Session 3 - Cranial Nerves

Question 1

What are the cranial nerves?

Answer 1

The cranial nerves are 12 pairs of named nerves that originate directly from the brain, instead of the spinal cord like other nerves. Each pair has its own function and leaves the skull via specific foramina. Understanding the individual characteristics of each pair of cranial nerves is important when testing them during a clinical examination.

The table on the next page lists the important characteristics of each cranial nerve. They are often referred to by their specific number in roman numerals, or by their names. All cranial nerves leave their respective point of origin from the central nervous system anteriorly, except the trochlear nerves (CN IV) which leave the midbrain posteriorly.

Question 2

What is CN1?

Answer 2

Olfactory Nerve (CN I)
Specialised nerve cells in the roof of the nasal cavity detect odour molecules and convey this information superiorly through the cribriform plate to the olfactory bulbs. The olfactory bulbs are located on the superior surface of the cribriform plate, either side of the crista galli. The olfactory tracts then carry smell information posteriorly along the underside of the frontal lobes towards the olfactory cortices in the temporal lobes.

I – Olfactory - Sensory - (origin)Cerebrum - Cribriform plate- Olfaction (smell).

Question 3

What is CN2?

Answer 3

Optic Nerve (CN II)
The optic nerve carries sight information from the retina to the optic chiasm, where some fibres cross over. This is explained in more detail in the next section

II – Optic - Sensory - (o)Diencephalon - Optic canal - Sight.

Question 4

What are CN 3,4,AND 6 SIMILARITIES?

Answer 4

Oculomotor, Trochlear and Abducens Nerves (CN III, IV, VI)
These nerves control the muscles that are responsible for movements of the eyes and diameter of the pupils. They are covered in more detail in session 2.

Question 5

What is CN 3 like?

Answer 5

III – Oculomotor - Motor (+ para-sympathetics) - Midbrain - Superior orbital fissure - Eye movements (SR, IR, MR, IO), eyelid opening (LPS), pupillary constriction and accommodation.

Question 6

What is CN 4 like?

Answer 6

IV – Trochlear
Motor
Midbrain
Superior orbital fissure
Eye movements (SO).

Question 7

What is CN 5 like?

Answer 7

The trigeminal nerve is the largest cranial nerve and divides into three branches shortly after it exits the pons. The branches are often named V1, V2, and V3.

V - Trigeminal - Both sensory and motor - (o)Pons
- V1 – ophthalmic – superior orbital fissure.
- V2 – maxillary – foramen rotundum.
- V3 – mandibular – foramen ovale

• V1 – sensation from the upper third of face.
• V2 – sensation from the middle third of face.
• V3 – sensation from the lower third of face, motor to the muscles of mastication, motor to the tensor tympani muscle.

Question 8

What are the nerves of CN 5?

Answer 8

Ophthalmic nerve (V1) – a purely sensory nerve that supplies the skin over the superior third of the face as far as the top of the of the head, and the anterior surface of the eye.
• Maxillary nerve (V2) – also a purely sensory nerve that supplies the middle third of the face, including most of the internal nasal cavity, upper teeth and palate.
• Mandibular nerve (V3) – a mixed motor and sensory nerve. The sensory fibres supply the inferior third of the face, including general sensation to the anterior 2/3 of the tongue, and the mandibular teeth and gums. The motor fibres of the mandibular nerve supply five muscles.

Question 9

What muscles does the motor fibres of the mandibular nerve supply?

Answer 9

One is the tensor tympani in the middle ear (see session 2) and the other four are the ‘muscles of mastication’ which are responsible for moving our mandible to chew our food:
o Temporalis
o Masseter
o Medial pterygoid
o Lateral pterygoid.

Question 10

What is CN 7 like?

Answer 10

Facial Nerve (CN VII)
The facial nerve takes an unusual route out of the skull. It first passes through the internal auditory meatus alongside the vestibulocochlear nerve (CN VIII). Shortly after this, it gives off a branch which supplies parasympathetic secretomotor function to the lacrimal gland.

As it progresses through the middle ear cavity, the facial nerve gives off a branch to the stapedius muscle and another branch known as the ‘chorda tympani’. This nerve supplies taste sensation to the anterior 2/3 of the tongue and parasympathetic secretomotor function to the submandibular and sublingual salivary glands. The facial nerve then leaves the temporal bone of the skull via the stylomastoid foramen which is located between the styloid and mastoid processes of the temporal bone, hence the name.

After exiting the skull, it gives of a small branch which carries sensory information from the ear and motor supply to some muscles of the scalp. The main body of the facial nerve then enters the substance of the parotid salivary gland (which it does not innervate), and within it, divides into five branches which spread out across the face to supply the muscles of facial expression. The five branches of the facial nerve are: temporal, zygomatic, buccal, marginal mandibular and cervical branches.

VII - Facial - Both (+ para-sympathetics) - Pons

• Internal auditory meatus, then stylomastoid foramen
• Motor to the muscles of facial expression, motor to the stapedius muscle, sensation from the ear canal, secretomotor function to the submandibular and sublingual salivary glands and lacrimal gland, taste from the anterior 2/3 of the tongue

Question 11

What si CN 8 like?

Answer 11

VIII – Vestibulo-cochlear
Sensory
Pons
Internal auditory meatus
Balance and hearing.

This purely sensory nerve carries information of the special senses hearing and equilibrium (balance) from the inner ear. This is covered in more detail in session 2.

Question 12

What is CN 9 like?

Answer 12

IX –
Glosso-pharyngeal
Both (+ para-sympathetics)
Medulla
Jugular foramen
Motor to the stylopharyngeus muscle, sensation from the pharynx and posterior 1/3 of the tongue, sensation from the carotid baroreceptors, secretomotor function to the parotid salivary gland, taste from the posterior 1/3 of the tongue.

The glossopharyngeal nerve has numerous functions. It supplies motor function to one muscle, the stylopharyngeus, which assists with facilitating swallowing. It carries general sensation from the middle ear, auditory tube, majority of the pharynx and both general and taste sensation from the posterior 1/3 of the tongue. It provides parasympathetic secretomotor supply to the parotid salivary gland and finally, it carries unconscious sensory information from the carotid chemoreceptors and baroreceptors towards the medulla.

Question 13

Wat is CN 10 like?

Answer 13

X - Vagus
Both (+ para-sympathetics)
Medulla
Jugular foramen
Motor to the muscles of the soft palate, pharyngeal muscles and internal laryngeal muscles, sensation from the external ear and ear canal, taste from the epiglottis and parasympathetics to the thoracic and abdominal organs.

Some of the main functions of this nerve have been discussed in previous modules (parasympathetic supply to thoracic and abdominal organs, and motor and sensory innervation to the larynx). However, it also supplies motor innervation to the muscles of the soft palate, palatine folds and pharyngeal constrictors, and carries general sensation from the ear canal and pinna. Finally, it carries taste sensation from the epiglottis of the larynx.

Question 14

What are CN 11 like?

Answer 14

XI - Accessory
Motor
Spinal cord C1-C5
Foramen magnum (in) and jugular foramen (out)
Motor to the trapezius and sternocleidomastoid muscles.

Accessory Nerve (CN XI)
This purely motor nerve follows an unusual route. It does not actually originate from the brain. Its fibres originate from the spinal cord levels C1-C5. They join each other to form the spinal accessory nerve and enter the skull through the foramen magnum, then leave the skull again via the jugular foramen. The accessory nerve only supplies two muscles: sternocleidomastoid which is responsible for turning the head and nodding, and trapezius which is a large muscle on the back of the torso responsible for shrugging and numerus movements of the scapula.

Question 15

What is CN 12 like?

Answer 15

XII - Hypoglossal
Motor
Medulla
Hypoglossal canal
Motor to the muscles of the tongue

Hypoglossal Nerve (XII)
Finally, the hypoglossal nerve is a purely motor nerve. It leaves the skull as the only structure to pass through its named foramen, the hypoglossal canal, and supplies motor innervation to all intrinsic and extrinsic muscles of the tongue (except palatoglossus which is supplied by the vagus nerve). Damage to a unilateral hypoglossal nerve will cause that side of the tongue to be paralysed and atrophy, meaning the other side will overpower it. This leads to a characteristic finding of the tongue pointing towards the side of the lesion.

Question 16

What is the CR of CN 1-4

Answer 16

I. Olfactory – the clinician can simply ask the patient if their sense of smell has changed, or they can ensure the patient is able to identify certain strong-smelling compounds, such as coffee, chocolate, vanilla, or cinnamon.
II. Optic – this nerve is tested in several ways. Firstly, visual acuity is tested using a Snellen chart (a chart with letters of decreasing sizes). Visual fields (peripheral vision), accommodation to near and far objects, colour vision, and the pupillary light reflex also test the optic nerve.
III. Oculomotor – this nerve is tested alongside CN IV (trochlear) and CN VI (abducens) by a clinician asking a patient to follow their finger as they move it across the patient’s field of vision. The clinician can observe the movements of the patient’s eyes to ensure they are moving as they should. The pupillary light reflex also tests the oculomotor nerve.
IV. Trochlear – see above

Question 17

What is the CR of CN 5,6?

Answer 17

V. Trigeminal – the motor and sensory parts of the trigeminal nerve are tested separately. Firstly, sensation is tested by simply ensuring the patient can feel a brush of cotton wool against their skin in the three regions of the face (forehead, cheek, jaw) on both sides. Sensation may be further tested by ensuring the patient is able to tell the difference between sharp and crude touch, or by testing the blink reflex when the cornea of the eye is touched. Motor function is tested by palpating a patient’s jaw muscles as they clench their teeth or asking the patient to forcibly open their mouth against resistance.
VI. Abducens – see above.

Question 18

What is the CR of CN 7-9?

Answer 18

VII. Facial – the motor function of the facial nerve is its only function that is routinely tested. This is achieved by asking the patient to perform a series of facial movements, such a raising their eyebrows, closing their eyes tightly, blowing their cheeks out or showing all their teeth. The clinician looks for any asymmetry in their movements.
VIII. Vestibulocochlear – the simplest way this nerve is tested is by blocking the one of the patient’s ears and whispering a number or word in the other ear and asking the patient to repeat it to check their hearing. The test is repeated with the other ear. The function of the ear, cochlear and vestibular system can be tested more thoroughly with other tests using tuning forks and dedicated equipment in specialist clinics.
IX. Glossopharyngeal – this nerve is tested by assessing the patient’s gag reflex. A tongue-depressor (a blunt wooden ‘lolly-stick’) is pressed against the oropharynx and a normal finding would be the patient ‘gagging’ or looking like they will vomit. This tests the sensory function of this nerve.

Question 19

What is the CR of CN 10-12?

Answer 19

X. Vagus – there are several ways to test the vagus nerve. The gag reflex above tests the motor function of the vagus nerve, but it can be further tested by asking the patient to open their mouth and say ‘ahhh’. This should cause elevation of the soft palate by muscles which are supplied by the vagus nerve. Coughing and swallowing also both require function of the vagus nerve.
XI. Accessory – to test the trapezius muscle, a clinician can ask the patient to shrug their shoulders, and to test the sternocleidomastoid, a clinician asks the patient to turn their head against resistance.
XII. Hypoglossal – finally, the hypoglossal nerve is tested by asking the patient to protrude their tongue. Deviation of the tongue to one side or the other may imply damage to one of the hypoglossal nerves.