Cranial nerves – revision.

cranial nerves

CN I – Olfactory nerve.

Sensory: Smell.

Path: Forebrain – Cribriform plate of Ethmoid bone (remains in skull).

Tests: Asking patient to close eyes and to block one nostril at a time, provide a commonly recognised scent (ie mint or coffee). Request the patient inhales through unblocked nostril and identifies the scent.

Pathologies: Head trauma, upper respiratory infections, sinus diseases, space occupying lesions (SOL).

olfactory-nerves-filaments

CN II – Optic nerve.

Sensory: Vision.

Pathway: Occipital cortex – optic tract – optic chiasm (crossing over) – exit via optic foramen – retina.

Tests: Visual fields (periphery), visual acuity (Snellen chart), accommodation (convergence and constriction with item approaching nose), direct and consensual light reflex.

Pathologies: Multiple Sclerosis (MS), optic neuritis, bitemporal hemianopsis, pituitary tumour, berry aneurysms of the internal carotid arteries.

optic-nerve-lateral-geniculate-nucleus-of-thalamus-optic-radiation-visual-cortex

CN III – Oculomotor nerve (see CN IV and CN VI).

Somatic Motor: Eye movement (all but lateral rectus and superior oblique extraocular muscles).

Path: Midbrain – between superior cerebellar and posterior cerebral arteries – cavernous sinus – superior orbital fissure – orbit.

Innervates all of the extraocular muscles except superior rectus and lateral oblique. (LR-6 SR-4).

Pathologies: Extradural haematoma, aneurysm of posterior cerebral or superior cerebellar arteries.

Test: H test (relevant to CN IV and VI). Ask patient to hold head still while watching your finger. Proceed to move finger in a H shape starting from central point.

oculomotor

CN IV – Trochlear nerve (see CN III and CN VI).

Somatic motor and proprioceptive to superior oblique (extraocular muscle).

Path: Dorsal surface of midbrain – passes anteriorly around brainstem – pierces dura mater – lateral wall of cavernous sinus – superior orbital fissure – orbit – superior oblique (inferiomedially).

Pathologies: Diplopia (double vision), severe head trauma.

Tests: see CN III (inferomedial).

trochlear nerve

CN V – Trigeminal nerve.

Somatic motor: muscles of mastication; mylohyoid, anterior belly of digastric, tensor tympani, tensor veli palatini.

Sensory: Dura of anterior and medial cranial fossa, mucous membrane of nasal cavity, paranasal sinuses and mouth.

Path: Lateral surface of pons – medial part of crest of petrous part of temporal bone – trigeminal cave.

** Has THREE branches:

Pathologies: Cranial trauma, space occupying lesions, aneurysms, meningeal infections, poliomyelitis, multiple sclerosis, dental trauma, herpes zoster opthalamicus, trigeminal neuralgia – episodic pain.

Tests: Sensory – Touch path of nerve along skin, gently with cotton wool.
– Corneal reflex – touch sclera gently to invoke blink.

Motor: Resist lateral movement of jaw.
Ask patient to clench jaw, palpate masseter.

trigeminal_nerve.251185132

CN VI – Abducens nerve (see CN III and CN IV).

Somatic motor to lateral rectus (extraocular muscle).

Path: Brainstem (between pons and medulla) – pontine cistern – straddles basilar artery – pierces dura – petrous part of temporal bone – cavernous sinus – orbit – lateral rectus muscle.

Pathologies: Diplopia present in all eye movements, complete paralysis (SOL), medial deviation, atherosclerotic internal carotid artery.

Tests: see CN III and CN IV.

abducens nerve

CN VII – Facial nerve.

Sensory : Taste in anterior 2/3 of tongue. Floor of mouth and palate.

Motor: Platysma, stylohyoid (posterior belly), digastric, stapedius. (proprioceptors to these muscles).

Path: Junction of pons and medulla – posterior cranial fossa – internal acoustic meatus – facial canal – stylomastoid foramen of temporal bone and parotid gland.
– Main trunk forms intraparotid nerve plexus.

Pathologies: Most frequently paralysed of all cranial nerves.
– Lesion near origin: loss of motor, sensory and autonomic functions.
– Central lesion: Paralysis of  inferior (contralateral) facial muscles.
Bells palsy: Upper motor neurone lesion (UMNL) – stroke/CVA in pons. Bi/Unilateral paralysis.
Lower motor neurone lesion (LMNL) – Usually stylomastoid foramen, trauma, herpes zoster, infection/inflammation.
Multiple sclerosis. Myasthenia gravis. Eye dryness, facial numbing, hearing sensitivity.

CN VIII – Vestibulocochlear nerve (auditory).

Sensory: Hearing, equilibrium and motion.

Path: Junction of pons and medulla – internal acoustic meatus – separates to vestibular and cochlear nerves.

- Vestibular: hair cells of vestibular portion of membranous labyrinth – establishes contact with a number of regions for control of posture, maintenance of equilibrium, coordination of head and eye movements – extends through brain stem and spinal cord – descending component is known as medial vestibulospinal tract (concerned with control of balance and posture) – ascending part connects to nuclei of abducens, trochlear and oculomotor (concerned with coordination of head and eye movements).

- Cochlear: Hair cells of the ‘Organ of Corti’ within cochlear duct of inner ear – inhibitory function, modulating transmission of auditory information.

Pathologies: Acoustic neuroma (benign tumour of 8th CN), tinnitis (peripheral lesion), vertigo (trauma), conductive deafness (external/ middle ear), sensorineural deafness (disease in cochlear or path from cochlear to brain).

Tests: Quick hearing – whisper behind patient or rub fingers together 5cm from each ear.

Webers test (512hz tuning fork ) – vibrate fork and place on the vertex of the patient’s skull
– equal = normal or bilateral conductive loss
– unequal = conductive loss in lesser.

Rinne test (512hz tuning fork) – Vibrating tuning fork base held at mastoid process
– compares superiority of air to bone transmission.
– Air conduction normally longer than bone.
– Reverse indicates loss of sensorineural transmission.

vestibulococh nerve

CN IX – Glossopharyngeal nerve.

Somatic motor: stylopharyngeus.

Visceral motor: Otic ganglion (parotid gland).

Sensory: Posterior 1/3 of tongue, pharynx, tympanic cavity, pharyngotympanic cavity, carotid body and sinus (chemoreceptors).

Path: Lateral medulla – anterior jugular foramen – follows stylopharyngeus – between superior and medial pharyngeal constrictor muscles – oropharynx and tongue.

Pathologies: Pain on swallowing, protruding the tongue, touching the palatine tonsil and eating.

Tests: Ask patient to say ‘ah’. Soft palate should rise up straight and in the midline.

glossopharyngeal nerve IXa

CN X – Vagus nerve.

Somatic sensory: Inferior pharynx and larynx, root of tongue and taste buds.

Visceral sensory: Thoracic and abdominal organs (to proximal 1/3 of transverse colon).

Somatic motor: Soft palate, pharynx, intrinsic laryngeal muscles, intrinsic tongue muscles, palatoglossus.

Proprioception: above muscles.

Path – Lateral medulla – jugular foramen – carotid sheath – root of the neck…
~ Right side: across subclavian artery – side of trachea – back of root of lung – posterior pulmonary plexus – (from lower plexus) descends on oesophagus – oesophageal plexus – abdomen via oesophagus – postero-inferior stomach surface – left side of celiac plexus and lienal (splenic) plexus.
Left side: Enters thorax between left subclavian and carotid arteries – crosses left side of aortic arch – behind root of left lung – posterior pulmonary plexus – anterior oesophagus – unites with right in oesophageal plexus – anterosuperior stomach surface – fundus – lesser omentum – hepatic plexus.

Pathologies: Dysphagia, anaesthesia of superior larynx, aneurysms or aortic arch (neck operations), dysphonia, aphonia, inspiratory stridor (high pitch noise), space occupying lesions (larynx/thyroid), L>R due to longer course.

Tests: Same as CN IX.

vagus-nerve-image

CN XI – (Spinal) accessory nerve.

Somatic motor: sternocleidomastoid (SCM) and trapezius.

Path: Anterior horn motor neurones of upper 5 or 6 cervical roots – foramen magnum – joins CN X at jugular foramen – descends along internal carotid artery – SCM – posterior cervical region – trapezius.

Pathologies: Surgery trauma.

Tests: Ask patient to shrug against resistance.
Rotation of head/neck can indicate contralateral nerve damage.

accessory nerve

CN XII – Hypoglossal nerve.

Somatic motor: muscles of the tongue (styloglossus, hyoglossus, genioglossus)

Path: Medulla – hypoglossal canal – joins cervical plexus somatic motor fibres (C1 and 2) – hyoid muscles – dura mater of posterior cranial fossa – angle of mandible – curves anteriorly to tongue.

Pathologies: Deviation to paralysed side on protrusion (ipsilateral to nerve injury).

Tests: Tongue protrusion test.
ask patient to push tongue against inner cheek while resisting on the outside.

hypoglossal

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One thought on “Cranial nerves – revision.

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