Cranial Nerves Flashcards
What is the general arrangement of the cranial nerves?
Rostral ———–> Caudal
CNI —————> CNXII
note: exception is that CNXII is just in front of CNXI
Which nerves are found in which cranial fossa?
ANTERIOR CRANIAL FOSSA =
- olfactory nerve (CNI)
MIDDLE CRANIAL FOSSA =
- optic nerve (CNII)
- oculomotor nerve (CNIII)
- trochlear nerve (CNIV)
- trigeminal ganglion (CNV)
POSTERIOR CRANIAL FOSSA =
- facial nerve (CNVII)
- vestibulocochlear nerve (CNVIII)
- glossopharyngeal nerve (CNIX)
- vagus nerve (CNX)
- spinal accessory nerve (CNXI)
- hypoglossal nerve (CNXII)
What is the purpose of the foramina of the cranium?
Reduce mass of the cranium & allow passage of nerves and blood vessels
However:
- make skull base weaker (freq. fractures)
- compresses nerves
- spread of infection/metastases to brain
Describe the location and functions of the first cranial nerve.
Olfactory nerve
Olfactory bulb gives off septal branches which enter the nasal cavities via the cribiform plate of the ethmoid bone, descends along medial wall of nasal cavities
- entirely sensory: receptive field territories = nasal cavity + space around the body
- left & right nasal cavities supplied by separate left & right olfactory nerves
- sense of smell (olfaction)
- somatic (???)
note: not a true nerve (runs along brain tract)
note: sensation conveyed directly to cerebral cortex without going through the thalamus (?smelling salts revive fainted people, ?olfactory hallucinations)
How is the function of the first cranial nerve tested?
Ask if sense of smell has changed recently
Test with specific odours e.g. coffee
note: sense of smell can be lost temporarily in upper respiratory tract infections (anosmia)
Describe the location and functions of the second cranial nerve.
Optic nerve
Optic canal
- entirely sensory
- vision
note: not a true nerve (runs through a brain tract?)
How is the function of the second cranial nerve tested?
Fundoscopy = use opthalmoscope to examine optic disc
- look for retinal vein engorgement & blurring of margins of optic disc (papilloedema) —> swelling of first part of optic nerve
Visual acuity = patient covers one eye and reads a sample of text (Snellen chart) (repeat with other eye, keep glasses on)
Visual fields =
- Sit opposite patient with eyes at same level, as patient to cover right eye with right hand and cover own left eye with left hand
- Whilst patient looks directly into exposed eye, extend right arm and move right hand slowly from periphery, whilst moving index & middle fingers
- Ask patient when they first see fingers (should see both at the same time)
- Repeat for all quadrants, and for other eye
Causes of defects:
- early sign of meningitis
- tumours of hypothalamus
- aneurysms of internal carotid artery (near hypothalamus)
- increased CSF pressure (secondary to increased intracranial pressure) transmitted to optic nerve & disc)
Colour vision = use Ishihara colour vision test (numbers and different coloured circles)
Pupillary reflexes =
- light reflex: shine light into right eye and look for constriction in the pupil of the illuminated eye (direct reflex), AND equal constriction in opposite eye (consensual reflex) & repeat for left eye
- accomodation: hold index finger ~1m from patient, ask patient to focus on finger whilst moving it towards their nose, watch for constriction of pupils in response to convergence/accomodation
Describe the location and functions of the third cranial nerve.
Oculomotor nerve: divided into anatomical oculomotor nerve proper and associated parasympathetic fibres
Anatomical oculomotor nerve proper:
- originates from oculomotor nucleus (midbrain)
- travels along lateral side of cavernous sinus, through tentorial notch (thickening of meninges that partitions the intracranial space), to the ciliary ganglion
- exits cranium via superior orbital fissure (dorsal and ventral branches)
- movement of extraocular muscles
Associated autonomic parasympathetic fibres:
- originates from Edinger-Westphal nucleus (midbrain)
- travels with anatomical oculomotor nerve proper
- exits cranium via superior orbital fissure
- joins inferior branch oculomotor nerve (inf. oblique) and ends at ciliary ganglion
- supplies sphincter pupillae muscle
- mixed nerve: somatic motor & autonomic
Movements of extraocular muscles =
- levator palpebrae superioris: opens upper eyelid
- superior rectus: elevation, adduction, & internal rotation of eyeball
- medial rectus: adduction of eyeball
- inferior rectus: depression, adduction, & external rotation of eyeball
- inferior oblique: pupil directs up and out
Pupils = ciliary muscle & sphincter pupillae muscle —> constriction of pupils & curvature of lens (accommodation)
What is oculomotor nerve palsy?
Unopposed actions of:
- lateral rectus —> abduction
- superior oblique —> pupil directed down & out
Denervation of levator palpebrae superioris —> complete ptosis of upper eyelid
Paralysis of sphincter pupillae —> dilated pupil & absent pupillary light reflex
Paralysis of ciliary muscle —> absent accommodation reflex
Associated with aneurysms of the posterior communicating artery & cavernous sinus thrombosis (compresses nerve)
Complete palsy of anatomical oculomotor nerve proper =
- e.g. diabetes causing unilateral CNIII palsy (pupil sparing 3rd nerve palsy), damage distal to ciliary ganglion (after parasymp. fibres have branched off)
- denervation of most muscles moving the eye (except lateral rectus & superior oblique) —> eye pulled “down & out”
- denervation of levator palpebrae superioris —> ptosis of upper eyelid
- no pupillary involvement
Palsy of autonomic parasymp. fibres associated with oculomotor nerve proper =
- e.g. damage to Edinger-Westphal nucleus only
- denervation of sphincter pupillae (parasymp.) —> unopposed action of dilator pupillae (symp.) —> unilateral pupil dilation
- loss of accomodation reflex of pupil
Describe the location and functions of the fourth cranial nerve.
Trochlear nerve
In cavernous sinus ???
Superior orbital fissure
Emerges from dorsal aspect of midbrain
- entirely motor
- supplies superior oblique (directs pupil down & out)
How is the function of the fourth cranial nerve tested?
Ask patient to look medially & downwards (as if walking down the stairs)
Damage to the trochlear nerve causes diplopia (double vision) due to lack of synergy between the eyes (medial rectus is still functional)
Describe the location and functions of the sixth cranial nerve.
Abducent nerve
In cavernous sinus (?)
Superior orbital fissure
- entirely motor
- supplies lateral rectus (abduction of eyeball)
Long intracranial course, therefore particularly vulnerable to damage due to increased intracranial pressure
Damage —> patient is unable to look outwards (squints)
How are the third, fourth, and sixth cranial nerves tested?
- inspect eyelids for ptosis
- note position of eyes in resting gaze
- ask patient to follow movement of index finger in an ‘H’ shape without moving head (avoid extremes of gaze, as this may cause physiological nystagmus)
- ask patient if they have double vision
- look for extraocular palsy & nystagmus (rapid involuntary eye movement)
+ additional tests for oculomotor nerve (CNIII)
Describe the location and functions of the fifth cranial nerve.
Trigeminal nerve (most widely distributed cranial nerve)
- opthalmic nerve (CNV1) = superior orbital fissure, in cavernous sinus (?)
- maxillary nerve (CNV2) = foramen rotundum, in cavernous sinus (?)
- mandibular nerve (CNV3) = foramen ovale
Mixed nerve: sensory & motor
Opthalmic nerve:
- sensory territories of scalp, forehead, upper eyelid, dorsum of nose
- sensations of cornea & conjunctivae
note: shingles spreading along the sensory territories of the opthalmic nerve can lead to blindness
Maxillary nerve:
- sensory territories of skin, lower eyelid, cheek, upper lip, mucosa of nasal cavity, paranasal sinuses, palate, roots of upper teeth
Mandibular:
- sensory territories of skin of temples, cheek, chin, mucosa of inner cheek, anterior 2/3 of tongue, roots of lower teeth
- temporalis = elevates mandible (closes mouth), retraction of mandible, side-to-side movements of mandible
- masseter = elevates mandible (closes mouth), side-to-side movements of mandible
- medial pterygoids = elevates mandible, side-to-side movements of mandible (closes mouth)
- lateral pterygoids = protrusion of mandible (opens mouth)
- anterior belly of digastric = depresses mandible (fixed to hyoid bone) - (opens mouth)
note: canthi of mouth is the boundary between the sensory territories of the maxillary and mandibular nerves
How is the function of the fifth cranial nerve tested?
Sensory:
- test for sensation of skin over front of cheek to light touch & pain (infraorbital nerve)
- test for sensation of skin over chin (mental nerve)
- test for sensation of skin on side of cheek (buccal nerve)
- test for general sensation to the front of the tongue (lingual nerve)
- test corneal reflex by touching the cornea (NOT SCLERAE) with cotton wool, should cause direct, consensual blinking (stimulates palpebral portion of orbicularis oculi)
(damage to sensory nerve —> neither eyelid blinks
if cornea of one eye produces blink in opposite eye —> defect in facial nerve)
Motor:
- ?obvious wasting of muscles of mastication
- ask patient to clench teeth & feel for contractions of temporalis muscles (temples) and the masseter muscles (angle of jaw)
- try and close patients jaw against pterygoids
- ask patient to move jaw from side-to-side (pterygoids) and look for equal movement
Describe the location and functions of the seventh cranial nerve.
Facial nerve + associated autonomic parasymp. fibres (nervus intermedius)
Branches of facial nerve: temporal, zygomatic, buccal, mandibular, cervical
Nervus intermedius:
- nucleus solitarius —> special sensory fibres (visceral afferents) —> ant. 2/3 of tongue & soft palate (taste)
- superior salivatory nucleus —> nervus intermedius fibres (autonomic) —> lacrimal, submandibular, sublingual, nasal, & palatine glands
- geniculate ganglion —> general sensory fibres (later join trigeminal afferents supplying auricle of the ear)
Internal acoustic meatus —> petrous portion of temporal bone —> facial canal —> IJV —> tympanic membrane —> stylomastoid foramen —> parotid sheath & gland —> maxillary artery
Passes through parotid gland but does not supply it
Closely associated with trigeminal nerve
Mixed nerve:
- motor = facial nerve
- sensory = nervus intermedius
- autonomic = greater petrosal nerve of nervus intermedius
Motor: muscles of facial expression & stapedius
- damage (on one side) —> loss of facial expression (forehead sparing; frontalis is supplied bilaterally), loss of sphincter function (inability to chew or blink correctly), loss of naso-labial fold, hyperacusis (over-sensitivity to normal sounds)
Sensory:
- general sensation of concha of auricle & behind ear
- special sensory: taste in anterior 2/3 via chorda tympani (damaged within posterior wall of tympanic cavity)
Autonomic:
- lacrimal, submandibular, & sublingual glands
- mucous membranes of nasopharynx, paranasal sinuses, hard palate, and soft palate
note: surgery on middle ear may damage the facial nerve within the labyrinthine wall of the tympanic cavity
note: tumours within the petrous part of the temporal bone will affect the facial nerve
How is the function of the seventh cranial nerve tested?
- inspect for asymmetry
- ask patient to raise eyebrows whilst pushing down (tests frontalis muscle)
- ask patient to screw up eyes and resist opening them against resistance (tests orbicularis oculi)
- ask patient to smile and show teeth (tests orbicularis oris)
- ask patient to blow out cheeks and tap to check for continence of air (tests buccinators)
Describe the location and functions of the eighth cranial nerve.
Vestibulocochlear nerve
Internal acoustic meatus
Entirely sensory:
- vestibular nerve (balance)
- cochlear nerve (hearing)
Damage to vestibular nerve:
- loss of balance
- vertigo
- nausea (input from eyes and ears do not match)
- nystagmus
- impairment of caloric response (cold water in ears induces eye movement)
note: nystagmus & impairment of caloric response can also occur due to cerebellar damage (test for ?brain death)
