Optics and Physiology of Vision Flashcards
What are the two part of the visual pigment molecule? What are their roles?
Opsin - determines the wavelength of light that the photopigment will absorb, thus determining color viison
Visual chromophore/retinal - uses the energy of the photon particle to undergo isomerisation and so starting conversion of light stimulus into electrical signal
what is the different between luminous intensity and luminance? And what are the units for each?
Luminous intensity - intensity of light source (candela)
Luminance - brightness reflected from a surface (candela/m2)
What are the 3 changes than light travelling in a vacuum undergoes when it strikes denser media?
- velocity is reduced
- wavelength shortens
- it bends (unless it struck the surface of the medium at a 90 degree angle)
what is an object that bends (or refracts) light called?
a lens
what happens to light rays when they strike a convex lens?
convergence
what happens when ray of light strike a concave lens?
divergence
what is unit for vergence power (amount of bending) of a lens?
diopters
what is the focal length of a lens?
the distance between the centre of the lens and the point at which parallel rays of light are brought into focus by that lens
True or false: the focal lens of a lens is directly proportional to it curvature radius
true
true or false: the vergence powers of lenses in a optical system are not addivitive
false, they are additive
what is accommodation?
rapid change in refractive power of the eye, intended to bring images of objects at different distances into focus on the retina
give the 4 mechanisms by which vertebrate eyes can accommodate
- changing the curvature or position of the lens
- changing the corneal curvature
- changing the distance between the cornea and the retina
- having 2 or more separate optical pathways of different refractive powers
true or false: accommodation is achieved in humans, primates and cats by changing the curvature of the lens
false: the feline lens capsule only has 5% of the elasticity in humans so cats are incapable of changing the lens curvature for accommodation
how do feline eyes accommodate and how is this possible?
translation - anteroposterior movement of the entire lens
abundance of meridonal (longitudinal) fibres in the feline ciliary body muscle with low amount of circular fibres
true or false: viewing distant objects is stimulated by the sympathetic nervous system
true
what 3 ciliary muscles control accommodative mechanisms in the chicken?
1/ anterior (cramptons muscle)
- intermediary (mullers muscle)
- posterior (bruckes muscle)
what are the unique anatomic adaptations that significantly increase the lenticular accommodative capability in chickens?
very large ciliary processes
ring of columnar epithelial cells at equatorial periphery of the lens (annular pad)
lenses are soft and malleable
corneoscleral sulcus permits a greater range of movement
true or false: accommodation in chickens occurs from direct squeezing of the lens
true
true or false: chickens cannot accommodate by changing the corneal curvature
false
how is corneal accommodation mediated in chickens?
anterior ciliary muscle (cramptons muscle)
true or false: chickens can accommodate independently in both eyes
true
Apart from changing corneal and lens curvature, what else contributes to accommodation in chickens?
changing choroidal thickness
does mydriasis increase or decrease the depth of focus of the eye?
decrease
what is an emmetropic eye?
one in which parallel light rays from a distant object are focused on the outer segments when the eye is disaccommodated
what is a myopic eye?
one in which parallel light rays from a distant object are focused on anterior to the retina (nearsighted)
what is hypermetropic/hyperopic eye?
one in which parallel light rays from a distant object are focused posterior to the retina (farsighted)
true or false: cycloplegia has a significant effect on refraction in most veterinary patients
false: it has no significance
true or false: aphakia leaves the eye severely myopic
false: severely hypermetrophic
what diopter power should the canine IOL have?
40-41.5D
why is a 41D IOL needed to correct 15D of hypermetropia?
IOL is placed in a capsular bag filled surrounded by aqueous humour. This environment results in a reduction of its overall refractive power due to the small difference in refractive indices between the AH and the IOL
what diopter power should the feline IOL have?
52-53D
what is scotopic vision?
dim-light vision
what are the characteristics of cone systems? (4)
high resolution of fine details
rapid responses
colour perception
low sensitivity to small fluctuations in light intensity
what are the characteristics of rod systems? (4)
poor visual resolution
no colour perception
extremely sensitive to minute changes in light levels
detection of movement
what is the fovea centralis in haplorrhine primates and many avian species?
a specialised, avascular area of the retina with higher cone density providing higher spatial resolution
what are the four morphological parts of rods and cones?
- outer segment
- inner segment
- nucleus
- synaptic terminal
what is the name of the rod photopigment?
rhodopsin
what is the function of opsin?
determines the wavelengths of light that the photopigment will absorb
briefly describe the phototransduction cascade
energy from photons transferred to chromophore
decrease in cyclic GMP levels leading to closure of the ion channels
hyperpolarisation of the photoreceptor
glutamate release decreased or terminated which affects second order neurons
true or false: photoreceptors are hyperpolarised in the dark causing them to release glutamate
false: they are depolarised
what are horizontal and bipolar cells?
second order neurons of the retina that connect the first order (photoreceptors) and third order (RGCs) neurons
