Lecture 8- CN I- IV Flashcards
cranial nerves stem from
the brainstem
- A- midbrain
- B- pons
- C- medulla oblongata
how many pairs of cranial nerves
12
cranial nerves supply the
- the head and neck (vagus nerve only nerve that works outside H&N), inc special sense organs (sight, taste, smell etc)
cranial nerves are part of which nervous system
peripheral
cranial nerves are associated with the
brainstem –> where nuclei are dound
- Some cranial nerves are like spinal nerves and have both
sensory and motor properties (mixed)
- Some cranial nerves will be entirely
motor or entirely sensory
- Some cranial nerves will carry parasympathetic fibres. How many?
- 4 cranial nerves
Signs and symptoms from structures innervated by cranial nerves
- Can arise due to
- injury or lesion involving
- The cranial nerve during its route outside of the CNS
- The brainstem (where CN nuclei are located- e.g. tumours and stroke in brainstem)
- Tracts within the forebrain which communicate with cranial nerves (primary motor cortex- contralateral relationship)
How to remeber cranial nerve topography
2244
- 2 CNs- forebrain
- CN I
- CN II
- 2 CNS- midbrain
- CN III
- CN IV
- 4 CNs – pons
- CN V
- CN VI
- CN VII
- CN VIII
- 4 CNs- medulla oblongata
- CN IX
- CN X
- CN XI
- CN XII
cranial nerve I is the
olfactory enrve
where does the olfactory nerve (CN I) arise
forebrain
special sense of olfactory nerve (CN I)
purely sensory
smell
olfactory nerve (CN I) testing
-
Not routinely tested when doing CN exam (ask about change in smell and taste)
- If tested one nostril at a time
- Absence or reduce sense of smell=
anosmia/ hyposmia
- Commonest cause of anosmia related to the olfactory nerve
= upper resp infection (cold), head injury, tumour in anterior cranial fossa tumours
- Associations with Parkinson’s disease, Alzheimer’s disease
route of the olfactory (CN I)
- Olfactory tract passes through cribriform plate of the ethmoid
- Once they pass the ethmoid the nerves becomes known as the olfactory bulb (x2 on each side)
- Received by the temporal lobe- where we make decisions about smell
how do colds cause asonmia
- When we have a cold, mucosa becomes swollen and inflamed limiting the chemical orders in the air that you smell from reaching the olfactory bulb and then propagated via the olfactory pathway to be perceived by the temporal lobe
Head injury’s and the olfactory pathway
CN nerve II
optic nerve
where does the optic nerve arise from (CN II- optic nerve)
the forebrain
- Carries extension of meninges and subarachnoid space
special sense of optic nerve (C N II)
vision- nopt pain or temp
- Testing the optic nerve (one at a time)
- Pupil size
- Response to light
- Visual acuity (Snellen chart) and visual fields
- opthalmoscopy
- if problem with optic nerve: patients will likely report
blurred vision involving in one eye or complete absence of vision in one eye
Optic nerve on clinical exam
- abnormalities in pupil size, response to light, poor visual acuity and visual fields etc
- evidence of pathology involving the optic nerve that is visible on ophthalmoscopy
- Any disease which affects the optic nerve will cause
- optic nerve dysfunction e.g. optic neuritis (inflamm. of optic nerve- first sign of MS, usually only last a few months), AION (anterior ischaemic optic neuropathy- arteritis)
Change in appearance of CN II on ophthalmoscopy (due to raised ICP)
- papilloedema
- If you squash optic nerve- optic disc becomes very swollen and not distinct
route of the optic nerve (CN II)
- Retinal ganglion cells- which get feedback from rods and cones
- Axons of the retinal ganglion cells form optic nerve
- Exits back of orbit via the optical canal
- Fibres cross and merge at optic chiasm
-
Visual pathway
- At this point mixing or left and right sided axons- form optic tract (mix of left and right axons)
- Optic tract becomes the laterial geniculate nucleus
- Then becomes the optic radiation until it reached the primary visual cortex in the occipital lobe
- There is a communication from the optic tracts wtih the brainstem (midbrain) to allow certain visual reflexes e.g. pupillary reflexes to light
- Retinal ganglion cells- which get feedback
from rods and cones
can tell the optic nerve is working if..
you shine a light in one eye and it causes both pupils to constrict (allowing that the oculomotor nerve doesnt have a lesion)
cranial nerve III
oculomotor nerve
what to think when you hear oculmotor nerve
down and out btiches
oculomotor nerve (CN III) arises from the
midbrain
target tissue of the occulomotor nerve
- Extraocular muscles 4 out of 6
- Levator palpebrae superioris
- Sphincter pupillae (parasympathetic)
oculomotor nerve (CN III) fibres are both
motor and parasympathetic
- Testing the oculomotor
- Inspection of resting gaze
- Eye movements, pupils and pupillary light reflexes, eyelid position (normal or droopinh)
- Lesion will present with eye looking down and outwards
- Other signs and symptoms
- Would causes dipoplia (double vision)
- Ptosis
- Pupil may or may not be dilalted (depending on cause)
-
Causes of CN III legions
*
- Microvascular ischemic causes
- Compressive causes (pupil involving)
- Microvascular ischemic causes of CN III lesions
- >50 yo
- Diabetes/ hypertension (pupil sparing- may have ptosis and double vision)
compressive causes of CN III lesions are
pupil involving
- Nerve squashed
- E.g. aneurysmal (PCA- posterior communicating artery): associated with headache/ retroorbital pain
- E.g. head injury – brain swelling, accumulation of blood
- Tentorial herniation e.g. secondary to ICP
Pathway of the oculomotor nerve (CN III)
- Leaves the midbrain, very close relationship to the tentorium cerebelli
- Can squeeze the CN III- impeding its function
- Tumour found between the midbrain and cavernous sinus also cause CN III pathology
- Pathology of the cavernous sinus e.g. thrombosis- CN III lesion
- Once the oculomotor nerve leaves the cavernous sinus it gets into the orbit via the superior orbital fissure
- Has a superior and inferior division
- Also innervates the LPS and the iris (parasympathetic fibres)
Parasympathetic fibres ( blue circles) of the oculomotor are found in the
more peripheral parts of the nerve
Further away from the vasa nervorum (blood supply through centre of nerves)
Also supplied by Pial blood vessels
Therefore less vulnerable if the vasa nervorum is jeopardised than the other motor fibres closer to the vasa nervorum
oculomotor CN III in the brain image
cranial nerve IV
trochnear nerve
were does the trochlear (IV) arise from
the dorsa (back) of the midbrain
fibres of the trochlear (CN IV) are all
motor
what does the trochlear nerve ( CN IV) control
eye movement
- onyl controls one muscle
which one muscle does the trochlear nerve (IV) innervate
superior oblique muscle
- Testing the trochlear
- Inspection of resting gaze
- Eye movement
if trochlear (IV) nerve legion
Absence of superior oblique muscle- upwards, inwards resting eye (may be very subtle)
- Double vision
- Head tilt to compensate strange eye position
- Causes of Trochlear (CN IV) lesions
- Congenital or acquired
- Acquired
- Microvascular ischamia –> 50 years (diabtes/ hypertension)
- Trauma (head injury- even minor_
- Tumour
- Acquired
trochlear nerve route (CN VI)
- Passes from the dorsal aspect of the midbrain into the cavernous sinus
- Any pathology in CS= trochlear nerve lesion
- Gets into the orbit via the superior orbital fissure (like CN III)
- Purely motor- so just supplies the superior oblique muscle
