Neuro - Visual system pt. 3 + 4 Flashcards
Where does the visual pathway start and end?
transmits signal form eye to visual cortex
What landmarks does the visual pathway go through?
eye → through optic nerve → optic chiasm → optic tract → lateral geniculate nucleus → optic radiation → primary visual cortex
What is the optic nerve?
ganglion nerve fibres
What is the optic chiasm?
half of the nerve fibres cross here (53% of fibres)
What is the optic tract?
ganglion nerve fibres exit the optic chiasm as the optic tract
What is the lateral geniculate nucleus?
→ located in thalamus
→ where the ganglion nerve fibres synapse
What is the optic radiation?
→ 4th order neurone
→ travel in pairs, taking information from homonymous quadrants of vision
What is the primary visual cortex or striate cortes?
→ centre within the occipital lobe
→ processes visual information
→ Extra-Striate Cortex for further higher visual processing
What are the different neurones that make up the visual pathway from the retina to the lateral geniculate nucleus?
→ 1st order neurones
→ 2nd order neurones
→ 3rd order neurones
What are the 1st order neurones?
rod + cone retinal photoreceptors
What are the 2nd order neurones?
retinal bipolar cells
What are the 3rd order neurones?
→ Optic Nerve
→ Partial Decussation at Optic Chiasma – 53% of ganglion fibres cross the midline
→ Optic Tract
→ Destination = Lateral Geniculate Nucleus (LGN) in Thalamus
What do lesions anterior to the optic chiasma affect?
visual field in one eye only
What do lesions posterior to the optic chiasma affect?
visual field in both eyes
Where do crossed fibres at the optic chiasma originate form?
→ predominantly originate from the nasal retina
→ responsible for the temporal half of the visual field in each eye
Where do uncrossed fibres at the optic chiasma originate form?
→ predominantly originate from the temporal retina
→ responsible for the nasal half of the visual field in each eye
What visual defect would a left optic nerve compression cause?
unilateral field loss in left eye
What visual defect would a optic chasmal compression from a pituitary tumour cause?
bitemporal hemianopia
What visual defect would a left cerebrovascular event or stroke cause?
homonymous hemianopia (contralateral to lesion)
What is quadrant-anopia?
→ loss in one of the quadrants of vision
→ usually homonymous
What can cause quadrant-anopia?
lesions to the optic radiations in the temporal or parietal lobes
What is macular sparing?
homonymous hemianopia with a small inner circle of clear vision (sparing of the macula of central vision)
What can cause macular sparing?
→ lesions to optic radiation in the occipital lobe?
→ stroke
→ damage to the primary visual cortex
What can typically cause a bitemporal hemianopia?
enlargement of the pituitary gland due to a tumour
What can typically cause a homonymous hemianopia?
stroke or cerebrovascular event
Why is the macula spared sometimes with homonymous hemianopia?
→ area representing the macula is well protected
→ receives dual blood supply from both right + left posterior cerebral arteries
What is the main pupillary function?
regulates light input to the eye
What does the pupil do in light?
→ pupillary constriction
→ decreases spherical aberrations and glare
→ increases depth of field
→ reduces bleaching of photo-pigments
→ mediated by parasymapthetic nerve (within CN III)
→ causes circular muscle to contract
What does the pupil do in the dark?
→ pupillary dilation
→ increase slight sensitivity in the dark by allowing more light into the eye
→ mediated by the sympathetic nerve
→ causes radial muscles to contract
What cells participate in the pupillary reflex pathway?
small sub-section of retinal ganglion cells
How do cells form the afferent pathway of the pupillary reflex?
→ Rod and Cone Photoreceptors synapsing on Bipolar Cells synapsing on Retinal Ganglion Cells
→ Pupil-specific ganglion cells at posterior third of optic tract before entering the Lateral Geniculate Nucleus
→ Afferent (incoming) pathway from each eye synapses on Edinger-Westphal Nuclei on both sides in the dorsal side of the brainstem
How do cells form the efferent pathway of the pupillary reflex?
→ Edinger-Westphal Nucleus → Oculomotor Nerve Efferent → Synapses at Ciliary ganglion → Short Posterior Ciliary Nerve → Pupillary Sphincter of the iris
Why do both eyes respond to the pupillary reflex?
only one eye needs to be stimulated, as the afferent pathway in just one eye can stimulate the efferent pathway in both eyes
What is a direct light reflex?
constriction of the pupil of the light-stimulated eye
What is the consensual light reflex?
constriction of the pupil of the other eye (that hasn’t been stimulated)
What could cause damage to the afferent pathways of the pupillary reflex?
damage to the optic nerve
What would be the effect of a right afferent pathway defect?
→ right eye light stim = no pupil constriction in both eyes
→ left eye light stim = normal pupil constriction in both eyes
What could cause damage to the efferent pathways of the pupillary reflex?
damage to the 3rd nerve
What would be the effect of a right efferent pathway defect?
any eye light stim = only left pupil constriction, no right pupil constriction
What is the swinging torch test?
→ best way to detect RAPD (relative afferent pupillary defect)
→ elicited by swinging torch from left eye to right eye
→ the side which doesn’t cause pupil constriction = where the defect is
How is eye movement facilitated?
→ by 6 extra ocular muscles
→ innervated by 3 cranial nerves (III, IV, VI)
What is duction?
eye movement of one eye
What is version?
simultaneous movement of both eyes in the same direction
What is vergence?
simultaneous movement of both eyes in opposite directions
What is dextroversion?
looking to the right with both eyes
What is levoversion?
looking to the left with both eyes
What is convergence?
simultaneous adduction (inward) movement in both eyes when viewing a near object
What is saccade?
→ short fast burst of eye movement
→ up to 900 degrees a second
What are the different types of saccade?
→ reflexive saccade to external stimuli
→ scanning saccade
→ predictive saccade to track objects
→ memory-guided saccade
What is smooth pursuit?
→ sustained slow movement
→ up to 60 degrees per second
→ driven by motion of a moving target across the retina
What are the 6 extra-ocular muscles?
Superior rectus Inferior rectus Lateral rectus Medial rectus Inferior Oblique Superior Oblique
What are the main purposes of the extra-ocular muscles?
→ attach eyeball to orbit
→ straight + rotatory movement
What are the 4 straight extra-ocular muscles?
Superior rectus
Inferior rectus
Lateral rectus
Medial rectus
Which muscles come out from a cone from the back of the orbit? Which ones don’t?
→ all of them come out from the back, except Inferior Oblique
→ inferior oblique comes in nasally
What is the purpose of the superior rectus muscle?
→ Attached to the eye at 12 o’clock
→ Moves the eye up
What is the purpose of the inferior rectus muscle?
→ attached to the eye at 6 o’clock
→ moves the eye down
What is the purpose of the lateral rectus muscle?
→ Also called external rectus
→ Attaches on the temporal side of the eye
→ Moves the eye toward the outside of the head (toward the temple)
What is the purpose of the medial rectus muscle?
→ Also called internal rectus
→ Attached on the nasal side of the eye
→ Moves the eye toward the middle of the head (toward the nose)
What is the purpose of the superior oblique muscle?
→ Attached high on the temporal side of the eye
→ Passes under the Superior Rectus
→ Moves the eye in a diagonal pattern down and out
→ Travels through the trochlea
What is the purpose of the inferior oblique muscle?
→ Attached low on the nasal side of the eye
→ Passes over Inferior Rectus
→ Moves the eye in a diagonal pattern - up and out
What extra-ocular muscles does the Superior branch of CN III (oculomotor) innervate?
→ superior rectus
→ levator palpebrae superioris
What does the levator palpebrae superioris do?
raises the eyelid
What extra-ocular muscles does the inferior branch of CN III (oculomotor) innervate?
→ inferior rectus
→ medial rectus
→ inferior oblique
What extra-ocular muscles does CN IV (trochlear) innervate?
superior oblique
What extra-ocular muscles does CN VI (abducens) innervate?
lateral rectus
What muscle controls abduction of the eye?
lateral rectus
What muscle controls adduction of the eye?
medial rectus
What muscle elevates + abducts the eye?
superior rectus
What muscle depresses + abducts the eye?
inferior rectus
What muscle elevates + adducts the eye?
inferior oblique
What muscle depresses + adducts the eye?
superior oblique
What eye movement does supraduction test?
elevation of one eye
What eye movement does supra version test?
elevation of both eyes
What eye movement does infraduction test?
depression of one eye
What eye movement does infraversion test?
depression of both eyes
What eye movement does right dextroversion test?
right abduction + left adduction
What eye movement does left dextroversion test?
right adduction + left abduction
What is torsion?
rotation of eye around he anterior-posterior axis of the eye
What is third nerve palsy?
→ weakness of the third cranial nerve
→ muscle innervated by third nerve don’t work properly
What are the features of third nerve palsy?
→ Affected eye down and out
→ Droopy eyelid (loss of elevator palpebrae superioris)
→ Unopposed superior oblique innervated by fourth nerve (down)
→ Unopposed lateral rectus action innervated by sixth nerve (out)
What is sixth nerve palsy? What are the features?
→ Affected eye unable to abduct and deviates inwards
→ Double vision worsen on gazing to the side of the affected eye
What is Nystagmus?
oscillatory eye movement
What is Optokinetic Nystagmus?
smooth pursuit + fast phase reset saccade
What is optokinetic nystagmus reflex?
useful in testing visual acuity in pre-verbal children by observing the presence of nystagmus movement in response to moving grating patterns of various spatial frequencies
What does presence of the Optokinetic Nystagmus reflex signify?
signifies that the subject has sufficient visual acuity to perceive the grating pattern
