Cranial Nerves I-VI Flashcards
What nervous system are cranial nerves part of?
What are the exceptions
Peripheral nervous system
Except CNs I + II
Where can injury of a cranial nerve arise?
- route outside the CNS (axon)
- brainstem (where CN nuclei are located)
- neurones within brainsteam + forebrain (e.g. upper motor neurones)
Outline cranial nerve topography
2,2,4,4
- 2 from forebrain: I, II
- 2 from midbrain: III, IV
- 4 from pons: V, VI, VII, VIII
- 4 from medulla: IX, X, XI, XII
What is CN I?
Olfactory nerve
1 nose
Outline the olfactory nerve
Cranial nerve I - arises from forebrain
- sense of smell (only sensory)
- test in one nostril at a time
What is anosmia?
Absence or reduced sense of smell
Commonest cause of anosmia
Common cold/upper respiratory tract infection
Pathway of olfactory nerve I
- olfactory receptors
- olfactory nerves travels up through holes in ethmoid bone (cribiform foramina)
- olfactory bulb
- olfactory tract
- temporal lobe
How does the olfactory nerve pass through the skull?
Cribiform foramina
Through ethmoid bone
What can cause a loss in sense of smell (anosmia)?
Start with most common
- common cold/URTI: most common
- head/facial injury: shearing olfactory neurones during passage through cribiform foramina
- anterior cranial fossa tumours: compression of olfactory bulb or tract
What is CN II?
Optic nerve
Outline optic nerve
Cranial nerve II - arises from forebrain
> carries extensions of meninges
- sense of vision (only sensory)
- pupillary size + response to light
How can you visualise optic nerve II?
Ophthalmoscopy
How can you test the function of optic nerve II?
Visual acuity (Snellen chart)
Signs + symptoms of optic nerve II lesions
- blurred visions/complete absence of vision
- poor visual acuity
- abnormal pupil size + response to light
- visible pathology on ophthalmoscopy
Pathway of optic nerve II
- retinal ganglion cells axons
- axons form optic nerve
- exit via optic canal in sphenoid bone
- left + right optic nerve merge at optic chiasm
- continue as right and left optic tracts
How do optic nerves ll travel through the skull?
Via optic canal
What is papilloedema?
Swollen optic disc
What is cranial nerve lll?
Oculomotor nerve
What is cranial nerve lV?
Trochlear nerve
What is cranial nerve Vl?
Abducens nerve
How many cranial nerves have parasympathetic fibres?
What are they?
4
Oculomotor III
Facial VII
Glossopharyngeal IX
Vagus X
How to test the olfactory nerve I?
- change in taste of smell (+ taste associated)
- one nostril at a time
Which cranial nerves innervate muscles within the orbit (move the eye)?
Oculomotor nerve III
Trochlear nerve IV
Abducens nerve VI
Outline the oculomotor nerve
Cranial nerve III - arises from midbrain
- somatic efferent fibres: motor innervation to superior, inferior + medial rectus + inferior oblique+ levator palpebrae superioris
- visceral efferent fibres: parasympathetic | sphincter papillae + ciliary body
Pathways of the cranial nerves that move the eye
Which are they?
- midbrain: III + IV
- caudal pons: VI
- cavernous sinus
- exits skull through superior orbital fissure into orbital cavity
- innervates muscles
Oculomotor III, trochlear IV, abducens VI
How do you test the Oculomotor nerve III?
- inspect resting gaze
- eyelid position
- eye movements H
- pupillary reflexes
Action of levator palpebrae superioris
Elevating eyelids (keeps eyelids open)
Action of sphincter pupillae
Constricts pupil
Signs + symptoms of oculomotor nerve III lesions
- ptosis
- abnormal position of eye down and out
- possible dilated pupil > if compression on parasympatheic fibres
- double vision
Why do you get ptosis in Oculomotor nerve III lesions?
Loss of innervation to levator palpebrae superioris
Why can you get a blown pupil in oculomotor nerve III lesions?
- external compression
- compression of parasympathetic fibres
- loss of parasympathetic signals to sphincter pupillae muscle
- oculomotor nerve has a close relationship to tenteorium cerebelli edge v
uncl herniation - brain herniates over edge of tentorium cerebelli
Why do patients present with ‘down and out’ position in oculomotor nerve III lesions?
- Paralysis of 4 extra ocular muscles
- Only lateral rectus + superior oblique function > ABduction + depression
Causes of Oculomotor nerve lesions
Pupil sparing:
- microvascular ischaemia
Pupil involving: compressive
- aneurysm
- head injury
- uncul herniation
What is ptosis?
Dropping eyelid
Outline the trochlear nerve
Cranial nerve IV - arises from midbrain
- supplies superior oblique muscle
Causes of trochlear nerve IV lesions
Congential or acquired
- microvascular ischaemia
- trauma: head injury
- intracranial tumour
Three risk factors for microvascular ischaemia
> 50 years old
Diabetes
Hypertension
How do you test the trochlear nerve?
- inspection of resting gaze
- testing eye movements
Signs + symptoms of trochlear nerve IV lesions
- dipoplia: double vision
- abnormal eye position up and in
- difficulty moving eye downwards when positioned inwards
What is dipoplia?
Double vision
Action of superior oblique muscle in eye
Intorsion
Depression
ABdction
What cranial nerve has the longest intracranial length?
Trochlear nerve IV
Outline the abducens nerve
Cranial nerve VI - arises from caudal pons
- supplies lateral rectus muscle in eye
- abducting the eye
Signs and symptoms of Abducens nerve VI lesions
- dipoplia - worse in lateral gaze
- abnormal eye position
- difficult lateral eye movement
What risk relating to raised intracranial pressure does the Abducens nerve VI have due to its pathway?
- arises from caudal pons > vertical route + ‘fixed’ at point of entry into cavernous sinus
- raised ICP > downwards displacement of brain > stretch abducens nerve route
- causing false localising sign
Which extraoccular muscles does the Oculomotor nerve supply?
Superior rectus
Medial Rectus
Inferior rectus
Inferior oblique
What is cranial nerve V?
Trigeminal nerve
Outline the trigeminal nerve
Cranial nerve V - arises from the pons
- general sensation to anterior 2/3 tongue
- muscles of mastication via mandibular branch
- sensory function to the face
- three branches from the trigeminal ganglion: ophthalmic, maxillary + mandibular
A cranial nerve lesion on the left side will effect what side of the patient?
Left side
Cranial nerves from brainstem (after the crossing)
What are the branches of the trigeminal nerve V?
Opthalamic Va
Maxillary Vb
Mandibular Vc
Outline the pathway of the ophthalmic Va branch of the trigeminal nerve
What does it supply?
- pass through cavernous sinus + superior orbital fissure
- general sensation to forehead, eye + nose
How do you test for the trigeminal nerve?
- light touch in Va, Vb, Vc dermatomes
- test muscles of mastication
- corneal reflex (ophthalmic division Va test)
Sensory supply of the ophthalmic branch Va of the trigeminal nerve
sensory: corneal reflex > involuntary blink reflex when cornea is touch
Signs and symptoms of trigeminal nerve V lesions
- trigeminal herpes zoster
- trigeminal neuralgia > causes headaches (shooting pains) due to compression from blood vessel
Pathways of the trigeminal nerve V
- ophthalmic: through cavernous sinus + superior orbital fissure
- maxillary: through cavernous sinus + foramen rotundum
- mandibular: through foramen ovale
What does the ophthalmic nerve Va pass through?
Cavernous sinus
Superior orbital fissure
What does the maxillary nerve Vb pass through?
Cavernous sinus
Foramen rotundum
What does the mandibular nerve Vc pass through?
Foramen ovale
What is Hutchinson’s sign?
- Vesicles involving tip of nose of trigeminal herpes
- Increased risk of front of eyeball being involved > potential loss of vision
What is the afferent and efferent part of the corneal reflex?
afferent: trigeminal Va
efferent: facial VII
How is the optic nerve affected by increased intra cranial pressure?
- surrounded by extension of the meninges > continuous with the subarachnoid space
- increased in ICP > increase pressure of CSF > increases tissue pressure within optic nerve
- axoplasmic flow + venous drainage interrupted
