Cranial nerves Flashcards
CNⅠ (name, origin, composition, osteology/route through skull, functions, clinical relevance)
Olfactory nerve
Extension of forebrain
Only sensory
Olfactory nerves pass into nasal cavity through small holes (cribriform foramina) in the cribriform plate of the ethmoid bone. The olfactory tract sends afferent signals from the olfactory bulb to the temporal lobe, where smell is perceived.
Responsible for special sense of smell - issues with CNⅠ present with anosmia/hyposmia (loss of smell and potentially taste)
Most commonly affected by URTI infection. Less commonly also ACF tumours, agressive impact/physical injury, also associated with parkinsons, alzheimers, etc
CNⅠⅠ (name, origin, composition, osteology/route through skull, functions, clinical relevance)
Optic nerve
Extension of forebrain
Only sensory
Retinal ganglion cells at back of eye go on to form optic nerve. Optic nerve exits orbit via optic canal. Both nerves from either side merge at optic chiasm - fibres cross here. Optic tracts then communicate with primary visual cortex within occipital lobe - vision is perceived. Optic tracts then communicate with midbrain - allows motor reflexes to occur
Responsible for special sense of vision - issues with CNⅠⅠ can present with poor pupillary reflex to light, poor visual acuity, and evidence of pathology seen on opthalmoscopy. Assess CNⅠⅠ with snellen chart and check visual fields.
If lesion is at level of eye/optic nerve, only one eye is affected
If lesion is at level of optic chiasm/tracts, both eyes are affected (eg pituitary tumours compressing optic chiasm, causing bitemporal hemianopia)
Affected by any disease involving CNⅠⅠ, eg optic neuritis (potential sign of MS) , AION (reduced blood supply to CNⅠⅠ)
CNⅠⅠ carries extension of meninges - any rise in ICP causes increased pressure in SA space - compresses CNⅠⅠ - affects venous drainage - papilloedema
(compression of CNⅠⅠ also affects flow of signals down CNⅠⅠ - axoplasmic flow)
CNⅠⅠⅠ (name, origin, composition, osteology/route through skull, functions, clinical relevance)
Oculomotor nerve
Ant. aspect of midbrain
Motor and PS
Passes through cavernous sinus, enters orbit through superior orbital fissure. Once CNⅠⅠⅠ is in orbit, it splits into a superior and inferior division (both contain motor fibres that innervate muscles) PS fibres continue travelling straight, and innervate muscles in iris that affect pupil size
Innervate 4/6 extra ocular muscles (muscles that move eyeball)
Also innervates levator palpebrae superioris (muscles that elevates eyelid)
PS fibres innervate sphincter pupillae (muscle in iris that surrounds pupil and changes its size)
Issues in CNⅠⅠⅠ can present with ptosis (drooping of eyelid). Eye can be in a down and out position. Pupil may be dilated.
CNⅠⅠⅠ can be affected by microvascular ischaemia (>50 years old, diabeties/HTN). This won’t affect the pupillary reflex, as the pial blood vessels can still supply the PS fibres with blood.
Other things that can affect CNⅠⅠⅠ are aneurysm of PCA (posterior communicating artery very close to CNⅠⅠⅠ - aneurysm can compress it), thrombosis within cavernous sinus, head injury (increased ICP) and tentorial herniation (CNⅠⅠⅠ very close to tentorium cerebelli - areas of brain herniating around edge of tentorium following increased ICP can compress CNⅠⅠⅠ).
All these other causes also affect pupillary reflex - are more concerning
CNⅣ (name, origin, composition, osteology/route through skull, functions, clinical relevance)
Trochlear nerve
Posterior aspect of midbrain
Only motor
Passes through cavernous sinus, enters orbit through superior orbital fissure.
Only innervates superior oblique muscle - issues with CNⅣ present with double vision (diplopia), and eye being upwards and inwards. Diplopia is worse than looking downward and medially (as SO muscle is primary depressor of eye when adducted) May be compensatory head tilt. Assess CNⅣ by looking at resting gaze and checking eye movements.
CNⅣ can be affected by microvascular ischaemia (>50 years old, diabeties/HTN). Also affected by trauma, tumour, thrombosis within cavernous sinus, etc.
CNⅤ (name, origin, composition, osteology/route through skull, functions, clinical relevance)
Trigeminal nerve
Halfway down pons, on its lateral aspect
Motor and general sensory
Trigeminal ganglion gives rise to three branches - Va, Vb, Vc. Va and Vb travel through cavernous sinus. Va passes through superior orbital fissure, Vb passes through foramen rotundum. Vc passes lateral to cavernous sinus and passes through foramen ovale
Recieves general sensory info from skin of face and scalp. Also from deeper structures like sinuses, ant 2/3rds of tongue, etc. Supplies muscles of mastication.
Va branch receives sensory info from orbital structures (frontal nerve - divides into supratrochlear and supraorbital nerves), forehead, down to tip of nose (nasociliary nerve)
Vb branch receives sensory info from skin below orbit (infraorbital nerve - cheek), upper row of teeth (superior alveolar nerve), etc
Vc branch innervates 4 muscles of mastication. Also receives sensory info from ant. 2/3rds of tongue (lingual nerve), ear and temple (auriculotemporal nerve), lower row of teeth (inferior alveolar nerve - affected by mandibular fracture)
Issues with CNⅤ present with sensory weakness in certain dermatomes, weakness in muscles of mastication, absence of corneal reflex (test Va branch by lighly touching surface of eye - patient should blink both eyes). Test CNⅤ by lighly touching dermatomes, feel for muscles of mastication while patient moves jaw, test corneal reflex.
CNⅤ affected by trigeminal herpes zoster (varicella zoster remains dormant in TG ganglion - travels down Va branch and affects entire dermatome on only one half of face). Hutchinson’s sign (vesicular rash on tip of nose) indicates this.
Distal branches affected by orbital/mandibular fractures.
CNⅤ affected by trigeminal neuralgia, PCF tumours
CNⅥ (name, origin, composition, osteology/route through skull, functions, clinical relevance)
Abducens nerve
Bottom of pons, @ pontomedullary junction (medial to CNⅦ)
Only motor
Travels steeply upwards (vulnerabe to increased ICP), passes through cavernous sinus, enters orbit through superior orbital fissure.
Innervates lateral rectus muscles - moves eye laterally. Issues with CNⅥ can present with diplopia that is worse in lateral gaze (when patient is looking to the side) Present with abnormal eye position (affected eye in a medial position), and difficulty looking laterally. Assess by inspecting resting gaze, and checking eye movements.
CNⅣ can be affected by microvascular ischaemia (>50 years old, diabeties/HTN). Also affected by trauma, tumour, thrombosis within cavernous sinus, etc. Affected by increased ICP of any cause (increased ICP pushes brainstem downwards, hence stretching CNⅥ - false localising sign)
CNⅦ (name, origin, composition, osteology/route through skull, functions, clinical relevance)
Facial nerve
Bottom of pons, @ pontomedullary junction
Motor, special sensory and PS fibres
Passes through internal acoustic meatus to enter petrous bone. Gives of three branches here
1st branch is greater petrosal nerve - PS fibres - supplies glands of eye/nose/mouth (lacrimal/mucosal glands in nasal and oral cavity)
2nd branch is nerve to stapedius - motor fibres - innervates stapedius muscle in ear
3rd branch is chorda tympani - sensory and PS fibres - receives taste from ant. 2/3rds of tongue, and supplies salivary glands (all except parotid)
Remainder of CNⅦ emerges from petrous bone via stylomastoid foramen - is completely motor - innervates muscles of facial expression
Issues with CNⅦ can present with unilateral facial droop. Other functions (taste, gland secretion, etc) may also be affected. Assess CNⅦ by examining muscles of facial expression.
CNⅦ can be affected by lesions in/around internal acoustic meatus, and PCF tumours. Basal skull fractures (involving petrous bone), middle ear disease, etc can also affect CNⅦ. Inflammation of facial canal (eg Ramsay-Hunt syndrome - relating to VZ in CNⅦ - causes vesicular rash in external ear). Parotid disease (CNⅦ passes through parotid)
CNⅧ (name, origin, composition, osteology/route through skull, functions, clinical relevance)
Vestibulocochlear nerve
Bottom of pons, @ pontomedullary junction
Only special sensory
Enters inner ear via internal acoustic meatus. Vestibular nerve responsible for balance, cochlear nerve responsible for hearing
Responsible for special sense of hearing and balance - issues with CNⅧ can present with hearing loss, dizziness (vertigo) and tinnitus (hearing noises that aren’t coming from an outside source). Test CNⅧ with hearing tests (whisper/finger rub), and tuning fork testing
CNⅧ can be affected by vestibular schwannoma (benign tumour of schwann cells surrounding CNⅧ - aka acoustic neuroma) and other PCF tumours. Also affected by basal skull fractures (involving petrous bone), and occlusion of artery that supplies CNⅧ (labyrinthine artery). Very rarely affected by pons lesions
CNⅨ (name, origin, composition, osteology/route through skull, functions, clinical relevance)
Glossopharyngeal nerve
Arises from medulla
Sensory, motor and PS fibres
Runs through PCF, exits base of skull via jugular foramen, enters carotid sheath but exits it soon after - closely related to ECA.
Mainly recieves sensory info from oropharynx/tonsils. Receives general and special sensory info from post. 1/3rd of tongue. Innervates 1 swallowing muscle (stylopharyngeus). Gives PS fibres to parotid. Forms afferent fibres from carotid sinus/body. Also sensory info from middle ear
Issues with CNⅨ and CNⅩ present with difficulty swallowing, weak cough, difficulty speaking/changes in voice. Gag reflex also affected (sensory = CNⅨ, motor = CNⅩ)
Affected by PCF tumours, medullary lesions, pathology/surgery within carotid sheath
CNⅩ (name, origin, composition, osteology/route through skull, functions, clinical relevance)
Vagus nerve
Arises from medulla
Sensory, motor and PS fibres
Runs through PCF, exits base of skull via jugular foramen, enters carotid sheath and remains in it - closely related to ICA
Motor innervation to muscles of pharynx and larynx - including soft palate. Sensory info from larynx/laryngopharynx. PS fibres to many tissues (heart, gut, etc)
Issues with CNⅨ and CNⅩ present with difficulty swallowing, weak cough, difficulty speaking/changes in voice. Gag reflex also affected (sensory = CNⅨ, motor = CNⅩ) CNⅩ also affects soft palate movement and uvula position - test this by asking patient to say ‘ahhhhh’ and see if uvula deviates. If there is a CNⅩ lesion, uvula deviates to the opposite side.
Affected by PCF tumours, medullary lesions, pathology/surgery within carotid sheath.
RLN branches of CNⅩ can be affected by thyroid surgery, or superior thorax/mediastinal pathology
CNⅪ (name, origin, composition, osteology/route through skull, functions, clinical relevance)
Accessory nerve
Medulla (CNⅪ also has some fibres from upper cervical spinal nerves)
Motor
Runs through PCF and exits skull via jugular foramen. Enters carotid sheath, but exits to travel towards posterior triangle (travels underneath SCM)
Innervates SCM and trapezius - test CNⅪ by asking patient to turn head to opposite side (test SCM) and shrug shoulders (test trapezius)
CNⅪ affected by surgery/pathology within posterior triangle and its structures. Also affected by PCF tumours, base of skull fractures (near jugular foramen), and medullary lesions
CNⅫ (name, origin, composition, osteology/route through skull, functions, clinical relevance)
Hypoglossal nerve
Medulla
Motor
Runs through PCF and exits skull via hypoglossal canal. Enters carotid sheath, but exits to travel towards tongue
Innervates muscles of tongue (involved in speaking, eating, etc) - test CNⅫ by observing tongue movements, and asking patient to protrude tongue
On protrusion of tongue, if CNⅫ lesion is present, tip of tongue will deviate towards the extended side
CNⅫ affected by surgery/pathology within carotid sheath and its structures. Also affected by PCF tumours and medullary lesions
