L9: cranial nerves part 2 - origin, route and function

Question 1

Describe the key branches of the divisions of the trigeminal nerve

Answer 1

Ophthalmic – frontal nerve (gives rise to supraorbital/supratrochlear) & nasociliary nerve
Maxillary – infra-orbital nerve and superior alveolar nerve
Mandibular – auriculotemporal nerve, lingual nerve, inferior alveolar nerve & mental nerve

Question 2

Describe the course of the ophthalmic nerve

Answer 2

Travels laterally through the cavernous sinus
Nerve then exits the cranium via the superior orbital fissure, where it divides into frontal, lacrimal and nasociliary nerves
Provide sensory innervation to the skin and mucous membranes of the structures derived from the frontonasal prominence

Question 3

Describe the course of the maxillary nerve

Answer 3

Passes through the lateral wall of the cavernous sinus, before leaving the skull through the foramen rotundum
Gives rise to numerous sensory branches: infraorbital & superior alveolar nerves (main important branches)

Question 4

Describe the course of the mandibular nerve

Answer 4

Does not go through the cavernous sinus, takes a more lateral route
Travels through the foramen ovale
Travels through the infratemporal fossa - gives off branches for muscles of mastication
Next splits into three branches: auriculotemporal, inferior alveolar & lingual nerves
Auriculotemporal nerve: sensory for ear & temple
Lingual nerve: anterior 2/3s general sensation of tongue
Inferior alveolar: sensation of lower gum and teeth, passes through mental foramen to become mental nerve (sensory of middle gum, teeth and skin of chin) & is vulnerable in mandibular fractures

Question 5

Describe cranial nerve VII

Answer 5

Facial nerve – originates from the pontine-medulla junction
Has motor, special sensory (TASTE) & parasympathetic function
Innervates muscles of facial expression, special taste sensation to the anterior 2/3 of the tongue, supplies many of the glands of the head and neck
Patients report with unilateral facial droop
Causes: lesions in/around internal acoustic meatus & posterior cranial fossa tumours, basal skull fracture & middle ear disease

Question 6

Describe the course of cranial nerve VII

Answer 6

Passes through internal acoustic meatus
Goes into petrous bone: in here it gives off three branches (greater petrosal nerve, nerve to stapedius & chorda tympani)
After giving branches, rest of facial nerve continues; close relationship with middle ear as it runs through facial canal
Emerges through base of skull via stylomastoid foramen & gives off several extracranial branches

Question 7

How can you differentiate the cause of a facial droop?

Answer 7

Stroke: central cause of facial droop – only affects one side of the brain at the time, sparing of forehead
-this is because only the upper part of the face has dual cortical innervation (being supplied by the ipsilateral primary motor cortex)
Facial nerve lesion: affects the whole side of the face (no sparing of the forehead muscles)
-this is because the lesion happens after the facial nerve leaves the brainstem

Question 8

Describe cranial nerve VIII

Answer 8

Vestibulocochlear nerve – originates from pontine-medulla junction
Special sensory function (hearing and balance)
Tested by: gross bedside hearing tests + tuning fork testing
Patients can present with hearing loss, dizziness, tinnitus
Caused by: vestibular schwannoma, occlusion of labyrinthine artery & base of skull fracture

Question 9

Describe the symptoms and signs of a vestibular schwannoma

Answer 9

Unilateral hearing loss
Tinnitus
Vertigo
Numbness, pain/weakness down one half of face (close association with facial nerve)

Question 10

Describe the course of cranial nerve VIII

Answer 10

Nerve emerges from the brain at the cerebellopontine angle & exits the cranium via the internal acoustic meatus of the temporal bone

Question 11

Describe cranial nerve IX

Answer 11

Glossopharyngeal – arises from medulla
Sensory: oropharynx, tonsils, posterior 1/3 tongue (& taste)
Motor: stylopharyngeus
Parasympathetic: parotid gland
Tested by: speech, swallow, cough, gag reflex
Patient may present with: difficulty with swallow, weak cough & difficulties with speech/changes in voice

Question 12

Describe the course of cranial nerve IX

Answer 12

Runs through posterior cranial fossa
Exits through jugular foramen
Enters into carotid sheath

Question 13

Describe cranial nerve X

Answer 13

Vagus nerve – arises from the medulla
Motor and sensory function – muscles of larynx & pharynx (including soft palate), sensory to larynx/laryngopharynx, parasympathetic to many tissues
Tends to be examined with cranial nerve IX – soft palate elevation examination (uvula deviates from the weaker side)
Causes of pathology: posterior cranial fossa tumours, brainstem lesions & pathology involving carotid sheath structures

Question 14

Describe the course of cranial nerve X

Answer 14

Runs through posterior cranial fossa
Exits through jugular foramen
Enters into carotid sheath
Key branches in neck – right recurrent laryngeal & left recurrent laryngeal nerves

Question 15

Describe cranial nerve XI

Answer 15

Accessory – originates from medulla & has some contribution from upper cervical nerves
Motor function of muscles SCM & trapezius
Tested by: test actions of SCM & trapezius
Causes: injuries, surgery or pathology involving posterior triangle

Question 16

Describe the course of cranial nerve XI

Answer 16

Spinal component – enters the cranial cavity via the foramen magnum, traverses the posterior cranial fossa to reach the jugular foramen, exits the skull, descends the internal carotid artery to reach SCM & moves across the posterior triangle of the neck to trapezius
Cranial component – leaves the cranium via the jugular foramen & combines with the vagus nerve

Question 17

Describe cranial nerve XII

Answer 17

Hypoglossal – arises from the medulla
Motor function – tongue movements & protrusion
Causes: surgery/pathology in proximity to/involving upper carotid sheath, internal & external artery + posterior cranial fossa tumours

Question 18

Describe the course of cranial nerve XII

Answer 18

Passes laterally across the posterior cranial fossa
Exits the cranium via hypoglossal canal
Passes inferiorly to the angle of the mandible & moves in an anterior direction to enter the tongue

Question 19

Describe the importance of the nasociliary branch of the ophthalmic division of the trigeminal nerve in Hutchinson’s sign

Answer 19

Ophthalmic shingles lie within the boundaries of the ophthalmic dermatome
If the tip of the nose or around the front of the eye has been affected -> positive Hutchinson’s sign and indicates that the nasociliary branch has been affected
This means there is a strong chance of complications to ocular function

Question 20

What are the functions of the three branches of the facial nerve given off in the petrous part of the temporal bone?

Answer 20

Greater petrosal (given off at geniculate ganglion) - goes through pterygopalatine fossa and provides parasympathetic innervation to lacrimal, nasal & oral mucosal glands 
Nerve to stapedius - motor branch to stapedius (tiny muscle in middle ear) 
Chorda tympani - special sensory taste for anterior 2/3s of tongue, parasympathetic fibres also hitchhike with this nerve to get down into oral cavity