Crainal Nerves

Question 1

Give an overview of the cranial nerves

Answer 1

Part of the peripheral nervous system
12 pairs
Relate to brainstem (except two which arise from [fore]brain)
Arise at irregular intervals from CNS (rather than segments as seen in spinal nerves)
Supply structures of the head and neck* (CNX supplies strictest in thorax and abdomen too)
Individual names + Roman Numeral (which relates to order that they arise rostral to caudal)
• General sensory • Special sensory • Motor • Autonomic
Mixed: sensory + motor
Cranial nerves carry 1000s of axons which may be
Purely motor
+/- autonomics (hitch-hike)

Question 2

What is the brain stem

Answer 2

Adjoins the brain to the spinal cord
Continuous with spinal cord caudally
Vital role in regulation of cardio-respiratory functions and maintaining consciousness
Ascending sensory and descending motor fibres between brain and rest of body run through the brainstem
Location of majority of cranial nerve nuclei

Question 3

Describe CN I

Answer 3

The olfactory nerve
Special Sensory (so starts at periphery)
Function: Olfaction (sense of smell)
Paired anterior extensions of forebrain rather than a ‘true’ cranial nerve

Olfactory nerves in roof of nasal cavity - smell - generates AP
Cribriform foramina
Olfactory bulb - cell bodies here
Olfactory tract
Temporal lobe - allows o perceive signal as smelll

Question 4

What are some clinical points about CN I

Answer 4

• Often not formally tested: if do, simply ask about difficulties/ changes in sense of smell. May report things tasting unusual
(sometimes can use smelling salts.
• Test one nostril at a time
• Loss of sense of smell = anosmia
• Commonest cause of anosmia? Cold - upper resp tract infection

• Head injury can also cause anosmia (secondary to shearing forces and/or basilar skull
fracture) • Tumours at base of frontal lobes (within anterior cranial fossa) may involve CN I

Question 5

Describe CN II

Answer 5

Optic nerve
Paired anterior extensions of forebrain rather than a ‘true’ cranial nerve
Part of the visual pathway
Impulses generated by cells within retina in response to light: generates action potentials which propagate along optic nerve
Via other components of the visual pathway they reach primary visual cortex where the are perceived as vision

Visual pathway tackles signals rom retina to back of brain to perceive signals as image.
Cells respond to light by generating AP which propagate along the optic nerve . Retinal ganglion cells translate impulses along - millions of axons form the optic nerve . Optic nerve is purely special sense vision - no motor no pain sensation

Question 6

What is teh root of CN II

Answer 6

Retinal ganglion cells
Axons form optic nerve
Exits back of orbit via optic canal
Fibres cross and merge at optic chiasm

Optic nerve: Carries sensory fibres from the one eye (retina)
Optic chiasm: Mixing of sensory fibres from right and left optic nerves
OPtic tract: Contain sensory information from part of the right eye and part of the left eye

Question 7

What are clinical points about CN II

Answer 7

Different lesions within the visual pathway give very different patterns of visual loss e.g. retinal detachment, optic neuritis, pituitary tumour, stroke
Pituitary tumours compress optic chiasm, causing bilateral visual symptoms: ”bitemporal hemianopia” - fibres already mixed)

• Testing of function involves visual acuity tests e.g. Snellen Chart, checking visual fields,
testing pupillary responses
• Optic nerve seen directly with ophthalmoscope (i.e. optic disc= point at which nerve enters the retina)
• Carry extension of meninges thus nerve can swell due to raised ICP: can see evidence of raised intracranial pressure on fundoscopy as a swollen optic disc (papillodema) - both eyes can be affected if its raised intercranial pressure ,

Can look at optic nerve using an opthalmoscope

Question 8

What allows visual reflexe

Answer 8

Visual pathway extends back from the retina towards the primary visual cortex found within the occipital lobe
There is communication from the optic tracts with brainstem (midbrain) to allow for certain visual reflexes e.g. pupillary reflexes to light

Question 9

Describe CN III

Answer 9

Oculomotor nerve
Motor (M & A) also carries autonomic parasympathetic fibres

Function
• Innervates most of the muscles that move the eyeball (extra-ocular muscles)
• Innervates the muscle of the eye lid (LPS)
• Innervates the sphincter pupillae muscle (which constricts pupil)

Question 10

What is teh root of the cult motor nerve

Answer 10

Midbrain - runs through lateral wall of cavernous sinus - superior orbital fissure

Question 11

What is CN III vulnerable to compression by

Answer 11

Oculomotor Nerve Vulnerable to Compression Between Tentorium Cerebelli and Part of Temporal Lobe When Intracranial Pressure is Raised

Question 12

What are clinical points about the ocuomotor nerve

Answer 12

• Testing of function involves inspecting the eyelids and pupil size, testing eye
movements and testing the pupillary reflexes (e.g. to light) • Pathology can cause pupillary dilation and/or double vision (diplopia) • ‘Down and out’ position with severe ptosis (eyelid droops)

Question 13

What re the causes for injury/patholy of cn iii

Answer 13

Causes for injury/pathology of the nerve include
- Raised intracranial pressure (tumour/haemorrhage)
causes external compression of nerve
- Aneurysms (posterior communicating artery)
- Cavernous sinus thrombosis
These cause external compression of the nerve
Vascular (secondary to diabetes/
hypertension: typically pupil sparing)

Question 14

Whatclinical signs can be seen from cn ii Edison

Answer 14

See slide

Question 15

Describe CN IV

Answer 15

Midbrain _> cavernous sinus -> superiors orbital issure 3
Purely motor
Innervates one of the muscle that move the eyeball
(extra-ocular muscles) • Superior oblique
Only 1 muscle

Question 16

What are clinical points about the trochlear nerve

Answer 16

• Test function by testing eye movements (test III, IV & VI at the same time)
• Diplopia (worse on downward gaze e.g. when reading, walking downstairs)
• Rare and often subtle (patients correct the diplopia with tilt of the head)
• Head injury* is most common cause of acute CN IV injury or any cause of raised ICP
• Congenital palsies (children) [cause uncertain]

Question 17

Describe the pathway of CN V

Answer 17

Trigeminal nerve
Pons -> trigeminal ganglion -> *

• -> Va ophthalmic -> cavernous sinus wall -> superior orbital fissure -> orbit
• -> Vb maxillary > cavernous sinus -> foramen rotundum -> pterygopalatine fossa
• -> vc mandibular -> foramen ovale -> infratemporal fossa

Question 18

Describe cn v

Answer 18

• Main sensory nerve supplying skin of face and part of scalp
• Sensory to deeper structures within the head e.g. paranasal air sinuses, nasal and oral cavity, anterior part of tongue (general sensation NOT taste), meninges
• Motor to muscles of mastication (Vc only)
General sensory, motor (vc)

Question 19

What are clinical points about cn v

Answer 19

• Tested by checking sensation (to touch) in areas of its dermatomes (Va , Vb,
Vc), testing muscles of mastication (jaw jerk) and testing corneal reflex
• Number of branches vulnerable in orbital/facial trauma and fractures
• Number of conditions can involve branches of the trigeminal nerve
o E.g. trigeminal neuralgia, shingles - front of eye also affected - sight threatening
- corneal ulcer

Question 20

What brands are given by the maxillary division

Answer 20

• Infraorbital nerve* runs through floor of orbit

• Carrying sensory from area of cheek and lower eye lid
• Susceptible to injury in orbital floor fractures
• test sensation under eye

• Superior alveolar nerve

• Carrying sensory from upper teeth and gums
• Nerve blocks e.g. by dentists, max fax

Question 21

What are branches of the mandibular division of the trigemial nerve

Answer 21

• Inferior alveolar nerve* runs through bony canal in mandible, exiting as mental nerve (via mental foramen)
- Carrying sensory from area of area mental protuberance (chin),
lower lip and gum
- Susceptible to injury in mandibular fractures - numbness to gums , chin and lip

• Lingual nerve carrying general sensory from the anterior part of the tongue - not taste
• Auriculotemporal carrying general sensory from part of ear, temple area/lateral side of head and scalp and temporomandibular joint
Note: branches of the ophthalmic
division not shown • Supraorbital and supratrochlear nerves
• Sensory from forehead/anterior scalp • Follow blood vessels of same name

Question 22

What is teh function of CN VI

Answer 22

Purely motor
• Innervates one muscle that moves the eye (extra ocular muscle
• Lateral rectus
Important to look laterally - abduct eye

Question 23

What os the route of the abducens nerve

Answer 23

Lower pons (junction between pons and medulla)
Runs upwards before being able to pass into centre of cavernous sinus
Enters into orbit via superior orbital fissure

Question 24

What are clinical points about the abducens nerve

Answer 24

From its exit at bottom of pons, has upwards route around bulbous front of pons. It then passes into cavernous sinus .so,…… Nerve can be easily stretched in raised ICP due to emerging anteriorly, at ponto-medullary junction before running under the surface of the pons upwards towards cavernous sinus
• Tested during eye movements (tests III, IV and VI)
• Patients present with diplopia
• Susceptible to injury in raised intracranial pressure e.g. due to bleed, tumour