Cranial Nerves Of Oculomotion Flashcards
Main aims of eye movements
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
Where do the cranial nerves of oculomotion leave the brain and skull?
Leave the brainstem, pass through the cavernous sinus and exit the skull via the superior orbital fissure,
What fibres does the Oculomotor nerve have and what do they supply?
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)
Where do somatic Oculomotor fibres arise from in the brainstem?
The oculomotor nucleus - at the level of the superior colliculus in the region of the periaqueductal grey. In the midbrain.
What does periaqueductal grey mean?
Around the aqueduct - surrounding the cerebral aqueduct of Sylvius
Where do the parasympathetic autonomic fibres of the oculomotor nerve come from?
The Edinger-Westphal nucleus.
What could damage to the cerebral aqueduct cause?
Damage to the oculomotor nerve
Is innervation from oculomotor nerve ipsilateral or contralateral?
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
Where do the parasympathetic fibres of CNIII terminate?
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.
Where does the trochlear nerve arise from?
The trochlear motor nucleus in the midbrain. Also located around the periaqueductal grey. Located at the level of the inferior colliculus.
Which side of the brain does the trochlear nerve leave from?
Superior part of brain unlike all other which leave via the inferior part.
Brief course of the trochlear nerve
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.
Is trochlear a mixed nerve?
No it is a somatic nerve only.
What does the trochlear nerve innervate?
SO4 - superior oblique muscle of the eye
Does the trochlear nerve innervate ipsilaterally or contralaterally?
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.
What would injury to the trochlear nucleus cause?
Contralateral symptom
What would injury to the nerve after crossing after the midbrain result in?
Ipsilateral symptom
What does damage to the trochlear nerve cause and how do you test this?
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.
Where does the abducens nerve arise from?
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.
What does the abducens nerve supply?
The lateral rectus muscle
What is the facial colliculus?
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.
Describe the brief course of the abducens nerve
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.
Where is the abducens nerve vulnerable to damage?
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
What happens when the abducens nerve is damaged?
The patient is unable to look outwards. Causes squinting on looking outwards.
