the visual system: eye and retina Flashcards
in what ways does light interact with the environment (optics)?
- reflection
- absorption
- refraction - bending due to difference in speed of light through different media
function of the pupil
lets light inside the eye
- black so every wavelength is absorbed
function of the iris
contains muscles which control the amount of light entering the eye
function of the cornea
glassy, transparent covering of the pupil and iris
- refracts light and focuses it to back of retina
function of sclera
tough, protective wall of eyeball to give it its shape
function of the extraocular muscles
move the eyeball , controlled by oculomotor nerve (cranial nerve III)
function of optic nerve (cranial nerve II)
carries axons (sensory info) from retina to brain
what is the optic disk?
origin of blood vessels and optic nerve, cannot sense light (blind spot but filled in by cortex)
what is the macula?
- region of retina for central vision
- devoid of large blood vessels to improve visual quality
what is the fovea?
area of highest visual acuity
what is the retina?
contains the sensory receptor cells and afferent neurons
divided into nasal and temporal retina (split by fovea)
what controls the lens?
suspended by zonal fibres which are attached to ciliary muscle enabling stretching of the lens
how are images formed?
light rays focused onto the retina (ideally fovea)
refraction occurs at the cornea (80%) and lens (20%)
what determines the degree of refraction?
- difference in refractive indices between the two media
- angle at which light hits the interface between these two media
how does refraction occur at the cornea?
- light arrives at cornea through air but cornea is mainly water
- light travels slowly through water due to density = refraction occurs
- distance from refractive surface to convergence of parallel light rays = focal distance
how does the lens accommodate for objects at different lengths?
distant objects - stretched thin as light rays almost parallel, relaxed ciliary muscle
closer objects - fattening of lens for additional refraction as rays are not parallel, contracted ciliary muscle
what is hyperopia?
see things at a distance well but blurry close up (shorter eyeball so less distance for refraction)
convex lenses makes rays more parallel before entering eye
what is myopia?
see things close up but blurry at a distance (longer eyeball so image too much refraction and image formed before retina)
concave lenses diverge rays before entering eye
what is the laminar organisation of the retina?
- light must pass through ganglion and bipolar cells before it reaches the photoreceptors
- no ganglion and bipolar cells at fovea so highest visual acuity
- light that passes all the way through the retina is absorbed by pigmented epithelium
cells of the retina
- ganglion cells - output from retina (only cells that produce AP)
- bipolar cells - connect photoreceptors to ganglion cells
- photoreceptors - sensory transducers (rods and cones)
how is light absorbed by photoreceptors?
membranous disks which contain light-sensitive photopigments that absorb light
what is duplicity theory?
can’t have high sensitivity and high resolution in a single receptor
(therefore there are rods and cones)
features of rod photoreceptors
- greater number of disks
- higher photopigment concentration
- 1000 times more sensitive to light than cones
- enable vision in low light conditions
- low visual acuity
features of cone photoreceptors
- fewer disks
- used during daylight conditions
- colour vision
- high visual acuity
- lower sensitivity
how does retinal structure vary with region?
fovea contains only cones
central retina = low convergence of cells, low sensitivity, high resolution
peripheral retina = high convergence of cells, high sensitivity, low resolution
