visual systems 1 Flashcards
what is the basic anatomy of the eye?
lateral: upper eyelid lower eyelid lateral canthus palpebral fissure
medial: pupil iris sclera medial canthus caruncle limbus (border between cornea and sclera)
how does the lacrimal system work?
three types of tears:
basal - constant level of tears
reflex - increased production in response to irritation
emotional
afferents: come from cornea via cranial nerve V (opthalmic branch of the trigeminal efferents: parasympathetic neurotransmitter is acetylcholine
tears are produced by the lacrimal gland (sits above the eye)
they drain through the two puncta
they flow through the inferior and superior canaliculi
they then gater in the tear sac
and exit the sac through the tear duct and into the nasal cavity
what is tear film?
the healthy cornea is constantly covered by the tear film, a thin layer of fluid
it maintains a smooth cornea-air surface
importance:
oxygen supply to cornea as it has no blood vessels
removal of debris during blinking
bactericide
what are the three layers of the tear film?
superficial lipid layer:
reduces tear film evaporation
produced by a row of meibomian glands along the lid margin
aqueous:
water
main bulk of fluid
delivers oxygen and bactericidal factors
mucinous layer:
on corneal surface
maintains surface wetting
mucin molecules act by binding water molecules to the hydrophobic corneal epithelial cell surface
what is conjunctiva?
thin transparent tissue that covers the outer surface of the eye
begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids
nourished by tiny blood vessels that are nearly invisible to the naked eye
unless there is a condition causing inflammation - conjunctivitis
what comprises of the coating of the eyeball?
(diameter of eye is 24mm)
3 layers (outside to in): sclera - hard an opaque (the white) protects and maintains the shape
choroid - pigmented and vascular
shields unwanted scattered light
retina - neurosenspry tissue
what is the sclera?
commonly known as the white of the eye
forms the tough opaque tissue that serves as the protective outer coat
has a high water content
what is the cornea?
(when the sclera seamlessly gets to the front of the eye)
the transparent window covering the front of the eye
low water content
powerful refracting surface
provides 2/3 of the eyes focusing power
5 layers (outside to inside): epithelium
bowmanns membrane
stroma (made of collagen fibres, its regularity contributes to transparency)
descemets membrane
endothelium (pumps fluid out of cornea and prevents corneal oedema) (also passage of glucose that nourishes the cornea)
it doesnt have blood vessels as it is clear:
oxygen - comes from air
glucose - comes through the endothelium
what happens if you hydrate the cornea?
it becomes white and cloudy
what is the uvea?
vascular coat of the eyeball
lies between the sclera and the retina in the choroid
comprised of three parts:
iris
ciliary body
choroid
they are intimately connected, so a disease of one part effects all the other parts, but in differing amounts
what is the choroid?
lies between the sclera and retina
composed of layers of blood vessels that nourish the back of the eye
what is the iris?
controls light levels inside the eye
round opening with pupil in the centre
embedded with tiny muscles that dilate and constrict the pupil (ciliary and radial muscles)
it is the foremost part of the uvea
what is the lense
structure:
outer acellular capsule
regular inner elongated cell fibres, these give transparency
transparency may go with age (cateracts)
function: transparency regular structure refractive power (1/3 of the eyes focussing power) accommodation (short and long vision) elasticity
what is the retina?
very thin layer of tissue that lines the inner surface of the eye
responsible of capturing light rays that enter the eye
light impulses are sent via the optic nerve to the brain for processing
what is the optic nerve
transmits electrical impulses from the retina to the brain
connects to the back of the eye near the macula
visible portion is called the optic dic
there is a blind spot where the optic nerve meets the retina
what is the macula
located roughly in the centre of the retina
a small highly sensitive part of the retina responsible for detailed central vision
the fovea is the very centre of the macula
the macula allows us to appreciate detail and perform tasks that require central vision such as reading
high concentration of cones (dim, but colour) low rods (detect light at a lower level but only black and white)
but only the fovea has a high enough concentration of cones to perceive in detail
what is central vision?
detail day vision, colour vision
FOVEA has the highest concentration of cones
reading, facial recognition
assessed by visual acuity assessment
loss of foveal vision - poor visual acuity
what is peripheral vision?
shape, movement, night vision
essential for navigation vision
assessed by 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 the structure of the retina?
outer layer:
photoreceptors (first order neurone)
detection of light
middle layer:
bipolar cells (second order neurones)
local signal processing to improve contrast sensitivity, regulate sensitivity
inner layer:
retinal ganglion cells (third order neurone)
transmission of signals from the eye to brain
what are rods?
photoreceptors
longer outer segment with photosensitive pigment
100 times more sensitive to light than cones slow response to light black and white responsible for night vision (scotopic) there are 120 million rods
what are cones?
less sensitive to light faster response daylight fine vision colour vision (phototopic) there are 6 million cones (5% of the number of rods)
how are the photoreceptors distributed?
rods: peripheral and night vision more photoreceptors more pigment higher spatial and temporal summation recognises motion
high levels either side of the macula, tails off as you go further out
none in blind spot
highest concentration 20-40 degrees away from the fovea
cones:
central and day vision
colour and detail
really high concentration in the macula
not many at all anywhere else
what is a frequency spectrum?
different colour are captured through different photoreceptors
S cones - blue
M cones - green
L cones - red
each of the pigments in these detect different wavelengths of light
what are colour vision deficiencies?
8% in males, 0.5% in women
deuteranomaly (aka daltonism) is the most frequent form of colour blindness
these people arent completely colour blind, but they dont perceive the colour red
full colour blindness (achromatopsia) only occurs in a very small percentage of the population
an ishihara test is used to assess colour blindness (can you see the number in the blobs of different colour dots)
