The Cranial Nerves

Question 1

What are the cranial nerves

Answer 1

-Cranial nerves are PNS components that connect directly to the brain rather than to the spinal cord
-There are 12 pairs of cranial nerves

Question 2

What do the number of cranial nerves correspond to

Answer 2

-The number corresponds to the level that they emerge from the brain

Low numbers = most anterior (cerebrum)
High numbers = most posterior (medulla)

Question 3

Describe CN I

Answer 3

The olfactory nerves

Special sensory (smell)
Origin: Receptors of olfactory epithelium
Pass through: Cribriform plate of ethmoid bone
Destination: Olfactory bulbs

Question 4

Pathway of CN I

Answer 4

Axons from special sensory neurons collect to form ~20 bundles that penetrate the cribriform plate

Olfactory afferents synapse in olfactory bulb

Axons of post-synaptic neurons proceed to cerebrum as the olfactory tracts

Question 5

Clinical test for CN I

Answer 5

Clinical test = hold strong smelling substance under each nostril

Question 6

What is CN II

Answer 6

Optic nerves

Special sensory (vision)
Origin: Retina of eye
Pass through: Optic canals of sphenoid bone
Destination: Diencephalon via optic chiasm

Question 7

Describe the nasal visual field

Answer 7

Images from nasal visual field hit temporal half of retina

Question 8

Describe the temporal visual field

Answer 8

Images from the temporal visual field hit nasal half of retina

Question 9

Describe the pathway of the optic nerves

Answer 9

Optic nerves converge at ventral , anterior margin of diencephalon, at the optic chiasma

Axons from nasal half of retina (temporal visual fields) cross at chiasma

Axons from temporal half of retina (nasal visual fields) remain uncrossed

-Information from both eyes passes into each optic tract

Question 10

Pathway of the optic tract

Answer 10

Some axons diverge as retinotectal axons to the midbrain for reflex activities of the eye (blinking, light-reflex and accomodation)

The majority of axons synapse in the lateral geniculate nuclei of the thalamus and continue as optic radiation to the visual cortex

Question 11

Points of damage or disease and their effects

Answer 11

Optic nerve (A) = blindness in same eye
Optic chiasma (B) = bitemporal hemianopia (loss of temporal fields)
Optic tract or radiation (C) = complex visual losses

Question 12

Clinical tests for visual loss

Answer 12

Simple test = Snellen charts (measures visual acuity)
Test visual fields (4 quadrants of each eye)

Question 13

What are CN III, CN IV and CN VI known as

Answer 13

The oculomotor (CN III), trochlear (CN IV) and abducens (CN VI) nerves

Question 14

Describe CN III, IV and VI

Answer 14

All three are motor nerves to extraocular muscles of eye
Oculomotor nerve (CNIII) also carries parasympathetic axons supplying pupillary constrictor muscles and ciliary muscles involved in eye reflexes
All three pass through the superior orbital fissures of sphenoid bone

Question 15

Name the extraocular muscles of eye

Answer 15

4 rectal muscles: lateral, superior, inferior and medial rectus

2 oblique muscles: superior and inferior oblique muscles

Question 16

Actions of extraocular muscles

Answer 16

Question 17

What do the 3 pairs of CN nerves innovate

Answer 17

Oculomotor supplies: Superior, medial, and inferior rectus muscles and inferior oblique muscle (also constrictors and ciliary muscles)

Trochlear supplies: Superior oblique (which passes through the trochlear)

Abducens supplies: Lateral rectus
3(LR6SO4) - oculomtor supplies all except LR and SO

Question 18

What does damage to oculomotor nerve do

Answer 18

Diplopia (double vision)

Lateral strabismus (squint) – lateral because LR and SO pull eye laterally

Dilated pupil in affected eye – due to unopposed symapthetic stimulation to dilator pupillae

Question 19

What does damage to abducens nerve do

Answer 19

Diplopia

Medial strabismus – because MR, SR and IR pull eye medially

Abducens has longest path in contact with bone of all cranial nerves - may be compressed against bone due to raised intracranial pressure (ICP) and is an early sign of raised ICP.

Question 20

What does damage to trochlear nerve do

Answer 20

Vertical diplopia (affected eye drifts up)

Torsional diplopia

Patient often inclines head to opposite side to fuse the different images

Question 21

Clinical tests for function of CN iii, iv and vi

Answer 21

All three nerves can be tested by asking patient to follow end of pen or pen torch as you move it across visual fields (vertically, horizontally and diagonally)

Question 22

What is an additional clinical test for oculomotor nerve

Answer 22

Additional test for oculomotor nerve is to shine torch into eyes and check for pupillary light reflex. Pupil in affected eye will not respond.

Question 23

What is CN V

Answer 23

The trigeminal nerve (CN V)
It arises from the pons
The trigeminal nerve is the major sensory nerve to the skin of the face and to underlying structures

Question 24

What are the 3 major divisions of the trigeminal nerve

Answer 24

Ophthalmic (sensory)
Maxillary (sensory)
Mandibular (sensory and motor to muscles of mastication)

Question 25

What is the trigeminal ganglion

Answer 25

Cell bodies of the sensory neurons are contained in the trigeminal ganglion

Question 26

What is the ophthalmic branch V1

Answer 26

Supplies sensory innervation to eye and upper part of nasal cavity

Question 27

What are the 3 major branches of the ophthalmic branch

Answer 27

Frontal – skin of forehead
Lacrimal – lateral skin of upper eyelid and conjuctiva (blinking)
Nasociliary – medial upper eyelid, conjuctiva, globe and upper part of nasal cavity

Question 28

Describe the maxillary branch V2

Answer 28

Almost entirely in bone
Sensory innervation to:
-Skin from lower eyelid to upper lip
-Maxillary teeth (superior alveolar nerves)
-Palate
-Most of nasal cavity

Question 29

Where does v2 exit the skull

Answer 29

Infraorbital foramen

Question 30

Describe the mandibular branch v3

Answer 30

Sensory innervation to skin of lower lip and chin
Sensory to lower teeth
Sensory to anterior 2/3 of tongue and floor of mouth
Motor to muscles of mastication (and other muscles eg mylohyoid)

Question 31

Describe the branches of v3 in the lower aspect of the mandible

Answer 31

Question 32

Clinical test for function of trigeminal

Answer 32

Sharp-blunt test over all divisions (forehead, cheek and jaw). Can the patient discern sharp and blunt?

For motor function, ask patient to clench teeth. Palpate temporalis and masseter muscles. Also, ask patient to protrude jaw, if there is nerve damage, it will deviate to the injured side.

Question 33

What is CN vii

Answer 33

The facial nerve

The facial nerve divides into 5 divisions in the parotid gland.

The facial nerve supplies:
-Motor innervation to muscles of facial expression

-Special taste sensation to anterior of tongue (chorda tympani)

-Parasympathetic secretomotor nerves to the lacrimal glands, and submandibular and sublingual salivary glands

Question 34

What is the postganglionic fibres

Answer 34

Postganglionic fibres
to submandibular and
sublingual salivary glands
/

Question 35

What is the chord tympani

Answer 35

Chorda tympani
(branch of facial nerve)
taste anterior 2/3 tongue

Question 36

Muscles of facial expression

Answer 36

Question 37

Test for clinical function with facial nerve

Answer 37

-Function can be tested by asking patient to make exaggerated facial expressions
-Inspect facial droop or asymmetry
-wrinkle forehead
-screw up eyes tightly
-Purse lips and puff out cheeks
-Stimulate taste on anterior 2/3 tongue

Question 38

What is the effect of Botox on CN vii

Answer 38

Botox blocks neurotransmission in the facial nerve, preventing the muscles of facial expression from creasing the skin

Question 39

What does damage to CN vii do

Answer 39

-Between brainstem and middle ear - all branches and functions are lost

-After middle ear – taste and lacrimation intact but facial expression lost on that side

-On face – individual muscle groups lost depending on branches damaged