Cranial Nerves Flashcards
Describe the route of the olfactory nerve (CN 1)
Begin in the route of the nasal cavity → Cribiform foramina (in plate of ethmoid bone) → Olfactory Bulb → Olfactory Tract → Temporal lobe
What is the key function of the olfactory nerve?
Sensory: smell
What is the most common cause of anosmia (loss of sense of smell)?
What other things can cause anosmia?
Most common: Respiratory Tract Infection
Other causes:
- Head injury - shearing force/ basilar skull fracture
- Tumour at base of frontal lobe
Describe the route of the optic nerve
From the retinal ganglion cells → axons form the optic nerve → exits the orbit via the optic canal → fibres cross and merge at optic chiasm
Where in the brain is vision pervieved and which nerve sends these impulses?
Impusles sent by the optic nerve to the primary visual cortex
A tumour in which location can cause bilateral visual symptoms?
Pituitary tumour → causes compression of the optic chiasm
How would you test the optic nerve?
- Visual acuity test (Snellen Chart)
- Visual Fields
- Pupillary light response
Explain what a papillodema is
Papillodema = swollen optic disc
Optic nerve carries extensions of the meninges so can be affected when ICP is raised (tumour/ bleeds)
Describe the route of the Occulomotor nerve (CN 3)
From the midbrain → Lateral wall of cavernous sinus → through superior orbital fissure → to extra ocular muscles
What is the function of the oculomotor nerve (CN3)?
Motor: to most of the muscles moving the eyeball (extra-ocular muscles) and the muscles of the eyelid (Levator Palpebrae Superioris)
Parasympathetic fibres: innervate the sphincter pupillae muscle causing contriction in bright light
Explain why damage to the occulomotor nerve can cause a blown pupil?
- The oculomotor nerve is vulnerable to compression between tentorium cerebelli and the uncus of the temporal lobe when ICP is raised
- Parasympathetic fibres are carried on the outside of the nerve, therefore compression squashes the parasymapthetic fibres
- Parasymapthetic fibres normally cause constriction → THEREFORE constriction allows unapposed dilation and ‘blown’ pupil appearance
A patient presents with this appearance. Which nerve is injured and explain why the eye looks like this
cranial nerve 3 - oculomotor nerve injured or compressed
Severe ptosis: normal function of oculomotor will be to raise the eyelid (levator palpabrae superioris) , a damaged oculomotor nerve will not be able to do this so eyelid droops
Blown pupil: due to compression of parasympathetic fibres → unapposed dilation of pupil
How would you test the function of the oculomotor nerve?
- Inspect eyelids and pupil size
- Test eye movement and pupillary light reflex
- Asses whether patient has double vision
What injuries/ pathology can cause injury to the oculomotor nerve. Which of these is pupil sparing?
- raised ICP from tumour or haemorrhage
- Aneurysm
- Cavernous sinus thrombosis
- Vascular (secondary to diabetes/ hypertension) is pupil sparing
Describe the route of the trochlear nerve
From the midbrain → lateral wall of cavernous sinus → through superior orbital fissure → to 1 muscle of teh eyeball suprior oblique
What is the function of the trochlear nerve (cranial nerve 4)?
Motor: Innervates 1 extra-ocular muscle: Superior oblique (abducts, depresses and internall rotates the eye)
Which of the cranial nerves has the longest intracranial course and why?
Trochlear nerve (CN4)
Has the longest route as it is the only one to originate from the dorsal aspect of the brainstem
How do lesions to the trochlear nerve present?
2 eyes don’t align causing double vision (diplopia)
Often rare and subtle- patients usually correct by tilting the head
Vulnerable to damage in raised ICP due to long route
Describe the route of the Trigeminal nerve (CN 5)
Originates in the pons → branches in the trigeminal ganglion where it splits into 3 branches:
- Va Opthalmic- through superior orbital fissure into the orbit
- Vb Maxillary - through foramen rotundum to pterygopalatine fossa
- Vc Mandibular- through foramen ovale to inframtemporal fossa
What is the function (s) of the Trigeminal Nerve (CN5)?
Sensory: main sensory supply to skin of face and scalp
Sensory to deeper structures within the head (paranasal sinuses), anterior part of tongue and meninges
Motor: to muscles of mastication Vc only
How would you test the function of the trigeminal nerve?
- Check sensation by touching facial dermatomes
- Test muscles of mastication (jaw jerk)
What is trigeminal neuralgia?
A sharp, excutiating pain of the face lasting s → 2 minutes
What is opthalmic shingles and why is this a sight threatening condition?
Shingles in the dermatome of the opthalmic branch of the trigeminal nerve
Sight threatening as scarring of the cornea can cause blindness
What are the branches of the Opthalmic Division of the Trigeminal nerve and what is the function? (Va)
Frontal, Lacrimal, Nasociliary
Main sensory fibres of the eye, conjunctive, orbital contents and structures deep to the dermatome
Frontal branch exits from orbit as supraorbital and supratrochlear to carry senory info from forehead
What are the branches of the Maxillary Division of the Trigeminal nerve and what is the function? (Vb)
There are many branches, most important ones:
Infraorbital nerve: runs through floor of orbit carrying sensation from cheek and lower eye lid susceptible to injury in orbital floor fractures
Superior Alveolar nerves: sensation from deep structures of face. Nerve blocked by dentists
What are the branches of the Mandibular Division of the Trigeminal nerve, what is its course and what is the function? (Vb)
Inferior alveolar nerve
Runs through bony canal of mandible and exits via the submental foramen as the mental nerve
Function: carries sensation from mental protuberance, lower lip and gum (susceptible to injury in mandibular fracture)
Lingual nerve: general sensory from anterior tongue
Auriculotemporal: general sesory from ear, temple and temperomandibular joint
Desribe the route of Abducens nerve (CN 6)
Lower Pons → Runs upwards before passing through cavernous sinus → enters orbit via superior orbital fissure
What is the function of the abducens nerve?
Purely motor: Innervates lateral rectus muscle of the eye (responsible for abduction)
What injury/ complications can affect the abducens nerve?
- Microvascular complications (diabetes/ hypertension)
- Raised ICP
Describe the route of the Facial Nerve (CN VII)
Exits laterally from the lower pons → enters the petrous bone via the internal acoustic meatus → branches within the petrous bone into 3 branches → exits through the base of the skull (stylomastoid foramen)
What are the functions of CN VII Facial Nerve?
Mixed nerve
Special Sensory: anterior 2/3rd of tongue (taste)
Autonomic (parasympathetic): Lacrimal gland (tears), mucosal glands, salivary glands (except parotid)
Motor: muscles of facial expression, nerve to stapedius (middle ear)
General sensory: small area of external ear
The facial nerve is desrcibed as having 2 roots at the cerebellopontine angle. What are these roots?
- Motor root (larger, just motor axons)
- Nervus intermedius (sensory and parasympathetic axons)
What are the 3 divisions of the facial nerve?
- Greater petrosal nerve
- Chorda tympani nerve
- Stapedius nerve
What functions does the Greater Petrosal nerve carry?
Parasympathetics to lacrimal, nasal and palatine glands
What functions does the chorda tyampani carry?
- Carries sympathetics to the salivary glands
- Carries taste from anterior 2/3 of tongue to brain
What is the function of the the nerve to strapedius?
Dampens down hard vibrations in the ear
How do you test the function of CN VII Facial Nerve?
- Test muscles of facial expression and corneal reflec
- Ask about other symptoms e.g. hyperacusis, dry eyes, altered taste
Where is the facial nerve vulnerable to damage?
- Middle ear pathology as nerve runs through the petrous bone
- Parotid gland as extracranial branches run through here
What are the afferent and efferent nerves involved in the corneal reflex?
AFFERENT: Opthalmic Branch of Trigeminal nerve
EFFERENT: Facial nerve - causes orbicularis oculi to close
Describe the route of the Vestibulocochlear nerve CNVIII
Cochlea and Semicircular canals → Vestibulocochlear nerve →Internal Acoustic Meatus → Lower pons
What is the function of the Vestibulocochlear nerve
Hearing and balance
How do you test the function of the vestibulocochlear nerve?
- Crude hearing test (99 in each ear)
- Enquire about balance
What is Presbyacusis
Old age related hearing loss (usually affects both ears)
What is an acoustic neuroma?
A benign tumour of the Schwann cells surround the vestibular component of CN VIII. Causes compression of the whole nerve +/- other nerves in close proximity (e.g. facial nerve)
What are some of the signs an symptoms of an acoustic neuroma?
- Unilateral hearing loss
- Tinnitus
- Vertigo
- Numbness, pain or weakness down one half of face
What structure do all 4 cranial nerves of the medulla enter as the exit the base of the skull?
The carotid sheath
(most quickly exit except Vagus which runs the legnth)
What are the functions of Glossopharyngeal Nerve (CN IX)
General Sensation: Palatine tonsils and oropharynx, Middle ear and tympanic membrane, carotid body and sinus
Special Sensory: Taste and General sensation from posterior 1/3 tongue
Autonomic: carries parasympathetics to innervate the parotid gland
Motor: supplies stylopharyngeus which assists in swallowing
How do you assess function of Glossopharyngeal Nerve?
- Tested in conjuntion with vagus nerve when asked to swallow
- Gag reflex (sensory limb) - would only test if there were concerns about swallowing
- Taste is not formally tested
Describe the route of the Vagus nerve (CN X)
Medulla → Jugular Foramen → Carotid Sheath → Through neck into thorax and abdomen
What are the functions of the Vagus nerve?
General sensory: lower pharynx and whole larynx. Small part of external ear and tympanic membrane
Motor: muscles of the soft palate, pharynx and larynx (involved in talking, coughing and swallowing)
Autonomic: Parasympathetics to the thorax (heart, tracheobronchial tree and abdominal viscera)
Describe the route of the recurrent laryngeal nerve (branch of vagus)
Right recurrent laryngeal : turns under the right subclavian artery
Left recurrent larygneal: turns under the aorta
How is the Vagus nerve tested?
- Noting patients ability to speak, cough, swallow
- Ask pt to say ‘Aah’ - looking for soft palate to rise on both sides
- Gag reflex - efferent limb
What is a common way the recurrent laryngeal nerve can be damaged? What effect can the damage have?
During thryoid surgery
Damage causes hoarseness and dysphonia
What is the function of CN XI Spinal accessory nerve?
Motor to sternocleidomastoid and trapezius
Describe the route of Spinal Accessory Nerve XI
From medulla → emerges via jugular foramen → passes deep to SCM providing innervation → Runs posterolaterally across posterior triangle → etners deep to trapezius
How do you test function of Spinal Accessory Nerve?
- Shrug shoulders against resistance (trapezius)
- Turn head against resistance (SCM)
How can the spinal accessory nerve be damaged?
- Runs through posterior triangle of neck (superficially)
- Can be damaged in lymph node biopsy, surgery, stab wound
What is the function of Hypoglossal nerve? (CN XII)
Motor: muscles of tongue (except one)
Describe the route of CN XII
Runs medial to angle of mandible → crosses internal and external arteries in neck
How do you test function of hypoglossal nerve?
Inspect the tongue by sticking out
Damage will cause weakness and atrophy of the tongue muscles on the ipsilateral side (Deviates towards the lesion)
