Cranial Nerves I-VI Flashcards
(39 cards)
Most of the cranial nerves arise from the brainstem.
Are they a part of the CNS or PNS?
PNS
How many cranial nerves are;
- Mixed motor and sensory
- Purely motor
- Purely sensory
Mixed: 4
Motor: 5
Sensory: 3
What sensory function is carried by the 3 purely sensory cranial nerves?
Special sensory function (as opposed to general- temperature, pain etc);
- Hearing and balance (CN VIII/ Vestibulocochlear)
- Vision (CN II/ Optic)
- Smell (CN I/ Olfactory)
How many cranial nerves carry parasympathetic function?
Only 4 (3, 7, 9 and 10)
List 3 places where cranial nerves can be damaged due to Injury/ lesion
- During its route outside CNS
- Brainstem (tumours, other pathology)
- Tracts within forebrain which communicate with cranial nerves
How are Cranial Nerves I and II atypical?
What is the significance of this?
- They are direct continuations of brain substance therefore can be classified as CNS (other CNs are of PNS)
- CNS nerves do not regenerate/ repair as easily as PNS therefore injury/ damage can be more significant if in these 2 nerves
How may cranial nerves arise from each section of brain?
Name them all
Remember: 2 2 4 4
- 2 from forebrain (Olfactory, Optic)
- 2 from midbrain (Oculomotor, Trochlear)
- 4 from pons (Trigeminal, Abducent, Facial, Vestibulocochlear)
- 4 from medulla (Glossopharyngeal, Vagus, Accessory, Hypoglossal)
The Olfactory nerve carries Smell and is not routinely tested, rather simply ask patient for any changes in sense of smell/ taste
How do you test it, if you wanted to?
Close one nostril and ask to smell something strong (1 nerve for each nostril)
Compare Anosmia and Hyposomia
List 3 causes
(These are associated with Parkinson’s and Alzheimer’s)
Anosmia; Absence of sense of smell
Hyposomia; Reduced sense of smell
- Common cold (most common)
- Head/ facial injury
- Anterior cranial fossa tumours
Describe the pathway of the Olfactory nerves from nasal cavity to brain
- Start as nerve fibres in nasal cavity
- Rise through Cribriform Foramina to form Olfactory Bulbs (1 on each side)
- Continues as Olfactory tract to Temporal Lobe
How can head/ facial injury cause absent/ reduced sense of smell?
- Very slight posterior displacement of brain
- Shearing of Olfactory nerves as they run through Cribriform Foramina
What are 2 specific things the Optic nerve is responsible for?
What are 3 things that can be used to test the nerve
- Pupillary size
- Pupil response to light
- Visual Acuity (Snellen Chart, at opticians)
- Visual Fields
- Opthalmoscopy (to directly see nerve)
What are 2 ways patients with optic nerve abnormalities may present
Blurred vision/ absence of vision in 1 eye
List 2 diseases that can affect Optic Nerve
- Optic neuritis (inflammation affecting nerve, may be a sign of MS in future)
- Anterior Ischaemic Optic Neuropathy (Can be caused by Temporal Arteritis)
How can raised intracranial pressure affect Optic nerve
- Increased pressure in Subarachnoid space, which is extended along by optic nerves (they are continuations of forebrain so carry meninges)
- Nerve compressed from outside-> Impaired flow of substances within axon
- Impeded blood flow to/ from optic nerves and retina
Describe the pathway of the Optic Nerve fibres
5 steps
- Rods and cones merge into Retinal Ganglion Cells
- RGC axons form Optic Nerve, with arteries and veins running in the middle
- Nerve exits orbit through optic canal
- Fibres from each nerve cross and merge at Optic Chiasm
- Optic tracts carry mixed information from both eyes towards Primary Visual Cortex (In Occipital lobe)
Describe the connection between the Optic Nerve pathway and the Brainstem
- Optic tract allows communication between optic nerve pathway and brainstem
- This contributes to certain visual reflexes to light (E.g pupil expands/ constricts in response to light)
The Oculomotor Nerve (CN III) has both Motor and Parasympathetic targets.
List them
Motor;
- 4 out of 6 extra ocular muscle
- Levator Palpebrae Superioris
- Ciliary muscles (ParaS too)
Parasympathetic;
- Sphincter Pupillae muscle (controls pupil size)
What are 4 ways we can test the Oculomotor Nerve
- Inspection of resting gaze
- Eye movements
- Pupil size and reflexes
- Eyelid position
How do patients with Oculomotor nerve dysfunction typically present?
What do these patients report?
- ‘Down and out’ pointing eyeball
- Drooping eyelid (Ptosis)
- Pupil may be dilated (depends on cause)
Report double vision (Dipoplia)
Compare 2 types of causes of Oculomotor nerve lesions
- Microvascular ischaemia;
- ‘Pupil sparing’ (No dilated pupil)
- >50, diabetes/ hypertension are risk factors - Compressive;
- ‘Pupil involving’ (Pupil dilated)
- Aneurysm of Posterior Communicating Artery compresses nerve (associated with headache/ retro orbital pain)
- Tentorial herniation (due to raised ICP)
Describe the pathway of the Oculomotor nerve in 3 steps
- Leaves midbrain, close to free edge of Tentorium Cerebelli (herniation around here may compress nerve)
- Passes through Cavernous Sinus (can get a thrombosis here)
- Passes through Superior Orbital Fissure to enter the orbit, nerve branches into Superior and Inferior divisions (responsible for keeping eyelid open)
Why doesn’t Microvascular Ischaemia generally affect the Parasympathetic fibres of CN III
- Motor fibres are affected by ischaemia of central Vasa Nervorum
- Peripheral parasympathetic fibres have their own vessels, the Pial vessels
(However compression would affect the Parasympathetic fibres first)
Is a Pupil Sparing or Pupil Involving CNIII lesion more urgent?
Why?
Pupil involving, as the suggests there is something exerting pressure on the nerve (this could be a tumour, haemorrhage, herniation etc.)
