The eye and seeing Flashcards
what are the 3 stages in seeing
optical part
conversion of light into visual code
cognition of the optical code
what is the anterior chamber
the fluid filled space between the iris and the cornea
what is the central retinal artery and vein
blood supply for inner layers of the retina
what is the choroid
vascularised nutritice layer between the sclera and the retina
what is the ciliary body
ring of ciliary epithelium that produces aqueous humour and the ciliary muscle which alters the shape of the lens for focussing
what is the cornea
tranparent tissue forming the anterior wall of the anterior chamber
important for refraction of light entering the eye
what is the fovea
centra part of the macula with the highest visual acuity
what is the fundus
posterior aspect of the interior of the eye, viewed with the opthalamoscope
what is the iris
diaphragm which regulates the amount of light entering the eye
what is the lens
adjustable element that allows focussing of light
what is the macula
central area of retina with the highest visual acuity
what is the optic disc
area with no photoreceptors so a blind spot
what is the retinal pigment epithelium
pigmented monolayer located betwen the inner choroid and outer photoreptor layer of the retina.
the RPE interacts with photoreceptors in the maintenance of visual function
what is the pupil
opening in the centre of the iris through which light passes
what is the retina
part of the CNS which contains light sensitive cells and other neurons which detect light and process light images
what is the sclera
tough outer covering of the eye
what is the vitreous cavity
space between the lens and the retina filled with clear gel
what is the suspensory ligament
Zonular fibres that transmit tension from the ciliary body to the lens to enable focussing of light passing through the lens
what is the conjunctiva
lines the inside of the eyelid and joins the cornea at the limbus
secretes mucin onto the cornea
what is bullous keratopathy
endothelial pump failure, e.g. secondary to complicated cataract surgery or degenerative
fluid is retained in the cornea- so a loss of transparency
painful eye with poor vision
what are the 3 layers of the tear film
oily lipid outer layer to reduce evaporation
aqueous middle layer containing lysozymes and antibodies
mucin layer containing glycoprotein, wets the cornea and allows good coverage
what is the function of the tear layer
wash the eye of debris
provide dissolved O2 to the cornea external epithelium
lubrication of the cornea
what is dry eye syndrome
dysfunction of either the lids and conjunctiva, lacrimal gland, or vit A deficiency
what structure is most responsible for refracting light so it focuses on the back of the eye
cornea because the greatest difference in refractive index is between the air and the cornea
the lens also refracts but to a lesser extent - fine tunes
what does the amount of difraction depend on
the difference between the two substances with
the refractive or focal power (diopters) given by
(n2 – n1)/r
where n1 and n2 are the refractive indices of the respective media and r is the radius
of the radius of the lens curvature (in meters)
how does the lens change when veiwing an object close up
lens bulges
(flattens for distant vision)
cilliary muscles contract which removes some of the radial tension on the lens provided by the elastic zonal fibres
what part of the nervous system is responsible for flattening the lens
sympathetic (distant viewing) cilliary muscles relax
describe the lens at rest
held in a flattened shape by elastic zonal fibres around the edge
what is presbyopia
with age, lenses lose their elasticity which makes it harder to focus an image on the retina (accomodation)
what is the other word for short sightedness
myopia
image focuses slightly anterior to the retina
what is the word for normal vision, neither long or short sighted
emmetropia
what is the other name for long sightedness
hyperopia
the image falls behind the retina
how is depth of field altered
altering the diameter of the pupil
how is myopia corrected
concave lens
how is hyperopia corrected
convex lens
what is astigmatism
a defect in the eye or in a lens caused by a deviation from spherical curvature, which results in distorted images, as light rays are prevented from meeting at a common focus.
what are cataracts
clouding of the lens usually due to the lens thickening with time
the lens epithelia differentiates into lens fibres which form the rest of the lens along with crystalin proteins
the lens becomes less elastic and the central portion clouds over due to being nutritionally deprived
corrective surgery is fairly straightforward
what structure produces aqueous humour
ciliary processes
what is normal intraocular pressure
10- 20 mmhg
what can lead to a rise in intraocular pressure
blockage in drainage (scleral venous drainage via canal of schlemm)
what are the possible consequences of glaucoma
severe optic nerve atrophy and irreversible loss of sight
what are the layers of the retina
outer nuclear layer ganglion cell layer plexiform layer outer nucleur layer pigmented epithelium
what is the ratio of rods to cones
20x as many rods
where are cones concentrated
in the macula
what is the convergence ratio
the number of rods or cones that synapse onto ganglion cells
what is the convergence ratio in the central retina of ganglion cells to rods or cones
1: 1
1: many
what protein derived from vitamin A is found in all rods and cones
retinal
what protein is retinal bound to under unstimulated conditions
opsin
what is the cause of colour blindness
inability to produce different forms of opsin
what happens when purple coloured retinal-opsin is hit by light
changes conformation and becomes detached- retinal appears bleached/ clear
what type of opsin is found in rods
rhodopsin
what form are photoreceptors in when in the dark
depolarised
hyperpolarised in the light
how do our eyes adapt to differering light levels
pupil constriction
difference in sensitivity of rods and cones
photoreceptors adapt to high levels of stimulation
levels of light are relative rather than absolute
