Cranial Nerves - III, IV & VI Flashcards
Which cranial nerves supply the extra-ocular muscles?
III - oculomotor nerve:
- medial rectus
- superior rectus
- inferior rectus
- inferior oblique
IV - trochlear nerve:
- superior oblique
VI - abducens nerve:
- lateral rectus
- All the extraocular muscles are supplied by GSE fibres*
Identify the cranial nerves
What is unique about the origin of the trochlear nerve?
it is the only cranial nerve to arise from the dorsal aspect of the brainstem
it arises at the level of the midbrain
Where is the trochlear nucleus located?
Describe the path the trochlear nerve takes after it arises from this nucleus
- the trochlear nucleus is located close to the central canal at the level of the midbrain and overlaps with the periaqueductal gray
- the trochlear nerve emerges from the nucleus and crosses over to the contralateral side as it exits the midbrain
- it loops around the periaqueductal gray
- it supplies the CONTRALATERAL superior oblique muscle
What is the result of damage to the trochlear nerve?
What symptoms will the patient complain of?
paralysis of the contralateral superior oblique muscle
- this results in outwards rotation of the eye as the action of the inferior oblique is unopposed
- patients complain of vertical diplopia and torsional diplopia
- double vision vertically and images being tilted/rotated
What fibre types are present in the oculomotor nerve?
What nuclei are they associated with?
- parasympathetic GVE fibres are associated with the Edinger-Westphal nucleus
- GSE fibres are associated with the motor nucleus of III
- these are both present at the level of the midbrain as III arises at this level
Where is the Edinger-Westphal nucleus located?
Describe the passage that GVE fibres take from this nucleus to reach the muscles that they supply
- it is located just anterior to the trochlear nucleus at the level of the midbrain
- fibres leave via the interpeduncular fossa and travel to the ciliary ganglion, where they synapse
- second order neurones travel to sphincter pupillae and ciliaris
- parasympathetic input causes contraction of these muscles
What fibres arise from the motor nucleus of III and what do they supply?
Where is this nucleus located?
- it is located lateral to the Edinger-Westphal nucleus at the level of the midbrain
- it gives rise to GVE fibres that pass out of the midbrain to supply the extraocular muscles
- these motor neurones do not synapse on their way to the muscle
Where does the abducens nerve arise from?
Where does it leave the brainstem and what does it go to supply?
- arises from the abducens nucleus at the level of the pons
- the abducens nucleus is situated dorsally and medially within the pons
- VI exits at the level of the pontomedullary junction and passes just lateral to the basilar artery
- it contains GSE fibres that supply the lateral rectus muscle
What happens if there is a lesion to the abducens nerve?
paralysis of the lateral rectus muscle
- the patient is unable to abduct their eye
- this leads to medial strabismus - the eye is orientated medially as the medial rectus is unopposed
What is the role of the lateral geniculate nucleus of the thalamus?
What fibre type does it receive?
- it is a visual relay centre
- SSA fibres travel in the optic nerve from the retina to reach the LGN
- LGN sends axons through the optic radiations to reach the primary visual cortex in the occipital lobe
- this allows for interpretation of visual information by the brain
Describe the afferent and efferent limb of the pupillary light reflex
Afferent limb - optic nerve:
- SSA fibres in II travel to the pre-tectal nucleus of the midbrain (within the superior colliculus)
- interneurones project from the pre-tectal nucleus to the Edinger-Westphal nuclei on BOTH sides of the brain
Efferent limb - oculomotor nerve:
- GVE fibres travel from the Edinger-Westphal nucleus to the ciliary ganglion, where they synapse
- post-ganglionic fibres travel from the ciliary ganglion to reach sphincter pupillae
As BOTH Edinger-Westphal nuclei are stimulated by interneurones, there is constriction of the pupil in BOTH eyes
What would be the effect on the pupillary light reflex if there was damage to II?
- no light information would be sensed and sent to the pre-tectal nucleus
- neither pupils would constrict
What would be the effect on the pupillary light reflex if there was damage to the oculomotor nerve?
- light information is still being sensed and sent to the pre-tectal nucleus via II
- interneurones are still relaying this information to both Edinger-Westphal nuclei
- information cannot travel down the damaged oculomotor nerve, but can still travel down the intact one
There is no direct response, but the consensual response will still occur
Why does the accomodation reflex need to occur?
- when light passes into the eye, it is concentrated onto the fovea to produce a clear image
- the light is refracted by the lens to focus it onto the fovea
- if an object is moved closer to the face, the light passes into the eye at a different angle and so needs to be refracted differently in order to be focussed onto the fovea
What are the 3 things that need to occur during accommodation and which muscles are involved in acheiving them?
Convergence of the eye (GSE):
- as an object gets closer, the eyes need to be drawn medially
- this is achieved through the actions of medial rectus
Increase in refractive power of the lens (GVE):
- contraction of the ciliary muscles makes the lens become more rounded in shape
Reduction in diameter of the pupil (GVE):
- achieved by constriction of sphincter pupillae
All 3 of these actions are brought about by GVE or GSE fibres of III
Describe the neurones involved in the accomodation reflex
Afferent neurone - optic nerve:
- as an object moves closer to the face, this is sensed by SSA fibres travellling in II to the LGN of the thalamus
- optic nerve synapses at the LGN and a branch is sent to the visual cortex via the optic radiations
- from the visual cortex, branch is sent to the pre-tectal nucleus of the midbrain
Interneurones:
- interneurones are sent from the pre-tectal nucleus to BOTH the Edinger-Westphal nucleus and the motor nucleus of III
- this is happening in both eyes, so inputs are arriving at both the ipsilateral and contralateral nuclei from the interneurones at the same time
Efferent neurone - oculomotor nerve:
- GSE fibres travel from the motor nucleus of III to the medial rectus muscle via III
- GVE fibres travel from the Edinger-Westphal nucleus to the ciliary muscles and sphincter pupillae via III
What is meant by convergence?
- when the 2 eyes are both moving closer to the midline
- the left eye is moving to the right and the right eye is moving to the left (different directions)
What is meant by divergence?
When does this usually occur?
- occurs when both of the eyes are moving away from the midline
- following convergence (looking at a close object), the eyes move back to their central position
- this is an example of divergence as both eyes are moving away from the midline
What is meant by vestibulo-ocular movements?
- when the head is moved, the eyes move in the opposite direction to continue to focus on a fixed point
- there is involvement of the vestibular system as the eyes are compensating for movements of the head
What is meant by the near reflex triad that occurs during vergence eye movements?
the near reflex triad describes 3 changes that occur when the gaze is transfered from a distant to a near object
1. accommodation:
- ciliary muscles contract to make the lens become more rounded
2. constriction of pupil:
- sphincter pupillae contracts to reduce the diameter of the pupil
3. ocular convergence:
- contraction of the medial rectus muscles leads to adduction of the eyeballs (movement towards the midline)
What are the 2 main “areas” involved in the pathway for horizontal saccades?
Frontal eye field:
- small area of the cortex located just anterior to the precentral gyrus
- activation of the frontal eye field leads to movement of the eye to the contralateral side
- it initiates eye movements at the level of the extraocular muscles
- involved in voluntary eye movements
Superior colliculus:
- involved in reflexive movements of the eye
What is the PPRF?
What is it important for?
paramedian pontine reticular formation
- it is a diffuse network of neurones that spans the length of the brainstem (not clustered into nuclei)
- at the level of the pons are either side of the midline, the diffuse network of neurones is called the PPRF
- it is important for horizontal gaze coordination (saccades)
Which nuclei are involved if we wanted to shift our gaze to the right?
- the left frontal eye field (BA 8) initiates this instruction
- a neurone travels from BA 8 to reach the right hand side of the brainstem
- it will descend to reach both the PPRF and NPH (nucleus prepositus hypoglossi)
- NPH located just above the hypoglossal nucleus in the medulla
- neurones from the PPRF and NPH both project to the abducens nucleus
What are the projections from the abducens nucleus if we want to shift the gaze to the right?
Lateral rectus:
- a neurone projects directly from the abducens nucleus to the ipsialteral lateral rectus muscle
- a neurone travels from the right abducens nucleus to the right lateral rectus, causing the right eye to look to the right
Medial longitudinal fasciculus (MLF):
- The MLF is a bundle of fibres that projects from the abducens nucleus to the contralateral oculomotor nucleus
- stimulation of the contralateral oculomotor nucleus leads to stimulation of the contralateral medial rectus
- contraction of the left medial rectus causes the left eye to look to the right
What would happen if someone with a lesion in the left MLF was asked to shift their gaze to the right and left sides?
- eyes appear normal when looking straight ahead
When asked to look to the left:
- both eyes are able to look to the left as both the left lateral rectus and right medial rectus are being stimulated
- left abducens nerve and right MLF are intact
When asked to look to the right:
- the right eye is able to abduct, but the left eye remains in the centre
- right lateral rectus is still being stimulated by CN VI
- left medial rectus is not being stinualted due to the lesion in the MLF (no stimulation of left oculomotor nucleus)
What is the role of the vestibular nuclei?
Where are these located and what projects to them?
- the vestibular system in the inner ear consists of 3 semi-circular ducts (as well as the utricle and saccular structures)
- from the semi-circular ducts, there are neurones which form the vesitbular division of the vestibulocochlear nerve (CN VIII)
- these axons enter the brainstem at the cerebellopontine angle and project to the 4 vestibular nuclei in the floor of the IVth ventricle in the caudal pons and rostral medulla
The role of the vestibular nuclei is to regulate balance and eye movements
What are the roles of the 3 semi-circular canals of the vestibular system?
Which receptors are stimulated when the head moves in a horizontal plane?
- Lateral - detects head movement in a horizontal plane
- Vertical - detects head movement in a vertical plane
- One that detects head movement in a coronal plane (head to ear)
- when the head is moved in the horizontal plane, fluid inside the lateral semi-circular canal moves and stimulates the hair cells
- hair cells are the receptors located within the semicircular ducts
- hair cell receptors connect to the vestibular division of CN VIII
Describe the horizontal vestibulo-ocular reflex if the head is moved to the right (and the eyes need to move to the left)
- hair cells in right semicircular canal begin to fire and the vestibular division of VIII is stimulated
- VIII travels to the vestibular nuclei on the ipsilateral (right) side
- fibres from the vestibular nuclei cross the midline to travel to the contralateral PPRF (left)
- fibres from the PPRF travel to the abducens nucleus on the same side (left)
From the abducens nucleus:
- projections to the lateral rectus on the same side (left) via the abducens nerve cause the left eye to look to the left
- projections to the right MLF (comes from left abducens nucleus but immediately crosses the midline) to the right oculomotor nucleus
- fibres travel via III to the right medial rectus to cause the right eye to look to the left
