Visual system Flashcards
what is refraction
as light goes from one medium to another, the velocity changes → the ratio of the two speeds is the new medium’s index of refraction (this can be used to identify the material)
Light bends when it reaches a new medium: the path changes
When light reaches a new medium, some of it reflects at the boundary and some refracts through
what is the equation for the refractive index
speed of light in material 1
/ speed of light in material 2
What is the function of convex lenses and name some examples of convex lenses
Convex lens: rays converge to a focal point (camera and normal eye)
Film = retina, diaphragm =iris, aperture = pupil, lens = lens, black paint = choroid
What is the function of concave lenses and name some examples of concave lenses
Concave lens: rays diverge and spread out (can be traced back to a focal point)
what is emmetropia
normal eyes
Emmetropia (normal sight) = adequate correlation between axial length and refractive power
parallel light rays fall on the retina (no accommodation- adjustment of the lens to keep object in focus on the retina, as the distance from the eye varies)
what is ametropia
Ametropia = mismatch between axial length and refractive power
Parallel light rays don’t fall on the retina (no accommodation).
4 types
what are the 4 types of ametropia
myopia ( near sightedness)
hyperopia ( farsightedness)
astigmatism
presbyopia
what is myopia ? wat are the 2 types
causes? symptoms?
treatments?
Myopia (nearsightedness): parallel rays converge at a focal point anterior to the retina Etiology unclear (genetic factor?)
Axial myopia: excessive long globe
Refractive myopia: excessive refractive power
Symptoms: blurred distance vision, squint to improve uncorrected visual acuity at a distance, headache
Treat: glasses (diverging lens), contact lens, surgery - remove lens to reduce refractive power
what is hyperopia
? wat are the 2 types
causes? symptoms?
treatments?
Hyperopia (farsightedness): parallel rays converge posterior to the retina
Unclear etiology - inherited?
Axial (short globe) or refractive (insufficient refractive power)
amblyopia : uncorrected hyperopia >5D
Symptoms: visual acuity at near tends to blur early - more noticeable when tired, low light or fine print
Nature of blue is varied from inability to read fine print to near vision is clear (but suddenly and intermittently blur)
Also (asthenopic): eye pain, headache (frontal), burning eyes, blepharoconjunctivitis
Treat: positive/converging lens - glasses, contacts, cataract extraction + positive lens, intraocular lens
what is ambylopia
amblyopia : uncorrected hyperopia >5D
what is astigmatism
? wat are the 2 types
causes? symptoms?
treatments?
Astigmatism: parallel rays come to focus in 2 focal lines instead of single focal point
Hereditary
Refractive media is not spherical: refract differently along one meridian instead of the perpendicular meridian
Dual focal points: punctiform object is represented as 2 sharply defined lines
Symptoms: headache, eye pain, blurred vision, distortion, head tilting and turning
Treatment: regular astigmatism (cylinder lenses with or without spherical lengs - convex or concave), Sx
rregular astigmatism: rigid cylinder lenses, surgery
what is the near response triad
Pupillary miosis (sphincter pupillae) increase depth of field
Convergence (medial recti from both eyes) to align both eyes towards a near object
Accommodation (circular ciliary muscle) to increase refractive power of lens for near vision
what is presbyopia
? wat are the 2 types
causes? symptoms?
treatments?
of accommodation (focus for near objects)
Onset from age 40 years
Distant vision intact
Comes from increased rigidity of lens and ciliary muscles
Treat: reading glasses (convex lenses to increase refractive power), bifocal glasses, trifocal glasses, progressive power glasses
Spectacle lens: monofocal (spherical), cylindrical, multifocal
Contact lens: higher quality of optical image and less influence on the size of retinal image than spectacle lenses
Indication: cosmetic, athletic activities, occupational, irregular corneal astigmatism, high anisometropia, corneal disease
what are the types of sceptical lenses
Spectacle lens: monofocal (spherical), cylindrical, multifocal
what are the types of contact lenses and why are they used
Contact lens: higher quality of optical image and less influence on the size of retinal image than spectacle lenses
Indication: cosmetic, athletic activities, occupational, irregular corneal astigmatism, high anisometropia, corneal disease
what are the pros , cons and complications of contacts
Contact lens: higher quality of optical image and less influence on the size of retinal image than spectacle lenses
Indication: cosmetic, athletic activities, occupational, irregular corneal astigmatism, high anisometropia, corneal disease
Contact lenses
Disadvantages: careful daily cleaning and disinfection, expense
Complication: infectious keratitis, giant papillary conjunctivitis, cornea; vascularisation, severe chronic conjunctivitis
when are intraocular lenses used , pros and cons?
Intraocular lenses
Replacement of cataract crystalline lens
Give best optical correction for aphakia ( no natural lens ) , avoid significant magnification and distortion caused by spectacle lenses
Disadvantages: monofocal =–> can see far away but will need glasses for close up
what are 3 types of surgery
Keratorefractive surgery ( aka laser surger)
Intraocular surgery
Clear lens extraction + intraocular lens
what is Keratorefractive surgery ? method? types?
Keratorefractive surgery ( aka laser surger) : RK, AK, PRK, LASIK, ICR, thermokeratoplasty
Preoperative eye Visible thin corneal flap prelaser treatment Initial cutting of corneal flap Cutting of corneal flap Flipping of corneal flap Photorefractive treatment (laser) Corneal stroma reshaped post laser Corneal flap back in position Treatment completed
what is the anatomy of teh external eye
Limbus = border between sclera and cornea
Conjunctiva = Thin, transparent tissue covering outer surface of eye - begins at outer edge of cornea, covers visible eye and lines inner eyelids
nourished by tiny blood vessels that normally can’t be seen unless in conjunctivitis
what can be seen in conjunctiva
Conjunctiva = Thin, transparent tissue covering outer surface of eye - begins at outer edge of cornea, covers visible eye and lines inner eyelids
nourished by tiny blood vessels that normally can’t be seen unless in conjunctivitis
what is the coat of the eye
Coat
Antero-posterior diameter of the eye - 24mm in adults. The eye resides within the anatomical space known as the orbit.
Enclosed by bony walls, laterally, medially, superiorly and inferiorly.
what are teh 3 layers of the eye
3 layers ( see below)
sclera - fibrous, protects and maintains shape (hard and opaque with high water content)
White of the eye
Uvea: Vascular coat of eyeball between sclera and retina. Made up of 3 pars
Iris - dilates and constricts pupil size
Round opening in centres of eye = pupil
Ciliary body
choroid- pigmented and highly vascular (circulation, shielding unwanted scattered light)
Comprise of layers of blood vessels that nourish the back of the eye
Disease of one portion can affect others (not always to same degree though)
retina
what are the 3 parts of the uvea an what are thier functions
Iris - dilates and constricts pupil size
Round opening in centres of eye = pupil
Ciliary body
choroid- pigmented and highly vascular (circulation, shielding unwanted scattered light)
Comprise of layers of blood vessels that nourish the back of the eye
where is teh lacrimal gland found and what does it do ?
lacrimal gland resides within the orbit, latero-superiorly to the globe: secretes tears at a constant rate even in the absence of irritation or stimulation (basal tears)
what 3 things can cause tear secretion
Tears can be formed by :
Basal
Reflex
Emotion
what is the tear film and what does it do
Tear film
Tear film drains through the two puncta, tiny openings on the upper and lower medial lid margin.
Puncta form opening of the superior and inferior canaliculi which converge and drain into the tear sac then tear duct and nasal cavity.
Structure: see below
what is the function of the tear film
Mot superficial part of eye
Maintains smooth cornea-air surface, provides oxygen supply to cornea (avascular normally), removal of debris, bacteriacide
what are the 3 layers of the tear film and what are thier functions
Superficial lipid layer: reduces tear film evaporation – produced by a row of Meibomian glands along the lid margin.
Aqueous (water) tear film (tear gland): delivers O2 and nutrients to surrounding tissues and factors to protect from bacteria
Mucinous layer corneal surface: maintains surface wetting.
The mucin molecules act by binding water molecules to the hydrophobic corneal epithelial cell surface.
what is the afferent pathway to the eye innervated by
Cornea is innervated by sensory fibres of cranial nerve V1 (Ophthalmic trigeminal) which relays signal to CNS
what controls the efferent pathway to the eye
Mediated by the parasympathetic nervous system (acetylcholine neurotransmitter), innervating the lacrimal gland
what is the cornea
Clear window: powerful refracting surface, ⅔ of eye focussing power
Transparent, low water content, covers front of eye, avascular
what are the 5 layers of the cornea
5 layers Epithelium Bowman’s membrane Stroma: regularity → transparency Descement’s membrane Endothelium: pumps fluid out of corneal, preventing oedema
what happens if you hydrate the cornea
it turns white
what is the lens
transparent biconvex structure suspended posterior to the iris
Funcion
focuses light rays onto the retina, ⅓ of eye focussing power (higher refractive index than aqueous fluid and vitreous
Accommodation
Elasticity
( it needs to be transparent and have a high refractive power for function
what is the structure of the lens
Outer acellular capsule
Regular inner elongated cell fibres - transparency (can be lost with age: CATARACTS)
what is the retina and what does it do
Thin layer of tissue responsible for capturing light rays which are sent to brain for processing via the optic nerve
what is the function of the optic nerve and what does it form in the eye
Transmits electrical impulses from retina to brain, connecting near macula
Visible portion: optic disc
Creates a Blind spot : Where the optic nerve meets the retina there are no light sensitive cells .
The corresponding landmark for the blind spot is the optic disk
what is the blind spot
Creates a Blind spot : Where the optic nerve meets the retina there are no light sensitive cells .
The corresponding landmark for the blind spot is the optic disk
what is the macula and what is it’s function
small and highly sensitive retinal region (roughly in centre pf retina , temporal to optic nerve) responsible for detailed central vision.
Macula allows us to:
appreciate detail
perform tasks that require central vision (reading)
contains the highest concentration of photosensitive cone cells (+ low concentration of rods).
what is the fovea and what is its function
Fovea is the centre of the macula
Most sensitive part of retina
Highest concentration of cones - to perceive in detail
low rod concentration - aka less sensitive to light which is why stars brighter out of corner of eye
what is central vision
detailed day and colour vision
the region of highest visual acuity that is concerned with the centre field of vision is the fovea (most cones)
Reading, facial recognition
how to assess central vision
Assessed by visual acuity assessment
what happens if you lose central vision
Loss of foveal vision, leads to poor visual acuity
what is peripheral vision
Shape, movement, navigation vision, ‘Night vision’
Assessed by visual field assessment
Extensive loss of visual field leads to inability to navigate in environment (white stick despite perfect visual acuity)
how to assess peripheral vision
Assessed by visual field assessment
what happens if you lose peripheral vision
Extensive loss of visual field leads to inability to navigate in environment (white stick despite perfect visual acuity).
what are the 3 layers of the retina
outer
middle
inner
what does the outer layer of the retina do/ contain
Photoreceptors (1st order neuron), detection of light
what does the middle layer of the retina contain / do
Bipolar cells (2nd order neuron), local signal processing to improve contrast sensitivity, regulate sensitivity
what is the inner layer and what does it do / contain
Retinal ganglion cells (3rd order neuron), transmission of signal from eye to brain
what are the 2 types of photoreceptors
rod
cone
what is the function of rods
more rod cells than cone cells
Longer outer segment with photo-sensitive pigment
100x more sensitive to light than cones
Slow response to light
Scotopic/night vision
Peripheral and night vision, more photoreceptors, more pigment, higher spatial and temporal (time) summation, recognises motion
Highest conc in periphery 20-40 degrees away from the fovia
what is the function of cone cells
Less sensitive to light, but faster response
Photopic vision: day light fine vision, colour vision
Central and day vision, recognises colour and detail
High conc in fovia
what is the frequency spectrum
The eye captures different coloured lights though different receptors
S cones: blue (<400 to 530, peak at 420nm)
M cones: green (400 to >600, peak at 540)
L cones: red (400 to >600, peak at 570)
Rods: not colour sensitive (night vision, spatial recognition, 400 to 600 peak at 500)
Cones used to see colour
what are the 2 types of colour blindness
deuteranomaly/ daltonism
achromatopsia / full colour blindness
what is deuteranomaly/ daltonism
Deuteranomaly/Daltonism: don’t perceive red but are not completely colour blind
caused by the shift of M-cone sensitivity towards that of the L-cone causing red-green confusion (Green appears red)
Most frequent
what is achromatopsia
full colour blindness ( only effect small pat of population )
what is the diagnosis of colour blindness
ishihara test
what is the function of visual pathway and what are teh landmarks
Transmits signal from eye to the visual cortex
Visual pathway landmarks
eye
optic nerve (ganglion nerve fibres),
optic chiasm (half of the nerve fibres cross here),
optic tract (ganglion nerve fibres exit as optic tract),
lateral geniculate nucleus (ganglion nerve fibres synapse at lateral geniculate nucleus),
optic radiation (4th order neuron)
primary visual cortex or striate cortes (within the occipital lobe)
what is the pathway of the visual pathway
Optic ganglion nerve fibres → Lateral geniculate nerve fibres → Optic radiations → Primary visual cortex
what are the 3 neurons of teh optic pathway and
First order neurons - road and cone retinal photoreceptors
Second order neurons - retinal bipolar cells
Third order neurons - retinal ganglion cells
what 3 things does the optic nerve form
Optic Nerve (CN II)
Partial Decussation at Optic Chiasma – 53% of ganglion fibres cross the midline
Optic Tract
Destinations
Lateral Geniculate Nucleus (LGN) in Thalamus – to relay visual information to Visual Cortex
what is the optic chiasm
Optic chiasma : important landmark in visual pathway
53% of ribres cross at OC
Contralateral fibres originate from nasal retina and carry modalities of peripheral vision (temporal visual field)
Uncrossed ipsilateral fibres are responsible for nasal visual field (originate from temporal retina)
what does a legion to the anterior optic chiasm disrupt
Lesions anterior to optic chiasma affect visual field in one eye only
what does a lesion at the optic chiasma effect
Damages crossed ganglion fibres from nasal retina in both eyes
Temporal Field Deficit in Both Eyes – Bitemporal Hemianopia
what does lesion to the posterior optic chiasm effect
Right sided lesion – Left Homonymous Hemianopia in Both Eyes
Left sided lesion – Right Homonymous Hemianopia in Both Eyes
what are 7 disorders of teh visual pathway
monocular blindness bitemporal hemianopia right nasal hemianopia homonymous hemianopia quadrantanopia macular sparing
what are the causes of bi temporal hemianopia
Bitemporal hemianopia: caused by pit gland tumour COMMON
what are the causes of homonymous hemianopia
Homonymous hemianopia: caused by stroke (cerebrovascular accident)
what are teh causes of macular sparing
(contralateral) homonymous hemianopia with macular sparing: caused often by stroke, damage to primary visual cortex (area representing macula receives dual blood supply from PCA from both sides)
what is the pupillary reflex
Pupillary function: regulates light input to the eye like a camera aperture
what happens to the pupil in light
In light: pupil constriction
Decreases spherical aberrations and glare
Increases depth of field - sew near response triad from previous lecture
Reduces bleaching of photo-pigments
Pupillary constriction mediated by parasympathetic nerve (within CNIII)
what happens to the pupil in no light
In dark: pupil dilation
Increases light sensitivity in the dark by allowing more light into the eye
Pupillary dilation mediated by sympathetic nerve
what are the 2 pathways of teh pupillary reflex
afferent
efferent
describe the afferent pathway of teh pupillary light reflex
Rod and cone photoreceptors synapse with bipolar cells which then synapse on retinal ganglion cells.
Pupil-specific ganglion cells exist at posterior third of the optic tract before entering the lateral geniculate nucleus synapsing upon the dorsal brainstem.
These pupillary retinal ganglion cells form the pupillary reflex afferent pathway.
Afferent pathway from each eye synapse on Edinger-Westphal Nuclei on both sides of the dorsal brainstem
Describe the efferent pathway of teh pupillary light reflex
Edinger-Westphal nucleus → Oculomotor nerve (Parasympathetic nerve)
Synapses at the ciliary ganglion upon the short posterior ciliary nerve pupillary sphincter.
what is the direct light reflex
Direct light reflex - constriction of pupil of light stimulated eye
what is teh consensual light reflex
Consensual light reflex - constriction of pupil of other (fellow) eye
what are the 4 types of defect
Afferent ( right and unilateral )
Efferent ( right and unilateral )
what is a right afferent defect
Damage to the optic nerve
No pupil constriction in both eyes when the right eye is stimulated with light (No consensual or direct reflect)
Normal pupil constriction in both eyes when the left eye is stimulated with light.
There is a consensual reflex in the right eye.
what happens in a unilateral afferent defect
Difference response pending on which eye is stimulated
what happens in a right efferent defect
Pupil constriction
Damage to right 3rd nerve
No right pupil constriction whether right or left eye is stimulated with light
Left pupil constricts whether right or left eye is stimulated with light
what happens in a unilateral efferent defect
Same unequal response between left and right eye irrespective which eye is stimulated
what is the test used to test light pupillary reflex
Swinging torch test: relative afferent pupillary defect
what is the method of teh swinging touch test
partial pupillary response is still present when the damaged eye is stimulated.
Elicited by the swinging torch test – alternating stimulation of right and left eye with light.
Both pupils constrict when light swings to left undamaged eye.
Both pupils paradoxically dilate when light swings to the right damaged eye, as a result of reduced drive for pupillary constriction in both eyes.
why do we need eye movement and what are the 2 different types
Voluntary or involuntary of movement of eyes
Necessary for acquiring and tracking visual stimuli
Facilitated by the six extraocular muscles innervated by the three cranial nerves (III, IV and VI)
what nerves supply the eye
Facilitated by the six extraocular muscles innervated by the three cranial nerves (III, IV and VI)
what is duction
version
vergence
convergence
Duction – Eye Movement in One Eye
Version – Simultaneous movement of both eyes in the same direction
Vergence – Simultaneous movement of both eyes in the opposite direction
Convergence – Simultaneous adduction (inward) movement in both eyes when viewing a near object
what are teh 2 speeds of eye movement and what are thier function
Saccade – short fast burst, up to 900°/sec Reflexive saccade to external stimuli Scanning saccade Predictive saccade to track objects Memory-guided saccade
Smooth Pursuit – sustain slow movement
Slow movement – up to 60°/s
Driven by motion of a moving target across the retina.
what are teh 6 muscles of the eye
Six muscles Attach eyeball to orbit Straight and rotary movement Four straight muscles Superior rectus Inferior rectus Lateral rectus Medial rectus
what is the function of teh superior rectus
Moves the eye up.
what is the function of teh inferior rectus
Moves the eye down.
what is the function of the lateral rectus
Also called the 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 function of the medial rectus
Also called the 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 function of the superior oblique
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 function of the inferior oblique
Attached low on the nasal side of the eye.
Passes over the Inferior Rectus.
Moves the eye in a diagonal pattern - up and out.
what does CN III innervate
Third Cranial Nerve (oculomotor) Superior Branch Superior Rectus – elevates eye levator palpebrae superioris - raises eyelid (not shown) Inferior Branch Inferior Rectus – depresses eye Medial Rectus – adducts eye Inferior Oblique – elevates eye Parasympathetic Nerve – constricts pupil
what does CN IV innervate
Fourth Cranial Nerve (trochlear)
Superior Oblique – depresses eye
what does cranial nerve VI innervate
Sixth Cranial Nerve (abducens)
Lateral Rectus – abducts eye
how do you test for eye muscles
Extraocular Muscle Testing – Isolate muscle to be tested by maximizing its action and minimizing the action of other muscles
Abduction – Lateral Rectus
Adduction – Medial Rectus
Elevated and Abducted – Superior Rectus
Depressed and Abducted – Inferior Rectus
Elevated and Adducted – Inferior Oblique
Depressed and Adducted – Superior Oblique
what are teh directions of eye movement
Up (Elevation) Supraduction – one eye Supraversion – both eyes Down (Depression) Infraduction – one eye Infraversion – both eyes Right – Dextroversion Right Abduction Left Adduction Left – Levoversion Right Adduction Left Abduction Torsion – rotation of eye around the anterior-posterior axis of the eye
what are symptoms of CNIII 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 are symptoms of cN VI palsy
Affected eye unable to abduct and deviates inwards
Double vision worsen on gazing to the side of the affected eye
what is the optokinetic nystagmus reflex ?
what does it test
Nystagmus – Oscillatory eye movement
Optokinetic Nystagmus = Smooth Pursuit + Fast Phase Reset Saccade
Optokinetic Nystagmus Reflex is 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
Presence of Optokinetic Nystagmus in response to moving grating signifies that the subject has sufficient visual acuity to perceive the grating pattern
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