Visual System Flashcards
when are tears produced by the lacrimal gland?
basal, reflex and emotional responses
what is the control of a tear reflex ie a bug lands on eye
afferent - CN V1 opthalmic trigeminal
efferent - parasympathetic
acetylcholine neurotransmitter
what is the route of a tear from lacrimal gland to nasal cavity?
lacrimal gland - drain through two puncta
flow through superior and inferior canaliculi
gather in tear sac
exit through tear duct into nasal cavity
what is the tear film?
most superficial layer of eye - maintains a smooth cornea-air surface
supplies oxygen to cornea
what is the role of the tear film?
supplies oxygen to cornea
bactericidal
removes debris and dust
what are the 3 layers of the tear film?
lipid outer layer, aqueous layer, mucinous layer on cornea surface
what is the conjunctiva?
thin, transparent tissue covering outer surface of the eye
lines eyelids and visible part of the eye
what are the 3 tissue layers of the eyeball?
sclera
choroid
retina
what is the sclera?
the white of the eye
tough, opaque outer coat
high water content
what is the cornea?
transparent, dome shaped window covering front of eye
low water content
provides 2/3 of focusing power
what are the layers of the cornea?
epithelium bowmans membrane stroma descemets membrane endothelium - pumps fluid out of cornea to prevent oedema
what is the uvea?
choroid, iris and ciliary body
most vascular part of the eye
what is the choroid?
lies between retina and sclera
layers of blood vessels
what is the iris?
muscular to dilate and constrict the pupil size
controls light levels inside the eye
what is the structure of the lens?
outer acellular capsule
regular inner elongated cell fibres
what is the function of the lens?
transparency
refractive power and 1/3 of focusing power
accomodation
elasticity
what is the retina?
thin layer of tissue lining the inner eye
captures light rays and send to optic nerve
what is the optic disk?
visible portion of the optic nerve
what is the blind spot and why does it occur?
where the optic nerve meets the retina
there are no light sensitive cells
where does the optic nerve join the eye?
back of the eye, near the macula
what is the macula?
the centre of the retina highly sensitive for detailed central vision
what is the centre of the macula called?
fovea
what is the role of the fovea?
appreciates detail and focuses central vision
what is central vision?
detail day/colour vision
reading, facial recognition
by fovea of macula
what is the fovea?
most sensitive part of retina - centre of macula
highest concentration of cones, low concentration of rods
what is peripheral vision?
shape, movement, navigation and night vision
how is central vision tested?
visual acuity assessment
how is peripheral vision tested?
visual field assessment
what happens with loss of central vision?
poor visual acuity e.g reading
what happens with loss of peripheral vision?
inability to navigate environment, patient may need white stick even with perfect visual acuity
what is the overall structure of the retina?
outer - photoreceptors (1st order neuron)
middle - bipolar cells (2nd order)
inner - retinal ganglion cells (3rd order)
what is the function of retinal photoreceptors?
detection of light
what is the function of bipolar cells of the retina?
local signalling processing to improve contrast sensitivity and regulate sensitivity
transmits from photoreceptors to retinal ganglion cells
what is the function of retinal ganglion cells of the retina?
transmission of signal from eye to brain
what are the classes of photoreceptors?
rods
cones
what are rods?
long outer segment
contains photo-sensitive pigment
slow response to light and responsible for NIGHT vision
which is more abundant, rods or cones?
rods 120mil to 6 mil cones
what are cones?
shorter outer segment
less sensitive to light as rods but faster response
day, fine vision and colour
which cones detect blue wavelengths?
s cones
which cones detect green light?
m cones
which cones detect red light?
L cones
what is deuteranomoly?
not being able to perceive the colour red
most common colourblindness
what is achromatopsia?
full colour blindness
what is the ishihara test used for?
to detect colourblindness
what is the index of refraction (n) and how is it calculated?
ratio of the speed of light before and after hitting a boundary
speed of light in vaccum (air)/speed of light in new medium
what happens when light meets a boundary?
some is reflected, some refracts through the boundary into the new medium
what are the two types of lenses and how do they differ?
convex - takes light rays at brings them to a point
concave - takes light rays and spreads them outwards
what is emmetropia?
basically normal, clear vision
adequate correlation between axial length and refractive power
parallel light rays fall on the retina - no accomodation
what is ammetropia?
mismatch between axial length and refractive power
parallel rays dont fall on retina
what are the types of ametropia?
myopia
hyperopia
astigmatism
presbyopia
what is myopia?
near-sightedness (far objects are blurry, close objects in focus)
focal point of light rays falls short of the retina
what are the causes of myopia?
excessive long globe (axial myopia) more common
excessive refractive power (refractive myopia)
what are the symptoms of myopia?
blurred distance vision
squint in an attempt to improve uncorrected visual acuity
headaches
how is myopia treated?
negative/diverging (concave) lenses
contact lenses
removal of lens
what is hyperopia?
far sightedness (close objects blurry, distance fine) focal point of light rays falls behind retina
what are the causes of hyperopia?
excessive short globe (axial hyperopia)
insufficient refractive power (refractive hyperopia)
what are the symptoms of hyperopia?
ranges from inability to read fine print to clear near vision but is suddenly and intermittently blurry
bad reading vision
eyepain, headache, burning eyes, blepharoconjunctivitis
amblyopia if uncorrected
how is hyperopia treated?
positive/converging lens (convex)
contact lenses
positive lens + cataract extraction
intraocular lens surgery
what is astigmatism?
parallel rays come to focus in two focal lines
hereditary condition
what are the causes of astigmatism?
non-spherical refraction media (cornea)
what are the symptoms of astigmatism?
headache, eye pain
blurred vision
distortion of vision
head tilting and turning
how is astigmatism treated?
cylindrical lenses, surgery
irregular astigmatism treated by rigid cylindrical lenses, surgery
what is the near response triad?
an adaption for near vision
pupils constrict - pupillary miosis (sphincter pupillae) to increase depth of field
convergence (medial recti from both eyes) to align eyes towards near object
accomodation (circular ciliary muscle) to increase lens’ refractive power
what is presbyopia?
naturally occuring loss of accomodation/focus of near objects
distant vision intact, onset from 40+
how is presbyopia treated?
reading glasses (convex) bifocal glasses trifocal glasses progressive power glasses contact lenses spectacle lenses - monofocal. spherical, cylindrical, multifocal
what are the drawbacks of contact lenses?
careful daily cleaning and disinfection needed
expense
risk of complications
what are the complications of contact lenses?
infectious keratitis
giant papillary conjunctivitis
corneal vascularisation
severe chronic conjunctivitis
what are intraocular lenses?
replacement of cataract crystalline lens
best optical correction for aphakia (no lens)
avoids significant magnification/distortion by spectacle lenses
what is the process of LASIK surgery?
initial cutting of corneal flap flipping of corneal flap photorefractive treatment (laser) corneal stroma reshaped post laser corneal flap put back in place
give an example of an intraocular lens and its use
Staar intra-collamer lens (ICL)
correction of myopia and astigmatism
describe how clear lens extraction happens
natural lens is removed using a phaco tip
artificial intraocular lens inserted
what retinal cells make up the optic nerve?
retinal ganglion cells
what is the role of the lateral geniculate nucleus?
ganglion nerve fibres synapse here to form the optic radiation (4th order neurons)
where do the retinal ganglion cell fibres decussate?
53% decussate in the optic chiasm (nasal retina)
how do lesions anterior to the optic chiasm present?
affect visual field in one eye only
how do lesions posterior to the optic chiasm present?
affect visual field in both eyes
which fibres decussate at the optic chiasm?
nasal retina aka temporal visual field (due to image flipping)
which fibres don’t cross at the optic chiasm?
temporal retina aka nasal visual field (due to image flipping)
how does a lesion at the optic chiasm present?
temporal field deficit in both eyes - bitemporal hemianopia
damages crossed ganglion fibres from nasal retina in both eyes
how does a right sided lesion posterior to the optic chiasm present?
left homonymous hemianopia in both eyes
how does a left sided lesion posterior to the optic chiasm present?
right homonymous hemianopia in both eyes
what are the causes of bitemporal hemianopias?
enlargement of pituitary gland tumour
what are the causes of homonymous hemianopia?
stroke
what is the cause of homonymous hemianopia with macular sparing?
damage to primary visual cortex ie stroke (appears contralaterally to hemisphere damaged)
what blood vessel supplies the part of the visual cortex which is responsible for representing the macula?
posterior cerebral arteries (dual blood supply from both sides arteries)
therefore macula is likely to be spared in strokes
how does pupillary constriction occur?
parasympathetic stimulation causes circular muscles to contract, radial muscles relax
describe the direct light pupillary reflex
optic nerve receives light signals from retina
synapses in pretectal nucleus and then Edinger-Westphal in midbrain
oculomotor nerve carries impulse through motor branches to the ciliary ganglion, short ciliary nerves constrict the sphincter pupillae
what is the direct pupillary reflex?
constriction of pupil in light stimulated eye
how does the consensual light reflex occur?
the optic nerve activates the efferent pathway in both eyes therefore
optic nerve receives light signals in one eye
synapses in pretectal nucleus and then Edinger-Westphal in midbrain for both eyes
oculomotor nerve carries impulse through motor branches to the ciliary ganglion, short ciliary nerves constrict the sphincter pupillae of both eyes
what is a right afferent defect? aka optic nerve damage
no pupil constriction in both eyes when right eye stimulated with light
normal pupillary response in both eyes when left eye stimulated
how do right afferent defects occur?
damage to optic nerve
what is a right efferent defect? aka oculomotor nerve defect
no right pupil constriction when either left or right eye stimulated
left eye still constricts when right eye is stimulated
how do right efferent defects occur?
damage to oculomotor nerve
what is a swinging torch test used for?
to test relative afferent pupillary defects
what are relative afferent pupillary defects?
partial pupillary response still present when damaged eye stimulated
so semi-damage to optic nerve
how to test for deuteranomaly
ishihara test
how would you test the back of someones eyes
fundoscopy
what are the 3 types of retinal cells
photoreceptors
bipolar cells
retinal ganglion cells
what are all of the types of eye movements
duction - one eye movement
version - both eyes move in same direction
vergence - bot eyes move in opposite directions
convergence - simultaneous adduction of both eyes
what are the 2 speeds of eye movement
saccade - short fast bursts (reflex/predictive)
smooth pursuit - sustained slow movement (tracking(
what is the optokinetic nystagmus reflex
when following a moving object, and it moves out of the field of vision, eye will snap back to the original viewing position
smooth pursuit then eyes reset to the middle using the fast reflex saccade
what are the movements of the eye muscles
SR - eye up IR - eye down LR - eye temporally MR - eye nasally IO - diagonally up and out SO - diagonally down and out
appearance of 3rd nerve palsy
affected eye down and out (unopposed SO and LR) droopy eyelid (loss of levator palpebrae superioris) dilated pupil
appearance of 6th nerve palsy
affected eye unable to adduct
on relaxation is deviated inwards
double vision on gazing to the side of affected eye
appearance of 4th nerve palsy
when looking towards unaffected eye, affected eye moves upwards (IO takes over from LR)
what are the pupillary changes in the near response triad mediated by
sphincter pupillae contracts (circular smooth muscle) stimulated by parasympathetic nerves travelling with the oculomotor nerve
what are the lens changes in the near response triad mediated by
ciliary muscle attached to the lens via suspensory ligament contracts
reduces tension on the suspensory ligaments, so the lens relaxes and becomes thicker, causing greater refractive power
how does pupillary dilation occur
sympathetic stimulation to radial muscles causes them to contract
increases sensitivity to light
describe the visual pathway
photoreceptors depolarise -synapse to bipolar cells depolarise - synapse to retinal ganglion fibres and travel out of eye as optic nerve
reach optic chiasm where nasal retinal fibres cross over, temporal dont, to form optic tracts
then synapse at lateral geniculate nucleus in thalamus
then travel via optic radiation to the primary visual cortex in occipital lobe
where does the right visual field travel to in the brain
left hemisphere primary visual cortex
includes nasal retina from right eye and temporal retina from left eye
how does the swinging torch test work
distinguishes relative and complete afferent defects
shine light in one eye, both pupils constrict, then quickly swing to other eye and hold it there
both pupils should constrict - if not, theres a relative afferent pupillary defect in the second eye tested
complete would be tested by eyes in isolation - not quickly changing lights
