visual system Flashcards
what is the lacrimal system responsible for
tears - basal, reflex and emotional (crying)
how are tears produced
produced by lacrimal gland
drain through two puncta, openings on medial lid margin
flow through superior and inferior canaliculi
gather in tear sac- exit through tear duct into nasal cavity
what is the lacrimal systems afferents
cranial nerve, V1 - Opthalmic trigeminal
what are the efferents in lacrimal system
parasympathetic
what are the neurotransmitters involved in lacrimal system
acetylcholine
what is the role of the tear film
mains smooth cornea
oxygen supply to cornea - normal cornea has no blood supply
removal of debris
bactericide
what are the 3 layers in tear film
superficial lipid layer - reduce tear film evaporation(produced by meibomian glands
aqueous water layer
mucinous layer - maintains surface wetting
what is conjunctiva
thin transparent tissue that covers outer surface of eye
- begins at outer edge of cornea, covers visible part of eye and lines inside of eyelids
- nourished by tiny blood vessels
what are the 3 layers/coats of eye
scalera - hard and opaque
choroid - pigmented and vascular
retina - neurosensory tissue
what is the scalera
aka ‘white of the eye’
tough, opaque tissue that serves as eyes protective outer coat
high water content
what is the cornea
transparent, dome shaped window covering front of eye
low water content
powerful refracting surface - providing 2/3 of eyes focusing power
what are the 5 layers of the cornea
- epithelium
- bowmans membrane
- stroma - regularity contributes towards transparency
- descemets membrane
- endothelium - pumps fluid out cornea and prevents corneal oedema
what is the uvea
vascular coat of eyeball and lies between sclera and retina
composed 3 parts: iris, ciliary body and choroid
- intimately connected and disease of one part also affects other portions though not necessarily to same degree
what is the choroid
lies between retina and scalera
composed layers of blood vessels that nourish back of eye
what is the iris
controls light levels inside eye similar to aperture on camera
round opening- pupil
embedded with tiny muscles that dilate and constrict
what is structure of the lens
outer acellular capsule
regular inner elongated cell fibres -
may lose transparency with age - cataract (clouding)
what is the function of the lens
transparency regular structure refractive power 1/3 of eye focusing power - higher refractive index than aqueous fluid and vitreous accommodation elasticity
what is the retina
very thin layer of tissue that lines inner part of eye
- responsible for capturing light rays that enter the eye
light impulses are sent to brain for processing via optic nerve
what is optic nerve
transmit electrical impulses from retina to brain
connects to back of eye near macula
visible portion is called optic disc
what is the macula
located roughly in centre of retina, temporal to optic nerve
small and highly sensitive part of retina responsible for detailed central vision
what is the fovea
centre of macula
macula allows us to appreciate detail and perform tasks that require central vision e.g reading
why is fovea most sensitive part of retina
has highest concentration of cones, but low conc of rods
- only fovea has conc of cones to perceive in detail
what is central vision
detail day vision and colour vision
reading, facial recognition
how is central vision assessed
by visual acuity assessment
loss of foveal vision = poor visual acuity
how is peripheral vision assessed
visual field assessment
extensive loss of visual field = unable to navigate in environment
- patient may need white stick even with perfect visual acuity
what is peripheral vision
shape, movement and night vision
and navigation vision
describe retinal structure
outer layer = photoreceptors(1st order neurons), detection of light
middle layer = bipolar cells (2nd order), local signal processing to improve contrast sensitivity, regulate sensitivity
inner layer = retinal ganglion cells (3rd order)
transmission of signal to brain
what are the 2 types of photoreceptors
rods and cones
what are rods
photoreceptors with longer outer segment with photo-sensitive pigment 100x more sensitive to light than cones slow response to light responsible for night vision are 120 mill rods
what are cones
photoreceptors that are less sensitive to light - but have faster response
responsible for daylight, fine vision and colour vision
6 mill cones
what is rod/scotopic vision
peripheral and night vision
- more photoreceptors, more pigment, higher spatial and temporal(time) summation
- recognises motion
what is cone/photopic vision
central and day vision
recognises colour and detail
where can you find highest conc of rod receptors in retina
20-40 degrees away from fovea
what are the different receptors that capture different colours
s cones - blue
m cones - green
l cones - red
what is deuteranomaly/ daltonism
most frequent form of colour blindness
- dont perceive colur red
what is full colour blindness called
achromatopsia
what is refraction
light travelling from one medium to another,
- velocity changes
- path changes
what is the equation for index of refraction
speed of light in a vacuum/ speed of light in a medium
what exactly is light doing when reaching new medium
some light reflects off boundary and some light refracts through boundary
- angle of incidence = angle of reflection
- angle of incidence > angle of refraction depending on direction of light
what is a converging/convex lens
takes light rays and brings them to a point
what is a diverging/concave lens
takes light rays and spreads them outwards
what is emmetropia
adequate correlation between axial length and refractive power
parallel light rays fall on retina
- no accommodation
what is ametropia
mismatch between axial length and refractive power
parallel light rays dont fall on retina
what is myopia
nearsightedness
parallel rays converge at focal point anterior to retina
etiology - not clear, genetic factor?
causes:
1. axial myopia = excessive long globe(more common)
2. refractive myopia = excessive refractive power
what are the symptoms of myopia
blurred distance vision
squint to improve uncorrected visual acuity when gazing into distance
headache
treatment for myopia
correction with diverging lens
correction with contact lens
correction with removal of lens to reduce refractive power of eye
what is hyperopia
parallel rays converge at focal point posterior to retina
etiology - not clear, inherited
what are the causes of hyperopia
- axial hyperopia = excessive short globe, more common
2. refractive hyperopia = insufficient refractive power
what are the symptoms of hyperopia
farsightedness
symptoms - visual acuity at near ends tend to blur relatively early
- could be inability to read fine print or vision is clear then intermittently blurry
when is blurred vision more noticeable
if person is tired,
printing is weak or inadequate light
what are some asthenopic symptoms
eyepain
headache in frontal region
burning sensation in eye
blepharoconjunctivitis
what is blepharitis
inflammtion along edges of eyelid
what is amblyopia
uncorrected hyperopia
>5d
what is treatment for hyperopia
correction with converging/positive lens
correction with positive lens and cataract extraction
correction with contact lens
correction with intraocular lens
what is astigmatism
parallel rays come to focus in 2 focal lines rather than a single focal point
what is the etiology for astigmatism
hereditary
what are the causes of astigmatism
refractive media is not spherical
- refract differently along one meridian than along meridian perpendicular to it
- 2 focal points
what are the symptoms of astigmatism
asthenopic symptoms - headache, eyepain
blurred vision
distortion of vision
head tilting and turning
what is the treatment for regular astigmatism
cylinder lenses with or without spherical lenses
surgery
what is the treatment for irregular astigmatism
rigid cylinder lenses
surgery
what is the near response triad
pupillary miosis (sphincter pupillae) to increase depth of field
convergence (medial recti from both eyes) to align both eyes towards near object
accommodation (circular ciliary muscle) to increase refractive power of lens for near vision
what is presbyopia
naturally occurring loss of accommodation
onset from 40 yrs age
distant vision intact
corrected by reading glasses
what is accommodation
focus for near objects
what is the treatment for presbyopia
convex lenses in near vision
- reading glasses
- bifocal glasses
- trifocal glasses
- progressive power glasses
what spectacle lenses are used for presbyopia correction
monofocal lenses, spherical lenses, cylindrical lenses
multifocal lenses
what are the advantages of spectacle lenses over contact lenses
higher quality of optical image and less influence on size of retinal image than spectacle lenses
what are the indications for contact lens use
cosmetic athletic activities occupational irregular corneal astigmatism high anisometropia corneal disease
what are the disadv of contact lenses
careful daily cleaning and disinfection
expensive
what are the complications of contact lens use
infectious keratitis
giant papillary conjunctivitis
corneal vascularisation
severe chronic conjunctivitis
what is apakia
not having lens in your eye
what are intraocular lenses
replacement for cataract crystalline lens
gives best optical correction for aphakia, avoid significant magnification and distortion caused by spectacle lenses
what are some examples of keratorefractive surgery
rk, ak, prk, lasik, icr, thermokeratoplasty
what are some examples of intraocular surgery
clear lens extraction ( with/without iol)
phakic iol
describe surgical correction of lens
inital cutting of corneal flap flipping of corneal flap photorefractive treatment - laser corneal stroma reshaped post laser corneal flap back in position
what is an icl
Staar intra-collamer lens inserted into eye for correction of myopia and astigmatism
what is the visual pathway in retina
1st order neurones - rod and cone retinal photoreceptors
2nd order neurones - retinal bipolar cells
3rd order neurones - retinal ganglion cells
- opitic nerve
- partial decussation at optic chiasm(53% of ganglion fibres cross midline)
optic tract
destination = lateral geniculate nucleus in thalamus - to relay visual info to visual cortex
describe optic chiasma
lesions anterior to chiasm affect visual field in one eye only
lesions posterior to chiasm afects visual field in both eyes
where do the crossed fibres in visual pathway originate from
nasal retina, responsible for temporal visual field
what do uncrossed fibres in visual pathway originate from
from temporal field and are responsible for nasal visual field
what can lesions at optic chiasm cause
damages crossed ganglion fibres from nasal retina in both eyes
temporal field deficit in both eyes - bitemporal heia..
what is bitemporal hemianopia typically caused by
enlargement of pituitary gland tumour
what is homonymous hemianopia typically caused by
stroke - cerebrovascular accident
area representing macula receives dual blood supply from posterior cerebral arteries from both sides
why do pupils constrict in light
decreases spherical aberrations and glare
increases depth of fiels
reduces bleaching of photopigments
pupillary constriction mediated by parasympathetic nerve
why do pupils dilate in dark
increases light sensitivity in dark by allowing more light into eye
pupillary dilation mediated by sympathetic nerve
what is the afferent pathway for pupillary reflex
rod and cone receptors synapsing on bipolar cells synapse on retinal ganglion cells
pupil specific ganglion cells exit at posterior third of optic tract before entering lateral geniculate nucleus
afferent pathway from each eye synapses on edinger-westpal nuclei on both sides in brainstem
what is the efferent pathway for pupillary reflex
edinger-westpal nucleus -> oculomotor nerve efferent -> synapses at ciliary ganglion -> short posterior ciliary nerve -> pupillary sphincter
what is the consensual light reflex
constriction of pupil of other eye
what is the neurological basis of consensual light reflex
afferent pathway on either side alone stimulates efferents on both sides
what would you see with right afferent defect in pupillary reflex
e.g damage to optic nerve
pupil constriction in both eyes when right eye is stimulated with light
normal constriction when left eye stimulated with light
what would you expect to see with right efferent damage in pupillary reflex
e.g damage to right 3rd nerve
no right pupil constriction with left or right eye stimulated with light
left pupil constricts whether right / left is stimulated with light
what would you expect to see with unilateral afferent defect in pupillary reflex
different response depending on which eye stimulated
what would you expect to see with unilateral efferent defect in pupillary reflex
same unequal response between left and right eye irrespective of which eye stimulated
what is the relative afferent pupillary defect
partial pupillary response is still present when damaged eye is stimulated
- both pupils constrict when light swings to left undamaged side
- both pupils paradoxically dilate when light swings to right damaged side
what are eye movements facilitated by
six extraocular muscles innervated by 3 cranial nerves - 3,4 and 6
what is duction
eye movement in one eye
what is version
simultaneous movement of both eyes in opposite direction
what is vergence
simultaneous movement of both eyes in opposite direction
what is convergence
simultaneous adduction movement in both eyes when viewing near object
what is saccade
short fast burst upto 900’/sec
what are some examples of saccade
reflexive saccade to external stimuli
scanning saccade
predictive saccade to track objects
memory guided saccade
what is smooth pursuit in context of speed of eye movement
sustain slow movement
upto 60’/s
driven by motion of moving target across retina
what are the 6 extraocular muscles
superior rectus inferior rectus lateral rectus medial rectus superior oblique inferior oblique
what is the superior rectus
attached to eye at 12 oclock
moves eye up
what is the inferior rectus
attached to eye at 6 oclock
moves eye down
what is the lateral rectus
also called external rectus
attaches on temporal side of eye
moves eye towards outside of head
what is the medial rectus
aka internal rectus
attached on nasal side of eye
moves eye towards middle of head
what is the superior oblique
attached on temporal side of eye
passes under superior rectus
moves eye down and out diagonally
travels through the trochlea
what is the inferior oblique
attached low on nasal side of eye
passes over inferior rectus
moves eye up and out in diagonal pattern
what is role of 3rd cranial nerve - oculomotor nerve in eye
1. superior branch: superior rectus - elevates eye levator palpebrae superiosis - raises eyelid 2. inferior branch: inferior rectus - depresses eye medial rectus - adducts eye inferior oblique - elevates eye parasympathetic nerve - constricts pupil
which cranial nerve is responsible for depressing eye via superior oblique
4th cranial nerve - trochlear
what nerve os responsible for abducting eye via lateral rectus
6th - abducens
what is 3rd nerve palsy
affected eye moves down and out
droopy eyelids - loss of elevator palpebrae superiosis
unopposed superior oblique innervated by 4th nerve,down
unopposed lateral rectus action innervated by 6th nerve, down
what is 6th nerve palsy
affected eye unable to abduct and deviate inwards
double vision worsens on gazing to side of affected eye
what is nystagmus
oscillating eye movement
what is optokinetic nystagmus
smooth pursuit and fast phase reset saccade
what is optokinetic nystagmus reflex
useful in testing visual acuity in preverbal children by observing presence of nystagmus movement in response to moving grating patterns of various spatial frequencies
what does presence of optokinetic nystagmus in response to moving grating signify
subject has sufficient visual acuity to perceive grating pattern
