Eye movements and pupillary reflexes Flashcards
What three cranial nerves control the eye movements?
Oculomotor (III), Trochlear (IV) and Abducens (VI)
What level is the oculomotor nerve?
Superior colliculus (midbrain)
What are the oculomotor nerve nuclei?
Main oculomotor nucleus (supplies extrinsic eye muscles)
Accessory oculomotor nucleus/EWN (gives rise to preganglionic parasympathetic fibres).
Where do projections from the EWN go?
Ciliary ganglion then to ciliary muscle and pupillary constrictor
Where do projections from the main oculomotor nucleus go?
Enters superior orbital fissure in tendinous ring from which recti arise, and divides into superior branch (supplies superior rectus and levator palpebrae) and inferior branch (supplies medial/inferior rectus and inferior oblique).
What muscles does the oculomotor nerve supply?
Medial rectus, superior rectus, inferior rectus, levator palpebrae, inferior oblique
What is the pupillary light reflex?
Reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye.
What is the afferent for the pupillary light reflex?
Photosensitive retinal ganglion cells, which convey information via the optic nerve.
Where does the optic nerve terminate for the pupillary light reflex?
Some axons of the optic nerve connect to the pretectal nucleus of the upper midbrain
Where do axons from the pretectal nuclei synapse to (pupillary light reflex)?
From the neuronal cell bodies in some of the pretectal nuclei, axons synapse on (connect to) neurons in the Edinger-Westphal nucleus.
What is the efferent for the pupillary light reflex?
Edinger-Westphal nuclei: Parasympathetic preganglionic neuronal axons in the oculomotor nerve synapse on ciliary ganglion neurons.
Ciliary ganglia: Short post-ganglionic ciliary nerves leave the ciliary ganglion to innervate the Iris sphincter muscle of the iris.
Is the pupillary light reflex consensual? What does this mean?
Yes, shining light in one eye will cause constriction of both eyes
What makes the pupillary light reflex consensual?
The pretectal nucleus innervates both EWN
What does an optic nerve lesion do to the pupillary light reflex?
Direct reflex in lesioned side is lost. Consensual reflex is also lost.
BUT on non lesioned side
Direct and consensual reflex is intact.
What does a CNIII lesion do to pupillary light reflex?
Direct lost but consensual intact on lesioned side
On non lesioned side, direct intact but consensual lost
What are Argyll Robertson pupils (AR pupils)?
Argyll Robertson pupils (AR pupils) are bilateral small pupils that reduce in size on a near object (i.e., they accommodate), but do not constrict when exposed to bright light (i.e., they do not react to light).
What causes Argyll Robertson pupils (AR pupils)?
Loss of pretectal light input to the EWN
Oculomotor nerve palsy?
‘down and out’ position of the affected eye (as lateral rectus and superior oblique still have tone). Also ptosis and mydriasis (pupil dilation)
Trochlear nerve level
Nucleus of origin in midbrain at level of inferior colliculus - dorsal emergence
What does the trochlea nerve innervate?
Superior oblique
What does the superior oblique cause?
Depression/intorsion of eye (i.e. moving eye out and down.)
Trochlea nerve palsy?
Vertical diplopia (overlapping images), most severe on looking downward and medially (because depression action of superior oblique is most pronounced when eye is abducted.
Difficulty looking down
Abducens nerve level
Lies just lateral to the midline in the floor of the 4th ventricle. Nucleus in caudal pons
What does the abducens nerve innervate?
Lateral rectus
Abducens nerve palsy?
Loss of lateral eye movement, diplopia on looking to side, convergent squint.
Superior rectus role
Main movement is elevation. Also contributes to adduction and medial rotation
Inferior rectus role
Main movement is depression. Also contributes to adduction and lateral rotation
Lateral rectus role
Abducts the eyeball (lateral movement).
Medial rectus role
Adducts the eyeball (medial movement).
Superior oblique role
Depresses, abducts and medially rotates the eyeball.
Inferior oblique role
Elevates, abducts and laterally rotates the eyeball.
Levator palpebrae role
Superior eyelid movement
What are saccades?
Saccades are rapid, ballistic movements of the eyes that abruptly change the point of fixation.
When do saccades occur and why?
Fixation eye movements when fixated at one point. Constant visual stimulus can make the photoreceptors or ganglion cells become unresponsive so eye movement constantly changes the stimuli.
Scanning a visual scene.
What are smooth pursuit?
Slower tracking movements of the eyes designed to keep a moving stimulus on the fovea.
Where are smooth pursuit movement coordinated by?
Frontal pursuit area of the frontal lobe
What is involved in the correction of velocity during pursuit?
Cerebellum
Alternating slow and fast movement of the eyes in response to such stimuli is called…
Optokinetic nystagmus.
What is vergence?
Aligns the fovea of each eye with targets located at different distances from the observer
Maintains binocular vision.
What two types of vergence movements are there?
Convergence of divergence
Object closer by, the eyes rotate towards each other (convergence), while for an object farther away they rotate away from each other (divergence).
What eye movement allows the eyes to see depth?
Vergence
What is the vestibulo-ocular reflex?
Stabilize the eyes relative to the external world, thus compensating for head movements.
What is the sensory information provided to the VOR?
Vestibular information from the vestibular system (semi-circular canals)
What directions does the VOR work in?
All directions
What are the sensory and afferent signals in the ipsilateral VOR?
Hair cells excites vestibular ganglion, left horizontal canal as head turns left
Vestibular neuron projects in vestibulocochlear nerve (VIII) to lateral vestibular nucleus.
CONTRALATERAL
What are the sensory and afferent signals in the contralateral VOR ?
Hair cell excites vestibular neuron
Vestibular neuron projects to the medial vestibular nucleus
What follows Ipsilateral VOR lateral vestibular nucleus?
Second order neuron to ipsilateral occulomotor nucleus
Third order neuron projects in the occulomotor nerve (III) to left medial rectus
Left eye moves medially (right)
What follows contralateral VOR medial vestibular nucleus (excitatory)?
Second order excitatory neuron projects to contralateral abducens nucleus
Third order neuron projects in abducens nerve (VI) to right lateral rectus (right eye moves laterally, towards the right)
Third order neuron travels in medial longitudinal fasciculus to synapse in the occulomotor nucleus and from there information flows to the left medial rectus.
What follows contralateral VOR medial vestibular nucleus (inhibitory)?
Second order inhibitory neuron projects to the ipsilateral abducens nucleus
Third order neurons project to the left lateral recuts
Third order neuron also project to the contralateral occulomotor nucleus which then inhibits the right medial rectus
What is the accommodation reflex?
Reflex action of the eye, in response to focusing on a near object, then looking at a distant object (and vice versa), comprising coordinated changes in vergence, lens shape (accommodation) and pupil size.
Accommodation reflex sensory and afferent?
Light information from the retina is carried in the optic nerve, chiasm and tract, LGN and visual cortex
What area of cortex interprets accommodation?
Peristriate area 19
Accommodation reflex efferents?
Cortex sends signals via the Edinger-Westphal nucleus and the oculomotor cranial nerve nucleus
Innervates ciliary muscle, sphincter pupillae muscle
and the medial rectus
If convergence what is the activity of ciliary, pupillae and medial rectus?
All contract
If divergence what is the activity of ciliary, pupillae and medial rectus?
All relax
Effects of lesion of sympathetic supply to the eye?
Horners syndrome, anhydrosis, ptosis, miosis,
What does the MLF do?
Medial longitudinal fasiculus linking superior colliculus, oculomotor nuclei and vestibular nuclei with gaze control centres
Where are horizontal saccades generated?
Paramedian pontine reticular formation
What nerve nucleus receives the saccade information from the PPRF?
Abducens
How does the abducens nucleus produce saccades?
Motor neurons which innervate ipsilateral lateral rectus
Interneurons cross midline and ascend in MLF to motor neurons for contralateral medial rectus (in the occulomotor nucleus)
To make a saccade to a peripheral visual target what must occur?
Activity at retinotopic locationin the superior colliculus (SC) must occur.
Keeps the eyes fixating the current location
Via omnipause neurons which inhibit burst neurons in the PPRF
Opsoclonus myoclonus
Rapid involuntary both horizontal and vertical, unpredictable conjugate fast eye movements without intersaccadic interval
Where are vertical saccades generated?
Mesencephalic reticular formation
