7. Cranial Nerve I-VI Flashcards
How many cranial nerves are there and what nervous system are they a part of?
12, part of the PNS
Where do the cranial nerves originate from?
Brainstem (except for 2 which arise from the forebrain)
What do the cranial nerves supply?
All supply structures in the head and neck, Vagus also supplies structures in the thorax and abdomen
How are the cranial nerves numbered?
Relates to order that they arise rostral to caudal
What is the brainstem?
Adjoins the brain to the spinal cord. Continuous with spinal cord caudally. Ascending sensory and descending motor fibres between brain and rest of body run through the brainstem
What neurones make up the cranial nerves?
- some are mixed
- some are entirely sensory
- some are entirely motor
- some will carry parasympathetic function
signs and symptoms from structures innervated by cranial nerves can arise due to injury or lesion involving which areas?
1) The cranial nerve during its route outside of the CNS
2) The brainstem (where CN nuclei are located)
3) The tracts within forebrain which communicate with cranial nerves
What is the cranial nerve topography?
2 2 4 4
2 - forebrain
2 - midbrain
4 - pons
4 - medulla
Which cranial nerves are not ‘true’ cranial nerve and why?
The first 2 cranial nerves, they are paired anterior extensions of the forebrain rather than a true cranial nerve
What is cranial nerve I?
Olfactory
What is the path of cranial nerve I?
olfactory nerves in Roof of nasal cavity → Cribriform foramina → Olfactory bulb → Olfactory tract → Temporal lobe
what is CN 1 an extension of?
forebrain
Which part of the brain do to the first cranial nerves go to?
Uncus of the temporal lobes
What type of fibres do the first cranial nerves have?
(Special) sensory fibres
How is CN1 function tested?
Not routinely tested in cranial nerve exam(if tested: one nostril at a time)
? Absence or reduced sense of smell = anosmia/hyposmia .
What is the most common and other causes of anosmia?
Upper respiratory tract infection (cold) is most common cause.
Head/facial injury- impact
Anterior cranial fossa tumours
…associations with Parkinson’s Disease, Alzheimer’s Disease
How might head injury cause anosmia?
Secondary to shearing forces and/or basilar skull fracture
anterior to posterior displacement of brain in skull
how might URT cause anosmia?
causes respiratory mucosa lining to become swollen and interferes with chemical odours being able to reach olfactory nerves
What is the second cranial nerve?
Optic
What type of fibres are in the optic nerves?
Special sensory
What are impulses in the second cranial nerve generated by?
generated by cells within retina in response to light: generates action potentials which propagate along optic nerve
Where are the impulses from the second cranial nerves sent?
Primary visual cortex in the occipital lobe.
What is the path of the second cranial nerve?
Retinal ganglion cells → Axons form optic nerve → exits back of orbit via optic canal → Fibres cross and merge at optic chiasm → Optic tracts
What happens at the optic chiasm?
Mixing of sensory fibres from right and left optic nerves
Give examples of different visual lesions.
retinal detachment, optic neuritis, pituitary tumour, stroke
What lesion is a common cause of optic chiasm compression and what type of visual impairment does it cause?
Pituitary tumour, causes bitemporal hemianopia
What is bitemporal hemianopia?
vision is missing in the outer half of both the right and left visual field
What tests can be done to measure function of second cranial nerve?
Pupillary size and response to light
Visual acuity (Snellen Chart) and visual fields
Ophthalmoscopy
What is the optic disc?
Point at which nerve enters the retina
Why is the second cranial nerve affected when ICP rises and what does this cause?
Optic nerve is an extension of the forebrain so has extension of meninges, and has ECF in subarachnoid space. Increased intracranial pressure increases pressure in subarachnoid space so can squash the optic nerve. Causes papillodema.
How may lesions of optic nerve present?
On clinical examination –
• Abnormalities in pupil size and response to light (may be very subtle
• Poor visual acuity
• Evidence of pathology involving the optic nerve that is visible on ophthalmoscopy
What may cause optic nerve lesions?
Any disease involving optic nerve e.g optic neuritis, AION
What is AION?
anterior ischaemic optic neuropathy - optic nerve starved of blood supply and function affected e.g temporal arthritis which affects superficial temporal artery and any branches of ECA and some if ICA which includes ophthalmic artery
What is optic neuritis?
inflammation affecting optic nerve - deterioration in visual function - temporary
Why is there communication between optic tracts and brainstem(midbrain)?
allow certain visual reflexes e.g. pupillary reflexes to light
WHat is the difference in visual loss if optic chiasm affected compared to optic nerve?
optic nerve - only affects eye involved
optic chiasm - affects both eyes as mixing of fibres from left and right eye
What is papilloedema?
Swollen optic disc
Why is there photophobia in meningitis?
Irritation of the meninges around the optic nerve.
What is the third cranial nerve?
Oculomotor
What is the path of the third cranial nerve?
Midbrain → cavernous sinus → Superior orbital fissure → Muscles of eye
What type of fibres are in the third cranial nerve?
Motor and autonomic (parasympathetic)
What does the third cranial nerve innervate?
- Most of the muscles that move the eyeball (extra-ocular muscles)
- Muscle of the eye lid
- Innervates the sphincter pupillae muscle
What is the muscle of the of eyelid?
Levator palpebrae superioris - controls elevation of eyelid
What is the functions of the sphincter pupillae muscle?
Constricts pupil
What do the parasympathetics of the third cranial nerve innervate?
Sphincter pupillae muscle
When is the third cranial nerve vulnerable to compression and by what?
Between temporal lobe and uncus, when there is a raised ICP which causes herniation of the uncus
- close relationship to edge of tentorium cerebelli
Where do the parasympathetic fibres lie in the third cranial nerve and what is the consequence of this?
Around outer edge of nerve, pupils are affected first (dilate) if this nerve begins to be compressed from the outside and then the rest of motor function affected
What is the blood supply to CNIII and what is the effect of diseases affecting the blood supply?
- vasa nervorum is the blood supply to most of the CN 3 and is found in the centre of the nerve so if diseases affect this blood supply e.g microvascular ischaemia, the motor function will be affected first.
- there are pial blood vessels around the outside of the nerve supplying the parasympathetic fibres therefore, in diseases such as microvascular ischaemia affecting vasa nervorum, the parasympatheic fibres are not affected as they have the pial back up blood supply. therefore, pupils are spared
What condition in the cavernous sinus can affect CN III?
thrombosis`
Which cranial nerves go through the cavernous sinus?
Oculomotor, Trochlear, Ophthalmic, Maxillary, Abducens
Which cranial nerves go through the superior orbital fissure?
Oculomotor, Trochlear, Ophthalmic, Abducens
What is checked to test function of the third cranial nerve?
Inspection of resting gaze
Eye movements, pupils & pupillary light reflexes, eyelid position
What signs/symptoms can pathology in the third cranial nerve cause?
Report double-vision (dipoplia) On examination - • Ptosis • Abnormal position of eye • Pupil may or may not be dilated (depending on cause)
What position will the eye assume in cranial nerve III palsy and why?
Down and out, unapposed lateral rectus (CNVI) and Superior oblique (CNIV)
What are some causes of injury to the third cranial nerve?
• Microvascular ischaemia: >50 years, diabetes/hypertension [pupil sparing]
• Compressive: (pupil involving)
- aneurysmal (PCA): associated with headache/retroorbital pain
- head injury,
- tentorial herniation e.g. secondary to increased ICP
- Raised intracranial pressure (tumour/haemorrhage)
- Cavernous sinus thrombosis
What is the fourth cranial nerve?
Trochlear
What type of fibres does the fourth cranial nerve have?
Purely motor
What is the function of the CNIV?
Innervates one of the muscle that move the eyeball
• Superior oblique
What is the function of the superior oblique muscle?
Abduction, medial rotation, depression
What is the path of the trochlear nerve?
Midbrain → Lateral wall cavernous sinus → Superior orbital fissure → Superior Oblique
How many cranial nerves emerge from the dorsal brainstem?
Only CNIV (has longest course of any cranial nerve)
What are causes of CNIV injury?
• Congenital or acquired • Acquired - microvascular ischaemia->50 years (diabetes/hypertension) - trauma (head injury- even minor) - tumour
What signs/symptoms can damage to CNIV cause?
Report double-vision (dipoplia)
On examination -
• Abnormal eye position (may be very subtle!) - upwards and inwards position
• Head tilt…
Why is a CNIV lesion often subtle?
Patients correct the diplopia with tilt of the head
What is CNV?
trigeminal nerve
What are the branches of CNV?
Va - Ophthalmic
Vb - Maxillary
Vc - Mandibular
What is the path of CNV?
Pons → Trigeminal Ganglion → 3 branches
What is the path of CNVa?
Pons → Trigeminal Ganglion → cavernous sinus→ Supraorbital fissure → branches
What is the path of CNVb?
Pons → Trigeminal Ganglion → cavernous sinus→ Foramen rotundum → branches
What is the path of CNVc?
Pons → Trigeminal Ganglion → Foramen ovale
What type of fibres does the fifth cranial nerve have?
General sensory, Motor only in mandibular branch to muscles of mastication
What are the functions of the trigeminal nerve?
- Main sensory nerve supplying skin of face and part of scalp
- Sensory to deeper structures within the head e.g. paranasal air sinuses, nasal and oral cavity, anterior 2/3 of tongue (general sensation NOT taste), and meninges
- Motor to muscles of mastication (Vc only)
How do you test CNV function?
Checking sensation (to touch) in areas of its dermatomes (Va, Vb, Vc), Test muscles of mastication (jaw jerk) and corneal reflex.
What is trigemnial neuralgia?
Condition characterized by attacks of severe pain occuring in an area of sensory distribution of the trigeminal nerve. Thought to be caused by nerve compression
How may trigeminal lesions present?
- Sensory deficits within the dermatomal regions (on affected side)
- Weakness in muscles of mastication (on affected side-if Vc involved)
- [Absent corneal reflex (CN V is sensory part of this reflex)]
Describe the corneal reflex?
involuntary blinking of the eyelids elicited by stimulation of the cornea (such as by touching)
sensation from front of eye is carried by ophthalmic division of trigeminal nerve. The sense of touch is carried to brainstem and communicated with facial nerve to close eye - orbicularis oculi
What are some causes of injury to the fifth cranial nerve?
- Trigeminal herpes zoster i.e. shingles (reactivation of VZ in trigeminal ganglion)
- Trigeminal neuralgia (compression from an aberrant blood vessel)
- Orbital and mandibular fractures (distal branches of CN V divisions)
- Posterior cranial fossa tumours
- [Brainstem infarcts/lesions [medulla /pons] e.g. SOL, MS]
What are some branches of CNVa?
Frontal, Lacrimal and Nasociliary (enter into orbit through superior orbital fissure)
What does the frontal nerve continue as?
Continues out of orbit and onto forehead as supraorbital and supratrochlear nerves
Which branch of CN Va innervates the front of eye and what is the clinical significance of it?
nasociliary nerve - accounts for why the tip of nose and front of eye is under Va dermatome
- ophthalmic shingles will only develop onto nose if this nerve is involved
What is the Hutchinson’s sign?
presence of crusting and vesicular rash on nose in ophthalmic shingles - increases the liklihood of front of eye being involved in opthalmic shingles
What are some branches of CNVb?
Infraorbital nerve which branches into superior alveolar nerves (anterior, middle and posterior)
What do the superior alveolar nerves innervate?
Upper teeth and gums - sensory
What is the route of the infraorbital nerve?
route through floor of orbit to get to superficial skin of face - carries sensory info from region of cheek below orbit
Which nerve is susceptible to injury in orbital floor fractures and what does it normally innervate?
Infraorbital nerve (branch of CNVb) runs along floor of orbit (exits at infraorbital foramen). Innervates lower eyelid and area of cheek
What are some branches of CNVc?
– Auriculotemporal nerve
– Lingual nerve (carrying general sensation from anterior 2/3 tongue)
– Inferior alveolar nerve which branch into mental nerve [mandibular #s]
What is the course of the inferior alveolar nerve and what does it innervate?
Enters bony canal in mandible via mandibular foramen and exits via mental foramen as mental nerve. Mental protuberance, lower lip and teeth and gum of lower jaw
In what type of fracture is the inferior alveolar nerve vulnerable?
mandible fracture
What does the lingual nerve innervate?
General sensory from the anterior 2/3 of the tongue
What does the Auriculotemporal nerve innervate?
general sensory from part of ear, temple area/lateral side of head and scalp and temporomandibular joint
What first comes of the Vc branch of trigeminal?
motor fibres to muscles of mastication
What is cranial nerve VI and where does it originate from?
Abducens nerve, from the medial lower pons (junction between pons and medulla)
What type of fibres are in CNVI?
Purely motor
What does CNVI innervate and what does it do?
Lateral rectus muscle (extra ocular muscle), abducts the eye
What is the path of CNVI?
pons → Runs up and through cavernous sinus → superior orbital fissure → lateral rectus
Which part of the cavernous sinus does CNVI go through, how is this different to others?
Goes through more centrally in close relation to ICA, others pass at lateral wall
How is CNVI tested?
Inspection of resting gaze, eye movements
What would a patient with CNVI lesion present with?
Report double-vision (dipoplia- worse in lateral gaze on side of lesion)
On examination -
• Abnormal eye position
• Difficulty/unable to move affected eye laterally
What are some causes of CNVI lesions?
Microvascular ischaemia (diabetes/hypertension) Head injury, tumour Raised ICP (of any cause) → false localising sign
Why is CNVI susceptible to injury in raised ICP and what is the false localising sign?
Can be easily stretched in raised ICP due to emerging anteriorly, at ponto-medullary junction before running upwards towards cavernous sinus - steep upward route makes it more vulnerable in raised ICP
- CNVI stretched due to pressure and not compressed by anything as not structure in close proximity to compress it - false localising sign
Which cranial nerves supply the eye muscles?
- oculomotor
- trochlear
- abducens
What is common in the route of the 3 cranial nerves su[pplying eye muscles?
Exit brainstem (slightly different levels) Pass through the cavernous sinus Enter into the orbit via the superior orbital fissure
