Session 5 - Cranial Nerves I - VII Flashcards
How many pairs of cranial nerves are there?
12
Where do the cranial nerves come from? What do they supply?
Relate to brain stem, arising from CNS.
Innervate structures in the head + neck.
What types of axons are carried by cranial nerves?
General sensory Special sensory Motor Autonomic (a mix of the above, or only one type)
What is the brainstem?
Brain structure connecting the brain to (continuous with) the spinal cord.
What are the brainstems main functions?
Regulation of cardio-repiratory functions, and maintaining consciousness.
Ascending sensory, and descending motor fibres run through it.
What are the cranial nerve nuclei, where are most found?
Nuclei = collections of cell bodies of nerve fibres.
In the brainstem.
How many Cranial nerves come from each part of the brainstem?
Forebrain = 2 CNs Midbrain = 2 CNs Pons = 4 CNs Medulla = 4 CNs
Name the two CNs from the forebrain.
Olfactory CN I
Optic CN II
What is the route, function and type of axons in the olfactory system?
Special Sense.
Olfactory > Cirbriform foramina > Olfactory bulbs > Olfactory tracts > forebrain
Function: Olfaction (smell)
How is olfaction tested?
Not formally tested.
“have you noticed any change in sense of smell”
Testing one nostril at a time, with salts, or any alternative etc.
What is loss of sense of smell called? What can cause it?
Anosmia
- A common cold is most common.
- Head injury (basilar skull fracture, shearing forces)
- Intracranial tumours, at base of frontal lobes, could interfere.
What is the route of the optic nerve, and its function?
Special Sense.
(complex pathway, goes through optic canal to eye)
Function: Vision
How is the optic nerve clinically tested for problems?
Test one eye at a time.
Visual test (snellen chart, visual fields)
Test pupil response/ reflexes using torch
What can cause issues with the optic nerve? How is it visualised?
Optic disc can be seen with ophthalmoscope.
- Raised intra-cranial pressure can cause nerve to swell (as they carry an extension of the meninges).
(can be seen on ophthalmoscope)
- Lesions involving retina or nerve can cause visual disturbances (affecting one eye)
What is the optic disc?
The point where the optic nerve enters the retina.
Where is vision processed in the brain? Why is the brainstem involved?
Visual pathway extends from retina towards primary visual cortex in occipital lobe.
Communication with brainstem allows reflexes, e.g. puillary reflex to light.
What are the two cranial nerves from the midbrain?
Oculomotor CN III
Trochlear CN IV
What is the route, and types of axons in the oculomotor nerve?
Motor (no sensory)
Autonomic (parasympathetic)
Route: From midbrain through cavernous sinus (sides of sella turcica) through superior orbital fissure in oribit.
What is the sella turcica?
A depression in the sphenoid bone containing the pituitary gland.
What is the function of the oculomotor nerve?
Motor
- Most extra-ocular muscles
- LPS (Levator palpebrae superioris)
Autonomic
- Sphincter pupillae (pupil)
- Ciliary muscle - lens
Why is the oculomotor nerve often compress with raised intracranial pressure?
Uncus of temporal lobe can slip/ move over tentorium cerebelli, which is in close proximity to the entry of the nerve, compressing it.
Under compression, why would autonomic function be affected first in the oculomotor? What is a sign?
Due to the autonomic (parasymp.) fibres being on the outer part of the nerve. (motor are inner).
Fixed dilated pupil (blown).
(continued compression, will compromise motor)
How is the oculomotor nerve CN III tested? What may be seen?
Inspect eyelids and pupils.
Test eye movements.
Pupillary light reflex.
Pupillary dilation and/or double vision
‘Down and out’ position of eyes, severe ptosis (eyelid droop)
Which things can cause oculomotor nerve damage?
- Raised IC pressure
- Aneurysms
- Vascular (2ndry to diabetes, hypertension/ often pupil sparing)
- Cavernous sinus thrombosis
What does ‘pupil sparing’ mean?
That the nerve damage/ compression is not affecting the pupillary response, but the eyelids, or other parts.
What is the route, and functions of the Trochlear nerve CN IV?
- Motor fibres.
Route: Midbrain > Cavernous sinus > Superior orbital fissure.
Function: one extra-orbital muscle (superior oblique)
Where does the trochlear nerve originate on the brainstem?
Emerges from the dorsal brainstem, has longest intracranial course.
(only CN to do so)
How is the trochlear nerve CN IV tested? Whar causes and signs are there of damage?
- Eye movements (more in session 6)
Signs: Diplopia (worse on downward gaze, when reading, or walking downstairs)
Causes: Head injury (most common), congenital palsies.
What are the names of the four cranial nerves from the pons?
Trigeminal (V)
Abducens (VI)
Facial (VII)
Vestibulocochlear (VIII)
What is the function of the trigeminal nerve?
What are its branches?
Main sensory nerve of the face.
Supplies muscles of mastication.
Split into three branches:
CN V 1 - Ophthalmic
CN V 2 - Maxillary
CN V 3 - Mandibular
How is the trigeminal nerve tested?
- Sensation to face (dermatomal areas)
- Muscles of mastication
- Corneal reflex (blink reflex when cornea stimulated, e.g. touch)
Which clinical conditions can affect the trigeminal nerve?
- Vulnerable to facial trauma
- Trigeminal neuralgia: severe facial pain, unknown causes. (over 50s, fairly rare)
- Shingles can affect the dermatomes.
What effects can shingles have on the face/ eyes?
Will cover one dermatome.
Can cause corneal ulcers, if dermatome affected includes nerves to part of eye.
How can a blow to the eye socket cause damage? How is it tested?
Blow causes increased intraorbital pressure.
This can damage the the infraorbital nerve.
Sensation would be reduced in the skin under the eyelid.
What is the route and function of the Abducens nerve CN VI?
- Motor Nerve only
Route: through cavernous sinus > Superior orbital fissure.
Function: 1 eye muscle! > Lateral rectus
How is the abducens nerve CN VI tested? What are some clinical signs/ causes?
Test eye movements. (more session 6)
- Diplopia
- Can be injured in > IC pressure.
What is the route, type, and function of the facial nerve CN VII?
Motor fibres + Autonomic
Route: Through petrous part of temporal bone
Function:
- Muscles of facial expression (5 branches)
- Taste - anterior 2/3 tongue
- Parasympathetic fibres to lacrimal + salivary glands.
How is the Facial nerve CN VII tested?
Test the muscles of facial expression.
Test corneal reflex (efferent limb)
Taste (not usually tested)
What is Bell’s palsy?
Idiopathic facial nerve palsy
Which nerve do dentists inject anaesthetic into in order to perform tooth extractions? How does it work?
The area which relates to the mandibular foramen.
Numbs the lower gums, jaw, teeth.
How would you check if someone had had a mandibular fracture?
Patient would present with numbness over the lip and the chin (teeth, lower gums).
Due to damage to the inferior alveolar branch of the mandibular nerver (itself a branch of the trigeminal), which also branches to the mental nerve (teeth/ lower gum).
Why would a local anaesthetic cause numbness of the tongue?
Anaesthetic may travel to the lingual nerve via trigeminal.
Can cause numbness of tongue.
(does not affect taste)
