3. SVM (Special Visceral Motor): provide motor innervation to skeletal muscles of pharyngeal arches

Question 5

Definition of Foramina?

Answer 5

Openings (holes) in the cranium where the cranial nerves pass through to exit the cranium and go to perform their sensory and motor functions

Question 6

What are the 3 Foramina where the cranial nerves pass through to exit the cranium?

Answer 6

1. Foramen Rotundum (Passage of Maxillary nerve “2nd division of Trigeminal nerve / V2 of CN V”)
2. Foramen Ovale (Passage of Mandibular nerve “3nd division of Trigeminal nerve / V3 of CN V”)
3. Internal Acoustic Meatus (Passage of Facial nerve “CN VII”)

Question 7

Answer below questions about the CN 1:
Name:
Arise from:
Function:
Clinical/Dental relevance:
How to examine:

Answer 7

Name: Olfactory

Arise from: Cerebrum

Function:
Sensory-
Transmits Smell info from Nose to Olfactory Area in Temporal Lobes (CNS)

Clinical/Dental relevance:
Limited relevance:
Anosmia (Partial or Full Loss of Smell)

How to examine:
*Ask if any changes in smell & taste
*Test each nostril (with the occluding of the other nostril) by asking to identify a specific smell (eg: coffee/mint) with eyes closed

Question 8

Answer below questions about the CN 2:
Name:
Arise from:
Function:
Clinical/Dental relevance:
How to examine:

Answer 8

Name: Optic

Arise from: Cerebrum

Function:
Sensory-
Transmits Vision info from Retinas (Eyes) to Visual Cortex (CNS)

Clinical/Dental relevance:
Blindness (to one of the eyes or both)

How to examine:
Using Snellen chart by an optometrist

Question 9

Regarding the Optic nerve:

Where do the paired optic nerves meet?

Answer 9

The optic chiasma:

• Nasal retinal axons will cross to the opposite side of the brain
• Whereas temporal retinal axons do not cross sides.

Question 10

Answer below questions about the CN 3:
Name:
Arise from:
Function:
Clinical/Dental relevance:
How to examine:

Answer 10

Name: Oculo-motor

Arise from: Junction of Midbrain and Pons

Function:
Motor-

1. GSM: Transmits info to 4 Extrinsic Eye muscles + Levator Palpebrae Superioris (Eyelid muscle to elevate the lid)
2. GVM: Transmits info to Pupillary Sphincter (constricts pupil)

Clinical/Dental relevance:
1. Oculo-motor nerve palsy/damage due to:
*Raised intra-ocular pressure
*Aneurysm
*Cavernous sinus infection
*Trauma

2. Down & Out position of the eye at rest due to paralysis of Some of the extrinsic muscles (Superior rectus, Inferior rectus, Medial rectus and Inferior oblique) without affecting Superior oblique and Lateral rectus activity
3. Ptosis (drooping of upper eyelid) due to paralysis of eyelid muscle (Levator Palpebrae Superioris) without affecting Orbicularis Oculi activity (Eyelid muscle to close the lid)
4. Dilated Pupil due to
   Unopposed action of dilator pupillae muscle (this means with bright light, Pupils will keep in dilating “no constriction”, so no light reflex)

How to examine:
*Ask pt if having changes in vision (Blurring, Diplopia)

*Test eye movements by asking them to follow you finger/pen in H-shaped movement

Question 11

Answer below questions about the CN 4:
Name:
Arise from:
Function:
Clinical/Dental relevance:
How to examine:

Answer 11

Name: Tro-chlear

Arise from: Midbrain

Function:
Motor-
Transmits info to Superior Oblique muscle (Eye muscle that depresses “sad eyes” and internally rotate the eyes)

Clinical/Dental relevance:
Tro-clear nerve palsy/damage due to:

*Raised ICP pressure
*Cavernous sinus thrombosis
*Micro-vascular damage (Hypertension & DM)

Leading to:
*Vertical Diplopia (Blurring or Double vision) that exacerbated when looking downwards & inwards (reading, walking downstairs)

*Head tilt away from the affected side

How to examine:
*Ask pt if having changes in vision (Blurring, Diplopia)

*Test eye movements by asking them to follow you finger/pen in H-shaped movement

Question 12

Answer below questions about the CN 5:
Name:
Arise from:
Function:
Clinical/Dental relevance:
How to examine:

Answer 12

Name: Trigeminal

Arise from: Pons

Function:
Both (S+M) & Some Para-sympathetic -

S: Transmits info from skin, mucous membranes (All intra-oral tissues) and facial sinuses to CNS

M (Only by the Mandibular division “V3” due to having motor fibers):
Transmits info to MOM

Supplies/Innervates all derivatives of the FIRST pharyngeal arch (Anterior belly of digastric, Mylohyoid, Tensor veli palatini (soft palate) and Tensor tympani (ear))

Clinical/Dental relevance:
1. Highly-related to dentistry during LA and 3rd molar surgery so be careful

2. Trigeminal neuralgia (Painful nerve malfunction in the distribution of Trigeminal nerve)
3. Unexplained or new onset numbness in the distribution of the Trigeminal nerve (Red flag!!)

How to examine:
1.Test of ophthalmic nerve (Corneal reflex): Using cotton roll to touch the cornea (Lack of a reflex indicates damage to either the trigeminal “opthalmic” or facial nerves)

2.Test of Sensory supply: Ask pt to close eyes & introduce a cotton to areas of the face supplied by CN5 (all 3 divisions) to detect tactile sensory competence

3.Test of Motor supply: Ask pt to clench jaw and you palpate Temporalis and Masseter contractions competence. Then ask pt to open mouth and deviate it to left & right to check Medial and Lateral Pterygoid competence

Question 13

Trigeminal nerve branches leave the cranium into 3 trunks (Divisions). Mention the 1st division and explain its function?

Answer 13

Opthalmic branch (V1):

Function:

1) S:

-General Somatic Sensory (GSS) innervation from Scalp, Forehead, Frontal, Ethmoid sinuses, Upper eyelids, Cornea and dorsum of the nose

-Corneal reflex: Opthalmic nerve detects a stimulus on the cornea, stimulating facial motor nerve to stimulate Orbicularis Oculi muscle (muscle to close eyelid) to contract and blink involuntary (So, the corneal reflex is detected by CN V but the reflex (reaction) will be produced by CN VII)

2) Para-sympathetic: Innervates Lacrimal gland

Question 14

How to test the corneal reflex?

Answer 14

The corneal reflex is tested as part of a cranial nerve exam by using cotton roll to touch the cornea (lack of a reflex indicates damage to either the trigeminal “opthalmic” or facial nerves). This isn’t a test that is often required, particularly in dental practice

Question 15

Trigeminal nerve branches leave the cranium into 3 trunks (Divisions). Mention the 2nd division and explain its function?

Answer 15

Maxillary branch (V2):

Function:

1) S:

-General Somatic Sensory (GSS) innervation from Lower eyelid, Cheeks, Nasal mucosa, Upper lip, Upper teeth and Palate

2) Para-sympathetic: Innervates Lacrimal gland

Question 16

What are the 6 branches of the Maxillary branch of the Trigeminal nerve? and explain the function of each branch.

Answer 16

► 6 Branches (MNGZI):

1) Middle Meningeal: supplies the Meninges

2) Nasopalatine: supplies Nasal septum mucosa and Palate around anterior teeth

3) Greater and Lesser Palatine: supply sensation and parasympathetic innervation to gingiva and mucosa near the hard palate

4) Zygomatic: supplies the skin over the zygomatic and temporal bones

5) Infra-orbital: supplies the lower eyelid and the upper lip

Question 17

Trigeminal nerve branches leave the cranium into 3 trunks (Divisions). Mention the 3rd division and explain its function?

Answer 17

Mandibular branch (V3):

Function:

1) S:

-General Somatic Sensory (GSS) innervation from External ear, Anterior 2/3 of tongue (Only general sensation “via lingual verve” as the special taste sensation is supplied by Chorda Tympani “CN 7”), Floor of mouth, Mucous Membranes, Lower teeth (Lower molars, canines and incisors), Lower Lip, Chin and skin over the mandible

2) M:

-Special Visceral Motor (SVM) innervation to MoM (Temporalis, Masseter, Medial and Lateral Pterygoid), Anterior Belly of Digastric muscle, Mylohyoid muscle (Suprahyoid muscle), Tensor veli Palatini and Tensor Tympani

3) Para-sympathetic: Innervates Parotid, Sub-mandibular and Sub-lingual glands

Question 18

4 Terminal Branches in the Infra-temporal fossa? and their function

Answer 18

1) Buccal nerve
2) Inferior Alveolar nerve
3) Lingual nerve
4) Auriculo-temporal nerve

-They innervate Skin, Mucous Membrane and Striated muscle derivatives of the mandibular prominence of the 1st pharyngeal arch (MoM)

Question 19

Answer below questions about the CN 6:
Name:
Arise from:
Function:
Clinical/Dental relevance:
How to examine:

Answer 19

Name: Abducens

Arise from: Junction of Pons and Medulla Oblongata

Function:
Motor -
Transmits info to Lateral Rectus muscle (eye muscle that abducts the eyes)

Clinical/Dental relevance:
Abducens nerve palsy/damage leading to:

*Diplopa
*Inability to abduct (so pt need to rotate the head to see sideways)

How to examine:
*Ask pt if having changes in vision (Blurring, Diplopia)

*Test eye movements by asking them to follow you finger/pen in H-shaped movement

Question 20

Answer below questions about the CN 7:
Name:
Arise from:
Function:
Clinical/Dental relevance:

Answer 20

Name: Facial

Arise from: Junction of Pons and Medulla Oblongata

Function:
Both (S+M), Glandular & Parasympathetic -

-S:
1) GSS sensation to small part of Conchea of external ear

2) SVS (via Chorda Tympani) to taste from anterior 2/3 of tongue, hard and soft palate

-M + Parasympathetic:
1) GVS to Lacrima, Nasal, Sub-mandibular, Sublingual and mucous glands

2) SVM to move facial expression muscles (Posterior belly of Digastric, Stylohyoid and Stapedius in the ear)

-Supplies/Innervates all derivatives of the SECOND pharyngeal arch

Clinical/Dental relevance:

1. Intra-cranial lesions, mostly due to external or middle ear infection but if there is no reason, the disease will be termed Bell’s Palsy, all leading to:

Facial nerve palsy (facial expression muscles will be paralysed and weakened) + other symptoms depend on the lesion location, if in:

• Chorda Tympani: Reduced saliva (dry mouth) and loss of taste on ipsilateral 2/3 tongue
• Nerve to stapedius: ipsilateral hyperacusis (hypersensitive to sound)
• Greater petrosal nerve: ipsilateral reduced lacrimal fluid production

2. Temporary facial nerve paralysis and weakness of all facial muscles due to misplaced ID block (into parotid gland where the CN7 is, rather than the IA nerve) but it will wear off with time. So make sure to hit the ramus and retract slightly before injecting!

Question 21

What is the main trunk of the facial nerve and how many branches it will split into?

Answer 21

The MOTOR ROOT of the facial nerve is the main trunk of the nerve that continues anteriorly and inferiorly into the parotid gland, splitting into 5 branches

Question 22

What are the 5 branches of the facial nerve?

Answer 22

1) Temporal branch

2) Zygomatic branch

3) Buccal branch

4) Marginal mandibular branch

5) Cervical branch

-These branches are responsible for innervating facial expression muscles

Question 23

Does the facial nerve innervate parotid gland?

Answer 23

Facial nerve only pass through the parotid gland but it doesn’t innervate it, as the parotid gland is supplied by the Glosso-pharyngeal nerve

Question 24

What is Chorda Tympani? its function?

Answer 24

A branch of the facial nerve (but not from the main 5 branches).

The chorda tympani para-sympathetic fibres travel with the lingual nerve from Infra-Temporal fossa, but the main body of the chorda tympani goes to innervate the special taste of 2/3 anterior tongue (SVS). Remember that lingual nerve (branch of V3 of CN V) only innervates anterior 2/3 tongue general taste sensation, whereas Chorda Tympani (branch of CN VII) innervates anterior 2/3 tongue special taste sensation

Question 25

Answer below questions about the CN 8:
Name:
Arise from:
Function:
Clinical/Dental relevance:

Answer 25

Name: Vestibulo-cochlear

Arise from: Junction of Pons and Medulla Oblongata

Function:
Sensory -
Transmit info from vestibular system and cochlear system to the brain
(Hearing and Balance)

Clinical/Dental relevance:
1. Hearing loss
2. Tinnitus
3. Dizziness and Nausea
4. Balance loss

Question 26

Answer below questions about the CN 9:
Name:
Arise from:
Function:
Clinical/Dental relevance:
How to examine:

Answer 26

Name: Glossopharyngeal

Arise from: Medulla Oblongata

Function:
Both (S+M)-
-S:
1. GSS to taste posterior 1/3 tongue, oropharynx, external and middle ear and Eustachian tube

2. GVS to carotid body and sinus
3. SVS to taste posterior 1/3 tongue

-M + parasympathetic:

1. GVM to parotid gland
2. SVM to stylopharyngeus

Clinical/Dental relevance:
Absent gag reflex will indicate a damage to glossopharyngeal nerve or vagus nerve

How to examine:
Ask pt to open their mouth and say “Ah” and look at uvula and tongue, both should be in midline position and the uvula should rise symmetrically when saying “Ah”, so any deviation will be a sign of nerve dysfunction

Question 27

Answer below questions about the CN 10:
Name:
Arise from:
Function:
Clinical/Dental relevance:
How to examine:

Answer 27

Name: Vagus

Arise from: Medulla Oblongata

Function:
Both (S+M) & Parasympathetic-

S:
1) GSS from external ear, Larynx and Pharynx

2) GVS from Larynx and pharynx

3) SVS taste from epiglottis region of tongue

M:
1) GVM to smooth muscles of larynx, pharynx and most of GIT

2) SVM to most muscles of larynx and pharynx

Parasympathetic: innervation of viscera

Clinical/Dental relevance:
Absent gag reflex will indicate a damage to glossopharyngeal nerve or vagus nerve

How to examine:
Ask pt to open their mouth and say “Ah” and look at uvula and tongue, both should be in midline position and the uvula should rise symmetrically when saying “Ah”, so any deviation will be a sign of nerve dysfunction

Question 28

Answer below questions about the CN 11:
Name:
Arise from:
Function:
Clinical/Dental relevance:
How to examine:

Answer 28

Name: Accessory
(Spinal accessory)

Arise from: Medulla Oblongata

Function:
Motor-
To sternocleidomastoid muscle (neck) and trapezius muscles

Clinical/Dental relevance: -

How to examine:
Ask pt to rotate head from side to side and shrug shoulders both normally and then against resistance. If there is longstanding damage may see muscle wasting on the affected side

Question 29

Answer below questions about the CN 12:
Name:
Arise from:
Function:
Clinical/Dental relevance:
How to examine:

Answer 29

Name:
Hypoglossal (under-tongue)

Arise from: Medulla Oblongata

Function:
Motor -
To all intrinsic and extrinsic muscles of tongue except palatoglossus (Innervated by vagus)

Clinical/Dental relevance:
Uncommon but can be caused by penetrating injury, H&N malignancy

How to examine:
Ask pt to protrude tongue and check for deviation, muscle twitching on the affected side and ask to push tongue against cheek and feel pressure and weakness against your hand