L7: cranial nerves part 1 - origin, route and function

Question 1

Describe cranial nerve I

Answer 1

Olfactory nerve – special visceral afferent nerve – transmits information about smell
Originates from the cerebrum
Not routinely tested – if tested, one nostril at a time
Absence/reduced sense of smell = anosmia/hyposmia
Common cause: upper respiratory tract infection
-less common causes include: head/facial injury, anterior cranial fossa tumours, associations with Parkinson’s disease & Alzheimer’s disease

Question 2

Describe the course of cranial nerve I

Answer 2

Sense of smell detected by olfactory receptors located within the nasal epithelium
Assemble into small bundles of true olfactory nerves – penetrate small foramina in the cribriform place in the ethmoid bone
Enter the cranial cavity = olfactory bulb
Olfactory tract = travels posteriorly on the inferior surface of the frontal lobe

Question 3

Describe cranial nerve II

Answer 3

Optic nerve – responsible for transmitting the special sensory information about vision
Originates from the cerebrum
Tested by: pupillary size & response to light, visual acuity, visual fields & ophthalmoscopy
Patients likely to report blurred vision involving one eye/complete loss of vision
Causes: any disease involving optic nerve (optic neuritis, AION), papilloedema

Question 4

Describe the course of cranial nerve II

Answer 4

Formed by convergence of axons from the retinal ganglion cells
Exits back of orbit via the optic canal
Fibres cross and merge at the optic chiasm (there is communication from the optic tracts with brainstem to allow for certain visual reflexes)

Question 5

Describe cranial nerve III

Answer 5

Oculomotor nerve – motor & parasympathetic (motor – innervates the majority of the extraocular muscles; parasympathetic – supplies the sphincter pupillae & ciliary muscles of eye)
Originates at midbrain-pontine junction
Tested by: inspection of resting gaze, eye movements, pupils, eyelid position & pupillary light reflexes
Patients report double-vision (diplopia); on examination – ptosis, abnormal position of eye, pupil may/may not be dilated
Causes: microvascular ischaemia (pupil sparing), compressive (pupil involving)

Question 6

Describe the course of cranial nerve III

Answer 6

Oculomotor nerve originates from oculomotor nucleus
Enters the lateral aspect of the cavernous sinus
Leaves via the superior orbital fissure – divides into superior & inferior branches

Question 7

Describe cranial nerve IV

Answer 7

Trochlear nerve – motor innervation of superior oblique
Originates at midbrain
Tested by: inspection of resting gaze, eye movements
Patients report double vision (dipoplia) & may have abnormal eye position and head tilt
Causes: congenital or acquired (microvascular ischaemia, trauma, tumour)

Question 8

Describe the course of cranial nerve IV

Answer 8

Arises from trochlear nucleus
Anteriorly and inferiorly within the subarachnoid space
Moves along lateral wall of the cavernous sinus & enters orbit of eye via superior orbital fissure

Question 9

Describe cranial nerve V

Answer 9

Trigeminal nerve – motor & sensory functions – skin, mucous membranes & sinuses of face (sensory), muscles of mastication (mandibular branch)
Originates at the pons
Tested by: light touch Va, Vb, Vc dermatomes, muscles of mastication & corneal reflex
Causes: trigeminal herpes zoster, trigeminal neuralgia, orbital and mandibular fractures & posterior cranial fossa tumours

Question 10

Describe the course of cranial nerve V

Answer 10

3 divisions
Ophthalmic and maxillary pass through the cavernous sinus
-ophthalmic passes through superior orbital fissure
-maxillary passes through foramen rotundum
Mandibular doesn’t pass through cavernous sinus – passes through foramen ovale

Question 11

Describe cranial nerve VI

Answer 11

Abducens nerve – provides innervation to the lateral rectus muscle
Originates at pontine-medulla junction
Tested by: inspection of resting gaze, eye movements
Patients report double vision (worse in lateral gaze on side of lesion)
Causes: microvascular ischaemia, head injury, tumour, raised ICP (false localising sign)

Question 12

Describe the course of cranial nerve VI

Answer 12

Arises from pontine-medullary junction
Steep upward route to take before entering the cavernous sinus (makes it vulnerable during raised ICP)
Enters the bony orbit via the superior orbital fissure

Question 13

Describe the different layers of the scalp

Answer 13

Skin – contains numerous hair follicles & sebaceous glands (common place for sebaceous cysts)
Dense connective tissue – richly vascularised & innervated
Aponeurosis – thin, tendinous tissue that connects occipitalis & frontalis muscles
Loose connective tissue – separates periosteum from aponeurosis
Periosteum – outer layer of skull bones

Question 14

What are emissary veins?

Answer 14

Connect veins in the dense connective tissue layer to veins deep to the skull (intracranial dural venous sinuses)
-traverses the loose connective tissue layer
In head injury: blood can spread freely in loose connective tissue layer & can accumulate under the orbicularis oculi muscle -> causes bruising around the eye (consider more dangerous causes first eg. orbital fractures & anterior cranial floor fractures)

Question 15

Describe infections of the scalp

Answer 15

Infections in the deep connective tissue layer won’t spread far – superficial, tender bump
Infections in the loose connective tissue layer can spread more freely and into intracranial structures via the emissary veins

Question 16

Why do deep lacerations to the scalp tend to bleed profusely?

Answer 16

1) Pull of the occipitofrontalis muscle prevents the closure of the bleeding vessel & surrounding skin
2) Blood vessels to the scalp are adhered to dense connective tissue, preventing the vasoconstriction that normally occurs in response to damage
3) Blood supply to the scalp is made up of many anastomoses, which contribute to profuse bleeding

Question 17

Outline the arterial supply to the scalp

Answer 17

Receives a rich arterial supply via the external carotid artery & ophthalmic artery – 3 branches of external carotid artery involved:
1) Superficial temporal – supplies the frontal & temporal regions
2) Posterior auricular – supplies the area superiorly & posteriorly to the auricle
3) Occipital – supplies the back of the scalp
Anteriorly & superiorly, scalp receives additional supply from two branches of ophthalmic artery – supraorbital & supratrochlear arteries

Question 18

Outline the venous drainage of the scalp

Answer 18

Superficial drainage follows the arterial supply: superficial temporal, occipital, posterior auricular, supraorbital & supratrochlear veins
Deep region of the skull is drained by the pterygoid venous plexus – situated between temporalis and lateral pterygoids & drains into maxillary vein
Emissary veins connect scalp and dural venous sinuses

Question 19

What two nerves innervate the scalp?

Answer 19

Trigeminal nerve

Cervical nerve roots

Question 20

Describe the branches of the trigeminal nerve which give innervation to the scalp

Answer 20

Supratrochlear– supplies the anteromedial forehead
Supraorbital – supplies a large portion between the anterolateral forehead & the vertex
Zygomaticotemporal – supplies the temple
Auriculotemporal – supplies skin anterosuperior to the auricle

Question 21

Describe the branches of the cervical nerve roots which give innervation to the scalp

Answer 21

Lesser occipital nerve – supplies the skin posterior to the ear
Greater occipital nerve – supplies the skin of the occipital region
Greater auricular nerve - supplies the skin posterior to the ear & over the angle of the mandible
Third occipital nerve – supplies the skin of the inferior occipital region