Cranial Nerves 1-v1 Origin, Route & Function

Question 1

What nervous system are cranial nerves part of? Where do they arise from?

Answer 1

Peripheral

relate to brain stem bar 2 which arise from forebrain (olfactory and optic CN1&2)

Midbrain - occulomotor and trochlear CN3&4

Pons - trigeminal, abducens, facial, vestibulocochlear CN5-8

Medulla-> carotid sheath - glossopharyngeal, vagus (runs length CS), spinal accessory, hypoglossal CN9-12

Question 2

Of the 12 pairs of cranial nerves how many are: purely special sensory, purely motor, missed sensory and motor, have autonomic function?

Answer 2

3 purely sensory
olfactory (smell), optic (vision), acoustovestibular (hearing & balance)

5 purely motor
Occulomotor (eye), trochlear (eye), abducens (eye), accessory (trapezius), hypoglossal (throat)

4 mixed sensory and motor
Trigeminal, facial, vagus, glossopharyngeal

4 also have autonomic function
Occulomotor, facial, glossopharyngeal

Question 3

Route of the olfactory nerve/ cranial nerve 1

Answer 3

Olfactory nerves in root of nasal cavity

Cribriform foramina

Olfactory bulb

Olfactory tract

Temporal lobe (close to onchus)

Question 4

What can cause problems with cranial nerve 1 and what would be a symptom?

Answer 4

Head injury can cause anosmia (loss of sense of smell) usually ok if just one e.g. basilar skull fracture

Tumours at base of frontal lobes May involve CN1

Commonest cause of anosmia is URTI

Question 5

Route of optic nerve/ cranial nerve 2

Answer 5

Impulses generated by cells in Regina in response to light

Axons form optic nerve

Exits back of orbit via optic canal

Fibres cross and merge at optic chiasm (just anterior to brainstem)

Question 6

Problems whic give unilateral and bilateral vision loss

Answer 6

Unilateral: retinal detachment

Bilateral: pituitary tumours - compress optic chiasm ‘bilateral hemianopia’

Question 7

Tests for vision (3)

Tool to see the eye and what it can show

Answer 7

Visual curry tests e.g. snellen chart

Visual fields (cover one eye)

Pupillary light response (pupils to constrict optic nerve has to work)

Seen directly with an ophthalmoscope (optic disc- point at which nerve enters retina)

Carry extension of meninges so nerve affected when raised intra-cranial pressure -> papillodema (swollen optic disc)

Optic disc- where optic nerve enters retina

Question 8

What is the visual pathway ?

Answer 8

Communication from optic tracts with brainstem (visual reflexes)

Retina 
Optic nerve 
Optic chiasm
Optic tract 
Laterial geniculate nucleus 
Optic radiation 
Primary visual cortex in occipital lobe

Question 9

Route of occulomotor nerve/ cranial nerve 3

Answer 9

Midbrain

Lateral wall of cavernous sinus

Superior orbital fissure

Question 10

Functions of the oculomotor nerve (CN3)

Answer 10

Motorand autonomic parasympathetic fibres

• Most of the muscles that move the eyeball (extra-ocular muscles)
• Muscles of the eye lid (levator palpebrae superioris)
• sphincter pupil muscle (constricts pupil)
• innervated ciliary muscle (controls size of lens)

Question 11

What is the pupillary light reflex?

Answer 11

Afferent optic nerve to oculomotor nerve to sphincter papillae muscle

Question 12

Where is the oculomotor nerve vulnerability,e to compression when intracranial pressure if raised and what is the first sign of this?

Answer 12

Between tentorium cerebelli and part of temporal lobe

Parasympathetic fibres affected first so pupils dilate

Question 13

What are signs there’s is damage to the oculomotor nerve, what are some examples of things that could cause damage?

Answer 13

Signs:
Pupillary dilation

Double vision (Diplopia) if eye muscles one side are weak)

Down and out position with severe ptosis (eyelid droops)

Causes for injury:
Raised ICP tumour/ haemorrhage

Aneurysms (posterior communicating artery)

Cavernous sinus thrombosis

Vascular (secondary to dinettes/ hypertension) typically pupil sparing

Question 14

Trochlear Nerve/ CN 4 route, what makes it different to other cranial nerves?

Answer 14

Midbrain
Lateral wall of cavernous sinus
Superior orbital fissure

Only nerve to emerge from posterior brainstem and has the longest intracranial course of any of the cranial nerves

Question 15

Function of trochlear Nerve

Answer 15

Innervated one of the muscles that moves the eyeball (extra-ocular muscles) superior oblique

Question 16

Signs of problems with the trochlear Nerve/ CN 4

Answer 16

Test eye movements (tests 3/4/6)

Diploplia

Rare and subtle tilt of head to correct diplopia

Congenital palsies

Head injury- long course makes vulnerable to raised ICP

Question 17

3 main branches and their roots of the trigeminal nerve / CN 5

Answer 17

Pons -> trigeminal ganglion ->

1. Va sensory (ophthalmic) superior orbital fissure-> orbit
2. Vb sensory (maxillary) foramen rotundum -> pterygopalatine fossa
3. Vc motor (mandibular) foramen ovale -> infratemporal fossa

Question 18

Functions of the trigeminal nerve / CN5

Answer 18

Main sensory nerve supplying skin of face and part of scalp

Sensory to deeper structures in head e.g. paranasal air sinuses, nasal and oral cavity, anterior tongue (general sensation) and meninges

Motor- muscles of mastication (Vc)

Question 19

Problems that can occur with cranial nerve 5 and how to check for them

Answer 19

Check sensation to dermatome areas

Muscles of mastication (jaw jerk) and corneal reflex

Branches vulnerable in orbital/ facial trauma

Conditions involving nerve: trigeminal neuralgia, shingles

Question 20

Branches of CN Va of the trigeminal nerve

Answer 20

Ophthalmic division

Frontal, lacrimal and nasociliary (enter into orbit through superior orbital fissure)

Frontal continues out of orbit as supraorbital and supratrochlear - carries sensory info from forehead

F
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N
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St

Question 21

Branches of CN Vb of the trigeminal nerve

Answer 21

Maxillary division

Many, important ones:

Infraorbital - runs through floor of orbit carries sensory info from area of cheek and lower eye lid (injured orbital floor fractures)

Superior alveolar (anterior, middle and posterior) - carries sensory info from deep structures of face (nasopharynx, nasal cavity, maxillary sinus, upper teeth and gums) nerve blocks by dentists

Question 22

Branches of CN Vc of the trigeminal nerve

Answer 22

Mandibular division

Inferior alveolar continues as mental

Auriculotemporal - Carries general sensory from part of ear, temple area of head and scalp and temporomandibular joint

Lingual - carries general sensory from anterior tongue

Question 23

Route of abducens Nerve/ CN 6

Answer 23

Lower pons (junction between pons and medulla)

Runs upwards before passing into cavernous sinus

Superior orbital fissure

Orbit

Question 24

Function of cranial nerve 6

Answer 24

Purely motor

One muscle that moves the eye - lateral Rectus

Question 25

Why can the abducens nerve be easily stretched in raised ICP?

Answer 25

It emerges anteriorly at ponto-medullary junction before running under surface of the pons upwards towards cavernous sinus

Question 26

What can cause problems for the abducens nerve and how can you test this? What Is a sign?

Answer 26

Test using eye movements (tests 3,4 and 6)

injury if raised ICP due to bleed/trauma

Microvascular complication with diabetes/ hypertension

Patients present with diplopia