Vision Flashcards
Light energy is defined in two ways. What two ways is light energy defined as?
- Particle - photon
- Wave - wavelength
What part of the electromagnetic radiation spectrum can humans best detect?
The visible light spectrum
Why are humans best able to detect visible light?
Most of the energy that is being put off by the sun is visible light, so its not a surprise that we can see these wavelengths
What is the purpose of the human eye?
To gather and focus light onto the fovea (involves refraction/bending of light)
The better we can focus light…
The better our visual acuity (measure of how well someone can see)
Convex lens (converges/diverges) light
Converges
Concave lens (converges/diverges) light
Diverges
(More/less) refraction is required to see objects that are close
- Explain why
More, because light rays coming from the object are diverging which requires more refraction
(More/less) refraction is required to see objects that are far
- Explain why
Les, because light rays coming from the object are mostly parallel to the back of the eye
Emmetropia
Normal vision
- Far source focused on retina without accomodation (shape change) of lens
- Near source focused on retina with accomodation of lens
Myopia
Nearsighted (able to see things that are close but not far away)
- Far source focused in front of retina (where retina would be in eye of normal length)
- Near source focused on retina without accommodation
How is myopia corrected? Explain how
Using a concave lens
- Diverges light rays before they reach the eye
- Far source focused on retina without accommodation
- Near source focus on retina with accommodation
Hyperopia
Farsighted (able to see things that are far but not close)
- Far source focused on retina with accommodation from lens
- Close objects focused behind retina, even with accommodation
How is hyperopia corrected? Explain how
Using a convex lens
- Converges light rays before they reach the eye
- Far source focused on retina without accommodation
- Near source focused on retina without accommodation
Presbyopia
“Old” sighted
- Hyperopia in older people due to lens losing flexibility
- Far source focused on retina with accommodation from lens
- Near source focused behind retina even with accommodation from lens
How is presbyopia corrected? Explain how
Corrected with a convex lens, which converges light rays before they reach the eye
- Far source focused on retina without accommodation
- Near source focused on retina with accommodation
Function of cornea
Does the majority of light refraction
- Static, can’t change shape
- Roughly symmetrical
What is astigmatism?
See double vision due to uneven refraction from the cornea (cornea is asymmetrical)
Anterior chamber
Fluid-filled and located behind the cornea
- Contains nutrients, bathes the tissues behind the cornea with O2-rich and nutrient-rich fluid
Anterior chamber is filled with…
Aqueous humour
What produces the aqueous humour?
The ciliary body
What does the aqueous humour drain into the anterior chamber through?
The canals of Schlemm
- connects with some of the vessels of the lymphatic system (gets recycled)
True or false: the aqueous humour is constantly being produced
True
- At a rate of 5 mL/day
Glaucoma
Due to increased intraocular pressure, due to narrowing of canals of Schlemm (amount of aqueous humour drained off is less than amount of aqueous humour produced)
- Darkening periphery because the outer axons of the optic nerve get damaged first (irreversible)
Iris function
Controls the amount of light that reaches the retina
- Controls the diameter of the pupil
- Important because too much light can damage the photoreceptor
Iris muscles are under control of the…
Autonomic nervous system
The iris is composed of which two muscles?
- Circular (constrictor) muscle runs circularly
- Radial (dilator) muscle runs radially
Parasympathetic stimulation of iris results in…
Pupillary constriction
- Circular (constrictor) muscle contracts
Sympathetic stimulation of the iris results in…
Pupillary dilation
- Radial (dilator) muscle contracts
Function of the lens
Responsible for the remaining refraction of the light (the light not refracted by the cornea already)
Lens is made of what material?
β crystalline proteins
What controls the lens shape?
Ciliary muscles bound to suspensory ligaments
When ciliary muscles contract…
Suspensory ligaments are slackened, so lens is round and refracts light more
- When looking at close objects
When ciliary muscles relax…
Suspensory ligaments are taut, so lens is longer and refracts light less
- When looking at far objects
Cataracts
Opaque lens when β crystalline proteins start to denature -> causes lens to get cloudy because proteins get folded up
- Lens becomes less flexible
True or false: there is no treatment for cataracts
False
- While there isn’t a lot of treatment for cataracts, lens replacements are still done although the prosthetic lens is much less flexible and less able to accomodate
Retina
Back layer of the eye where all the photoreceptors are located
Fovea
Divit in retina that has the greatest visual acuity
Optic nerve
Bundle of axons that carries all of the light information
Optic disc
Where the blood vessels enter the eye
True or false: there is no transduction of light energy at the optic disc
True
- We have a blind spot here
Which order of cells is activated as light travels through the eye?
Light -> photoreceptor
-> bipolar cells -> Ganglion cells
Function of amacrine and horizontal cells?
Increase our ability to detect slight differences in the light striking the eye (by changing the firing activity of surrounding cells)
The axons of the ganglion cells…
Converge into the optic nerve
Cones are ___chromatic, rods are ___chromatic
Trichromatic (there’s 3 types of cones), monochromatic
There are more (cones/rods) than (cones/rods) in the eye
More rods than cones
Cones are more concentrated around the…
Fovea
Rods are more concentrated around the…
Periphery of the eye
Intense light is needed to activate the (cones/rods), while (cones/rods) can activate even with low light
Cones, rods
(cones/rods) are responsible for most of our night vision
Rods (vision in the dark when there’s very little sources of light)
Phototransduction occurs in the…
Membrane of the outer segment of the rods and cones
Describe the structure of the outer segment in rods and the implications of this
The outer segment membrane on rods contains discs, which are bulges of phospholipid bilayer which increase surface area. This increased surface area allows for more packing of more photopigments.
Describe the structure of the outer segment in cones and the implications of this
The outer segment membrane on cones has repeating folds, which increases the surface area (space) for photopigments
The synaptic terminals of rods and cones are responsible for…
Releasing the neurotransmitter glutamate
- The amount of glutamate released depends on the membrane potential of the cells themselves
True or false: rods and cones generate action potentials
False
- rods and cones do not generate action potentials themselves, but they do change their membrane potential, which changes the amount of glutamate released.
Visual pigment(s) found in rods
Rhodopsin
Visual pigment(s) found in cones
Blue(short wavelength): cyanolabe
Green (medium wavelength): chlorolabe
Red (long wavelength): Erythrolabe
What two things is rhodopsin made up of?
Retinol (derivative of vitamin A) + opsin
What can a deficit in vitamin A in your diet result in?
Loss of night vision due to decreased retinol
When light is absent, rhodopsin is in what configuration?
When light is present, rhodopsin is in its 11-cis configuration
When light is present, rhodopsin is in what configuration?
Rhodopsin is in its all trans version (metarhodopsin II)
What are the steps to the phototransduction pathway? (8 steps)
- Light enters eye and hits rhodopsin.
- Rhodopsin changes configuration to all trans version (metarhodopsin II)
- When rhodopsin is activated, this activates G protein transducin
- Transducin activates cGMP phosphodiesterase, which converts cGMP into 5’GMP
- The decreased cGMP content in the photoreceptor decreases the activity of cGMP-gated channels, which pump Na+ into the cell
- Less influx of Na+ makes the cells hyperpolarize
- Hyperpolarizatoin results in less glutamate release
- Change in neurotransmitter release changes the signals of bipolar/ganglion cells to change the perception of light
What is the “dark current”?
In the dark when there’s no light present, we have a constant influx (current) of Na+ through cGMP channels
True or false: K+ is always flowing out of the photoreceptors
True
In photoreceptors, what is working to maintain the membrane potential, especially in the dark when so much Na+ is flowing into the cell?
Na+/K+ ATPase
What is the membrane potential of the photoreceptors at rest (i.e. when no light is present)?
~-50 mV (a little more positive than other RMPs)
How do we see different colours if we only have 3 types of cone pigments?
The different cones absorb light maximally at different wavelengths. If a wavelength activates different cones (at different levels), these colours will mix to give a new colour
What causes colourblindness?
Colourblindness is the result of disruption in any of the cones (lose the ability to distinguish between colours)
Where are horizontal cells located?
Between photoreceptors and bipolar cells
Where are amacrine cells located?
Between bipolar cells and ganglion cells
