lecture 25: vision Flashcards
(20 cards)
the electromagnetic spectrum
- only a small band comprises visible light
- the visible spectrum is wavelengths of approx 400-750nm
- it is the job of the visual system to detect this visible radiation and to us it as the basis for formation of visual perceptions of the world to guide behaviour
what is the eye
the organ that detects light and processes visual information to create visual perceptions and to guide behaviour
what are the two basic components of the eye
- an optical component that has the main job of collecting and focusing light into the plane of the retina
- a neural component that converts energy of light into patterned changes of membrane potential that other parts of the brain can decode to generate visual perceptions
what does the optic disc do
where all our neurons pass their axons down to the brain, also where all our blood vessels come out (feeds the cell) optic disc only part of the eye with no photoreceptors
what does the fovea do
- has the highest acuity
- where most the colour photoreceptors are
- where the eye wants to focus on if its of interest to you, shift the light onto the fovea, as want as many details as possible
refraction in the eye
- occurs when light travels from a medium of one refractive index to a medium of a different refractive index
- results in a change of direction of light, and is related to the difference between the refractive indices of the two media and to the curvature of the refractive surface
what are the three processes that occur as you shift your gaze from distant to near objects
- accommodation
- contraction/relaxation of ciliary muscle to alter lens shape and change refractive power - constriction of pupil
- improved depth of focus, fewer optical aberrations by excluding edges of lens - convergence of eyes
- objects remain in register on corresponding parts of the two retinae (esp. foveae)
mechanisms of accomodation in optics
- when looking at distant objects the ciliary muscle is relaxed due to low parasympathetic activity, so the zonular fibres are taut, and the lens is flattened
- when gaze shifts to close objects parasympathetic activation of ciliary muscle increases, ciliary muscle contracts, tension removed from zonular fibres, lens becomes more spherical due to natural elasticity (eyes total refractive power increases from about 60 up to about 75 diopters)
emmetropia
normal eye
myopia
- eyeball is too long
- nearsighted
- doesnt matter how much the lens accomodates/flattens out, the lens can’t make up for that differential
hyperopia
- eye ball is too short
- fixed by convex lenses which increases convergence
astigmatism
- curvature of cornea and/or lens is aspherical
- different amount of refraction in different planes
- corrected with a cylindrical lens
presbyopia
= age related loss of accommodation
- lens loses elasticity, accommodation falls from about 15 diopters in children, to 2 diopters at around 45-50 yrs, to essentially 0 at 75yrs
- in consequence, near point recedes from 10cm at 20 yrs to 80cm at 60yrs
- corrective lens needed (convex) to restore near vision
cataract
- lens becomes opaque, especially with age
- lens is broken up inside lens capsule, debris is removed and a plastic intraocular lens installed
- ability to accommodate is lost post surgery
photoreceptors aspects
- rods
- 120m per retina
- few in fovea
- function in low light (night vision)
- dont report to colour processing regions - cones
- 8m per retina
- many in fovea
- require relatively high light levels (day and twilight vision)
- 3 types, each most sensitive to red, green, blue light
what are photopigments
photoreceptors contain photopigments that give photoreceptors the capacity to respond to light
what are the 2 components of the photopigment
- a membrane spanning protein called an “opsin”
- rods have rhodopsin
- cones have either S(blue), M(green), L(red) photopsin - a chromosome called retinal, a Vitamin A derivative
phototransduction
the process by which photoreceptor cells in the retina convert light energy into an electrical signal that can be understood by the nervous system
phototransduction in the dark
- no light
- retinal non-activated (inactive 11-cis isoform)
- lots of cGMP
-cGMP-gated channels open - much Na and Ca influx (dark current)
- photoreceptor depolarised (-35mV)
- lots of glutamate released onto bipolar cells
phototransduction in the light
- light energy
- retinal changed (to active all-trans isoforms)
- G protein activates cGMP phosphodiesterase
- cGMP phosphodiesterase breaks down cGMP
- less cGMP, cGMP-gated channels close
- less Na and Ca influx
- photoreceptor hyperpolarised (-60mV)
- less glutamate released onto bipolar cells
