Cranial Nerves Of Oculomotion

Question 1

Main aims of eye movements

Answer 1

To position the fovea of the retina for maximum capture of the visually significant signals
Move the head and eyes away from harms way - reflex movements

Question 2

Where do the cranial nerves of oculomotion leave the brain and skull?

Answer 2

Leave the brainstem, pass through the cavernous sinus and exit the skull via the superior orbital fissure,

Question 3

What fibres does the Oculomotor nerve have and what do they supply?

Answer 3

Somatic motor fibres - supply the extraocular muscles of the eye, except lateral rectus and superior oblique. And the levator palpebrae superioris muscle of the upper eye.
Autonomic (parasympathetic) fibres supplying the constrictor pupillae (circular muscles)

Question 4

Where do somatic Oculomotor fibres arise from in the brainstem?

Answer 4

The oculomotor nucleus - at the level of the superior colliculus in the region of the periaqueductal grey. In the midbrain.

Question 5

What does periaqueductal grey mean?

Answer 5

Around the aqueduct - surrounding the cerebral aqueduct of Sylvius

Question 6

Where do the parasympathetic autonomic fibres of the oculomotor nerve come from?

Answer 6

The Edinger-Westphal nucleus.

Question 7

What could damage to the cerebral aqueduct cause?

Answer 7

Damage to the oculomotor nerve

Question 8

Is innervation from oculomotor nerve ipsilateral or contralateral?

Answer 8

Edinger-westphal nucleus supplies ipsilateral fibres, for pupil constriction
Inferior rectus, inferior oblique, medial rectus and levator palpebrae superioris muscle all receive ipsilateral innervation
Superior rectus muscle receives contralateral innervation

Question 9

Where do the parasympathetic fibres of CNIII terminate?

Answer 9

They continue to travel with the inferior portion of the oculomotor nerve before terminating in the ciliary ganglion, as preganglionic parasympathetic fibres. The relay fibres are postganglionic parasympathetic fibres that supply the constrictor pupillae muscles of the eye, constituting the short ciliary nerves.

Question 10

Where does the trochlear nerve arise from?

Answer 10

The trochlear motor nucleus in the midbrain. Also located around the periaqueductal grey. Located at the level of the inferior colliculus.

Question 11

Which side of the brain does the trochlear nerve leave from?

Answer 11

Superior part of brain unlike all other which leave via the inferior part.

Question 12

Brief course of the trochlear nerve

Answer 12

Motor nucleus in the midbrain. Nerve fibres cross the midline and it bilaterally decussate. Passes forward in the subarachnoid space. Pierces the dura mater to lie in the lateral wall of the cavernous sinus. Enters the orbital via the superior orbital fissure.

Question 13

Is trochlear a mixed nerve?

Answer 13

No it is a somatic nerve only.

Question 14

What does the trochlear nerve innervate?

Answer 14

SO4 - superior oblique muscle of the eye

Question 15

Does the trochlear nerve innervate ipsilaterally or contralaterally?

Answer 15

It innervates the superior oblique muscle on the opposite side (contralateral) from its origin. All of its fibres cross over the other side before it exits over the dorsum of the midbrain.

Question 16

What would injury to the trochlear nucleus cause?

Answer 16

Contralateral symptom

Question 17

What would injury to the nerve after crossing after the midbrain result in?

Answer 17

Ipsilateral symptom

Question 18

What does damage to the trochlear nerve cause and how do you test this?

Answer 18

Diplopia when looking downwards and inwards.
Test by asking patient to look medially and downwards - they should be able to look medially but not downwards.

Question 19

Where does the abducens nerve arise from?

Answer 19

From the abducens motor nucleus in the pons, on the floor of the fourth ventricle at the level of the facial colliculus. It is close to the midline.

Question 20

What does the abducens nerve supply?

Answer 20

The lateral rectus muscle

Question 21

What is the facial colliculus?

Answer 21

Axons from the facial nerve lip around the abducens nerve, creating a slight bulge, the facial colliculus, that is visible on the dorsal surface of the floor of the fourth ventricle.

Question 22

Describe the brief course of the abducens nerve

Answer 22

Leaves the brainstem at the pontomedullary junction. When it emerges from the brainstem it enters the subarachnoid space. It runs up between the pons and Clovis and pierces the dura to run between the dura and skull through Dorello’s canal. At the tip of the petrous temporal bone it makes a sharp turn to enter the cavernous sinus, where it runs alongside the ICA. It then enters the orbit through the superior orbital fissure and innervates the lateral rectus muscle of the eye.

Question 23

Where is the abducens nerve vulnerable to damage?

Answer 23

Fractures of the porous temporal bone
Aneurysms of the intracavernous carotid artery
Mass lesions that push the brainstem downward can damage the nerve by stretching it between the point where it emerges from the pons and where it hooks over the petrous temporal bone
Can be at risk of damage during surgery of the skull base
Raised intracranial pressure

Question 24

What happens when the abducens nerve is damaged?

Answer 24

The patient is unable to look outwards. Causes squinting on looking outwards.

Question 25

Briefly describe the course of the oculomotor nerve

Answer 25

Fascicular part. Basilar portions relating to the basilar artery. Intracavernous portion, sitting on lateral wall of cavernous sinus. Infraorbital portion when it leaves the intracranial cavity. Enters the orbit of eye via superior orbital fissure.