AH&N- The Cranial Nerves

Question 1

Which cranial nerves are sensory only?

Answer 1

Olfactory
Optic
Vestibulocochlear

Question 2

Which cranial nerves are sensory and motor?

Answer 2

Oculomotor
Trochlear
Trigeminal
Abducens
Facial
Glossopharyngeal
Vagus
Accessory
Hypoglossal

Question 3

Which is the shortest cranial nerve?

Answer 3

Olfactory nerve

Question 4

Which two cranial nerves do not join with the brainstem?

Answer 4

Olfactory nerve and optic nerve.

Question 5

What is the embryological origin of the olfactory nerve?

Answer 5

The optic placade (a thickening of the ectoderm layer), also gives rise to the glial cells which support the nerve.

Question 6

What are the two processes of the olfactory nerve?

Answer 6

Peripheral olfactory process (receptors) in the olfactory mucosa.
Central process that return the information to the brain.

Question 7

Describe the olfactory mucosa.

Answer 7

Senses smell and more advanced aspects of taste.
In the roof of the nasal cavity it is composed of pseudostratified columnar epithelium which contains a number of cells.
- basal cells: form new stem cells from which the new olfactory cells can develop
- sustentacular cells: tall cells for structures support, analogous to the glial cells located in the CNS.
- olfactory receptor cells: bipolar neurones which have 2 processes, a dendritic process and a central process. The dendritic process projects to the surface of the epithelium where they project a number of short cilia, olfactory hairs into the mucous membrane. These cilia react to odours in the air and stimulate olfactory cells. The central process (axon) projects in the opposite direction through the basement membrane
- bowmans glands present

Question 8

Name the 12 cranial nerves.

Answer 8

Olfactory
Optic
Occulomotor
Trochlear
Trigeminal
Abducens
Facial
Vestibulocochlear
Glossopharyngeal
Vagus
Accessory
Hypoglossal

Question 9

What is anosmia?

Answer 9

The absence of a sense of smell- temporary, permanent, progressive or congenital.
Temporary: caused by infection or by local disorders of the nose
Permanent: caused by head injury or tumours of the olfactory groove (meningioma)
Progressive: Can occur as a result of neurodegenerative conditions, such as Parkinson’s or Alzheimer’s. Precedes motor symptoms but is often not noticed by the patient.
Congenital: also a feature of an umber of genetic conditions such as kallman syndrome (failure to start or finish puberty) & primary ciliary dyskinesia (defect in cilia causing it to be immobile).

Question 10

How do you test the olfactory nerve?

Answer 10

Ask the patient if they have noticed any changes in food taste or sense of smell.
Examining the nerve involves testing each nostril in turn, asking the patient to identify a certain smell (i.e. Peppermint, coffee)

Question 11

Describe the anatomical course of the olfactory nerve.

Answer 11

Once the axon penetrates through the basement membrane it joins other non-myelinated processes to form the fila olfactoria
They enter the cranial cavity through the cribiform plate of the ethmoid bone, once in cavity the fibers enter the olfactory bulb (an ovoid structure containing mitral cells), which lies in the olfactory groove, within the anterior cranial fossa. Olfactory nerve fibers synapse with the mitral cells forming synaptic glomeruli, from the glomeruli second order nerves then pass posteriorly into the olfactory tract.
Olfactory tract runs inferiorly to the frontal lobe, as tract reaches the anterior perforated substance it divides into medial and lateral stria:
Lateral stria carries axons to the olfactory area of the cerebral cortex.
Medial stria carries the axons across the medial plane of the anterior commissure where they meet the olfactory bulb of the opposite side.

Question 12

Where does the primary olfactory cortex send nerve fibers to?

Answer 12

Piriform cortex, the amygdala, olfactory tubercle and the secondary olfactory cortex.
These areas are involved in the memory and appreciation of olfactory sensations.

Question 13

What is the embryological origin of the optic nerve?

Answer 13

Developed from the optic vesicle, an outpockeging of the forebrain therefore the entirety of the nerve can be considered an out pocketing of the CNS and as a consequence examining the optic nerve enables an assessment of intracranial health to be made.

Question 14

What is the optic nerve surrounded by?

Answer 14

Cranial meninges (not by epi-, peri- & endoneurium like most nerves)

Question 15

Describe the anatomical course of the optic nerve.

Answer 15

Formed by convergence of axons from the retinal ganglion cells, these cells in turn receive impulses from the photoreceptors of the eye. Then leaves bony orbit via optic canal (through sphenoid bone), enters cranial cavity, running along the surface of the middle cranial fossa (close proximity to pituitary gland), where both nerves unite to form the optic chiasm.
At the chiasm fibers from the medial half of each retina cross over forming the optic tracts:
Left optic tract- contains fibers from the left lateral retina and the right medial retina
Right optic tract- right lateral retina and left medial retina
Each optic tract travels on that side to reach lateral geniculate nucleus (LGN) a relay system located in the thalamus, fibers synapse here. Axons from LGN carry visual info by:
Upper optic radiation: carries fibers from superior retinal quadrants, travels through the parietal lobe to reach the visual cortex.
Lower optic radiation: carries fibers from the inferior retinal quadrants, travels through the temporal lobe via Meyers loop to reach the visual cortex.

Question 16

What is a pituitary adenoma? What effects does it have on cranial nerves? What treatment is there?

Answer 16

Tumour of the pituitary gland
Lies close to optic chiasm therefore enlargement can affect the functioning of the optic nerve, compression particularly affects the fibers that are crossing over from the nasal half of each retina.
Produces visual defect affecting peripheral vision in both eyes known as bitemporal hemianopia.
Surgical intervention is commonly required, to access the gland the surgeon uses a transsphenoidal approach, accessing the gland via the sphenoidal sinus.

Question 17

What are the motor functions of the oculomotor nerve?

Answer 17

Superior branch:
Superior rectus- elevates the eyeball
Levator palpabrae superioris- raises the upper eyelid
Sympathetic fibers innervate the superior tarsal muscle which acts to keep the eyelid elevated after the levator palpabrae superioris has raised it.
Inferior branch:
Inferior rectus- depresses the eyeball
Medial rectus- adducts the eyeball
Inferior oblique- elevates, abducts & laterally rotates the eyeball

Question 18

What are the parasympathetic functions of the oculomotor nerve?

Answer 18

2 structures in the eye:
Sphincter pupillae- constricts the pupil reducing the amount of light entering the eye.
Ciliary muscle- contracts, causes the lens to become more spherical and therefore more adapted to short range vision.
Parasympathetic fibers travel in the inferior branch of the oculomotor nerve, within the orbit they branch off & synapse in the ciliary ganglion. The fibers are carried from the ganglion to the eye via short ciliary nerves.

Question 19

Describe the anatomical course of the oculomotor nerve.

Answer 19

Originates from the anterior aspect of the midbrain and moves anteriorly, passing below the posterior cerebral artery and above the superior cerebellar artery.
Passes through the dura mater and enters lateral aspect of cavernous sinus inside which it receives sympathetic branches from the carotid plexus. These fibers do not combine with the oculomotor nerve but merely travel in its sheath.
The nerve leaves the cranial cavity via the superior orbital fissure where it divides into superior and inferior branches.

Question 20

Oculomotor nerve lesion
What are the main causes?
What are the clinical signs?

Answer 20

3 main anatomical causes:
Increasing intracranial pressure- compresses nerve against temporal bone
Aneurysm of the posterior cerebral artery
Cavernous sinus infection or trauma
Pathological causes:
Diabetes, MS, myasthenia gravis & giant cell arthritis.
Signs:
Ptosis (drooping of eyelid)- paralysis of levator palpabrae
Eyeball: down & out- paralysis of superior, inferior & medial rectus & inferior oblique
Dilated pupil- unopposed action of dilator pupillae muscle.

Question 21

Describe the anatomical course of the trochlear nerve.

Answer 21

Arises from the trochlear nucleus of the brain, emerging from the posterior aspect of the midbrain.
Runs anteriorly & inferiorly within the subarachnoid space before piercing the dura mater adjacent to the posterior clinoid process of the sphenoid bone. Then moves along the lateral wall of the cavernous sinus before entering the orbit of the eye via the superior orbital fissure.

Question 22

What is the motor function of the trochlear nerve?

Answer 22

Innervates only the superior oblique- depress and intort eyeball

Question 23

Describe an examination of the trochlear nerve.

Answer 23

Examined in conjunction with the oculomotor & abducens nerves by testing the movements of the eye.
The patient is asked to follow a point with their eyes without moving their head. The target is moved in an H-shape & the patient is asked to report any blurring of vision or diplopia.

Question 24

What are the common causes and clinical signs of trochlear nerve palsy?

Answer 24

Most common cause is congenital fourth nerve palsy, a condition in which the development of the trochlear nerve or nucleus is abnormal- may be curable by surgery.
Other causes include diabetic neuropathy, thrombophlebitis of the cavernous sinus and raised intracranial pressure (e.g. Due to haemorrhage or oedema) therefore rare to be affected in isolation
Patients will present with diplopia when looking down and in

Question 25

Describe the anatomical course of the abducens nerve.

Answer 25

Arises from the abducens nucleus in the pons of the brain and exits the brainstem at the junction of the pons and the medulla .
It then exits the subarachnoid space and pierces the dura mater to run in a space known as dorellos canal, the nerve travels thorough the cavernous sinus at the tip of the petrous temporal bone, before entering the orbit, the abducens nerve terminates by innervating the lateral rectus muscle.

Question 26

What does the abducens nerve innervate?

Answer 26

The lateral rectus.

Question 27

How is the abducens nerve examined.

Answer 27

In conjunction with the oculomotor and trochlear nerves by testing the movements if the eye.
The patient is asked to follow a point with their eyes without moving their head, the target is moved in a H-shape and patient is asked to report any blurring of vision or diplopia.

Question 28

What are some causes of abducens nerve palsy?

Answer 28

Any pathology that leads to downward pressure on the brain stem (e.g. Brain tumour, extra dural haematoma) can lead to the nerve becoming stretched along the divus of the skull: wernicke-korsakoff syndrome (caused by thiamine deficiency and generally seen in alcoholics)
Other causes include diabetic neuropathy and affected thrombophlebitis of the cavernous sinus- rare for abducens nerve to be isolated

Question 29

How will patients with abducens nerve palsy present?

Answer 29

With diplopia and a medially rotated eye which cannot be abducted past the midline. The patient may attempt to compensate by rotating their head to allow the eye to look sideways.

Question 30

What is the largest cranial nerve?

Answer 30

The trigeminal nerve

Question 31

What are the trigeminal nerves sensory and motor functions?

Answer 31

The three terminal branches innervate the skin, mucous membranes and sinuses of the face.
Only the mandibular branch has motor fibres, innervating the muscles of mastication: medial pterygoid, lateral pterygoid, masseter & temporalis. It also supplies other 1st pharyngeal arch derivatives: anterior belly of digastric, tensor veli palatani and tensor tympani.

Question 32

Describe the anatomical course of the trigeminal nerve.

Answer 32

Originates from 3 sensory nuclei: mesencephalic, primary sensory, spinal nuclei of trigeminal nerve
And one motor nucleus: motor nucleus of the trigeminal nerve
At the level of the pons the sensory nuclei merge to form a sensory root
The motor nucleus forms motor root
In middle cranial fossa the sensory root expands into the trigeminal ganglion, lateral to the cavernous sinus in the trigeminal cave
The peripheral aspect of the trigeminal ganglion give rise to 3 divisions: opthalmic, maxillary and mandibular.
The motor root passes inferiorly to the sensory root along the floor of the trigeminal cave, it’s fibres are distribute to the mandibular division.
The opthalmic nerve and maxillary nerve travel lateral to the cavernous sinus exiting the cranium via the superior orbital fissure and foramen rotundum respectively.
The mandibular nerve exits via the foramen ovale entering the infra-temporal fossa.

Question 33

What are the three terminal branches of the opthalmic nerve and what do they innervate?

Answer 33

Frontal, lacrimal and nasociliary which innervate the ski. And mucous membrane of derivatives of the frontonasal prominence derivatives

Question 34

What is the parasympathetic supply of the opthalmic nerve?

Answer 34

Lacrimal gland: post ganglionic fibres from the pterygopalatine ganglion (derived from the facial nerve) htravel with the zygomatic branch of V2 and then join the lacrimal branch of V1.

Question 35

Describe the corneal reflex.

Answer 35

The involuntary blinking of the eyelids- stimulated by tactile, thermal or painful stimulation of the cornea
The opthalmic nerve acts as the afferent detecting stimuli and facial nerve is efferent causing contraction of the orbicularis oculi muscle.
If the corneal reflex is absent it is a sign of damage to the trigeminal/opthalmic nerve or the facial nerve.

Question 36

What does the maxillary nerve innervate?

Answer 36

14 terminal branches which innervate the skin, mucous membranes and sinuses of derivatives of the maxillary prominence of the 1st pharyngeal arch.
Lower eyelid and its conjunctiva
Cheeks and maxillary sinus
Nasal cavity and lateral nose
Upper lip
Upper molar, incisor and canine teeth and the associated gingivitis
Superior palate

Question 37

What is the parasympathetic supply of the maxillary nerve?

Answer 37

Lacrimal gland: post ganglionic fibres from the pterygopalatine ganglion ( derived from the facial nerve) travel with the zygomatic branch of V2 and then join the lacrimal branch of V1
Nasal glands: parasympathetic fibres are also carried to the mucous glands of the nasal mucosa. Post-ganglionic fibres travel with the nasopalatine and greater palatine nerves.

Question 38

What are the four terminal branches of the mandibular nerve in the infra-temporal fossa? What do they innervate?

Answer 38

Buccal nerve, inferior alveolar nerve, auricotemporal nerve and lingual nerve. Innervate the skin, mucous membrane and striated muscle derivatives of the mandibular prominence of the 1st pharyngeal arch.

Question 39

What are the sensory and motor innervations on the mandibular nerve?

Answer 39

Sensory:
Mucous membranes and floor of the oral cavity
Anterior 2/3 of tongue
External ear
Lower lip
Chin
Lower molar, incisor and canine teeth and the associated gingiva
Motor:
Muscles of mastication
Anterior belly of the digastric muscle and the mylohyoid muscle
Tensor veli palatini
Tensor tympani

Question 40

What is the parasympathetic supply of the mandibular nerve?

Answer 40

Submandibular and sublingual glands: post-ganglionic fibres from the submandibular ganglion (from the facial nerve), travel with the lingual nerve to innervate.
Parotid gland: post-ganglionic fibres from the otic ganglion (derived from the glossopharyngeal nerve CN1X) travel with the auricotemporal branch of the V3 to innervate the parotid gland.

Question 41

What is inferior alveolar nerve block?

Answer 41

The inferior alveolar nerve, a branch of V3, travels through the mandibular foramen and mandibular canal.
Within the mandibular canal the inferior alveolar nerve forms the inferior dental plexus, which Innervates the lower teeth.
A major branch of this plexus, the mental nerve, supplies the skin and mucous membranes of the lower lip, skin of the chin and the gingiva of the lower teeth.
In some dental procedures which require a local anaesthesia, the inferior alveolar nerve is blocked before it gives rise to the plexus.
The anaesthetic solution is administrated at the mandibular foramen, causing numbness of area supplied by the inferior alveolar nerve.
The anaesthetic fluid also spreads to the lingual nerve which originates near the inferior alveolar nerve, causing numbness if the anterior 2/3 of the tongue.

Question 42

How do you examine the trigeminal nerve?

Answer 42

Testing sensory: ask the patient to close their eyes and introduce a cotton wisp to areas of the face supplies by the 3 divisions of the trigeminal nerve to detect tactile sensory competence.
Testing motor: ask the patient to clench their jaw as you palate superior to the zygomatic arch to feel for contraction of the temporal is and then repeat palpating inferiorly for the masseter. Ask the patient to open their mouth and deviate their mandible to the right and left to check for competence of the medial and lateral pterygoid muscle.
Test for corneal reflex.

Question 43

Describe the anatomical course of the facial nerve.

Answer 43

Intracranial:
Nerve arises in the pons as 2 roots ( large motor & small sensory)
They travel through the internal acoustic meatus (1cm long opening in the petrous part of the temporal bone)close proximity with the inner ear
Still within the temporal bone, the roots leave the internal acoustic meatus and enter into the facial canal. The canal is Z-shaped. Within the facial canal:
Two roots fuse to form the facial nerve, the nerve forms the geniculate ganglion, and the nerve gives rise to the greater petrosal nerve, the nerve to stapedius and the chords tympani
Facial nerve exits canal via the stylomastoid foramen just posterior to the styloid process of the temporal bone.
Extracranial:
After exiting the skull the facial nerve turns superiorly to run just ️anterior to the outer ear.
Posterior auricular nerve branches off and provides motor innervation to some of the muscles around the ear. Immediately distal to this motor branches are sent to the posterior belly of the digastric muscle and to the stylohyoid muscle.
The motor root of the facial nerve continues anteriorly and inferiorly into the parotid gland where it splits into: temporal, zygomatic, buccal, marginal mandibular and cervical branches. Facial expression.

Question 44

What are the motor functions of the facial nerve?

Answer 44

All muscles are derivatives of the second pharyngeal arch.
First motor branch arises in the facial canal, passes through the pyramidal eminence to supply the stapedius muscle in the middle ear.
Between the stylomastoid foramen and the parotid gland 3 branches:
Posterior auricular nerve- ascends in front of mastoid process and Innervates intrinsic and extrinsic muscles of the outer ear and occipital part of occipitofrontalis muscle.
Nerve to the posterior belly of the digastric muscle- Innervates a suprahyoid muscle of the neck, raises the hyoid bone.
Nerve to the stylohyoid muscle- same as posterior belly of digastric
In parotid gland:
Temporal- Innervates frontalis, orbicularis oculi & corrugator supercilli
Zygomatic- Innervates the orbicularis oculi
Buccal- orbicularis oculi, buccinator & zygomatious
Marginal mandibular branch- mentalis
Cervical- Platysma

Question 45

What are the special sensory functions of the facial nerve?

Answer 45

The chorda tympani branch of the facial nerve is responsible for the taste innervation of the anterior 2/3 of the tongue.
The nerve arises in the facial canal and travels across the bones of the Middle ear, exiting via the petrotympanic fissure and entering the infratemporal fossa.
Here the chords tympani hitchhiker with the lingual nerve, parasympathetic fibres stay with lingual nerve but main body leaves to innervate ️anterior 2/3 of tongue.

Question 46

What are the parasympathetic functions of the facial nerve?

Answer 46

Carried by the greater petrosal and chords tympani branches.
Greater petrosal: arises immediately distal to the geniculate ganglion within the facial canal, moving in anteromedial direction, exiting the temporal bone into the middle cranial fossa, it then travels across the formamen lace rum combining with the deep petrosal nerve to form the nerve of the pterygoid canal.
It then passes through the pterygoid canal to enter the pterygopalatine fossa and synapses with the pterygopalatine ganglion. Branches then provide innervation to the mucous glands of the oral cavity, nose, pharynx and lacrimal gland
Chords tympani

Question 47

What does intracranial lesion of the facial nerve result in?

Answer 47

Muscles of facial expression will be paralysed of severely weakened. Other symptoms depend on location of lesion.
Chords tympani: reduced salivation and loss of taste on 2/3
Nerve to stapedius: ipsilateral hyperacusis (hypersensitive to sound)
Greater petrosal nerve: ipsilateral reduced lacrimal fluid production
Most common cause: middle ear pathology such as tumour or infection
Extracranial:
Only the motor function is affected resulting in paralysis or severe weakness of the muscles of facial expression.
Parotid gland pathology-e.g. Tumour, parotitis, surgery
Infection of the nerve- particularly herpe
Compression during forceps delivery- neonatal mastoid process is not fully developed & does not provide complete protection of the nerve.
Idiopathic- Bell’s palsy

Question 48

Describe the anatomical course of the vestibulocochlear nerve.

Answer 48

Vestibular component- arises from the vestibular nuclei complex in the pons and medulla
Cochlear component- arises from the ventral and dorsal cochlear nuclei, situated in the inferior cerebellar penduncle.
Both combine in pons to for vestibulocochlear nerve, the nerve emerges from the brain at the cerebellar online angle and exits the cranium via the internal acoustic Meaton of the temporal bone