CNS vision Flashcards
What is the sclera?
White portion of the eye
What is the cornea?
The sclera becoming clear in the middle of the eye
What is the pupil?
The opening that lets light in. Its size can be controlled for more or less photons
What is the vitreous humour?
Some sort of gelly that keeps the shape of the eye
What is the retina?
It’s the back of the eye where the photoreceptors are lined
What is the fovea centralis? why?
The space with the highest visual acuity. The cell bodies of the retinal circuitry are moved out of the way to provide a clearer image.
What is the optic disk?
It is the blind spot of the eye because it is the entry point of the optic nerve. Because of that, there are no receptors there.
When light reaches the eye, it […]. Explain why.
It refracts. This is because it’s striking the boundary between air and the cornea or lens, which have different properties.
What is happening when your eye focuses?
Light is being bent to show up at a single point behind the lens, at the back of the eye. (focal point)
Which parts of the eye are responsible for refracting the incoming light? Which one is dominant?
The cornea and the lens. The cornea refracts light more than the lens does.
What is accommodation in the eye? Which parts of the eye are responsible for it?
The lens of the eye can change its shape depending on the distance of the focal object from the eye, thus changing the amount of refraction. Notably, the cornea is fixed and cannot be accommodated.
Explain how accommodation in the eye works and what muscles are involved.
When an object moves out of your focal range, the lens will change shape to refract the light such that the object can get back into focus. This is controlled by the ciliary muscles, which can pull and release the lens.
What does it mean to be myopic?
It means that the person is nearsighted.
When a person is myopic, what is wrong with the eye?
The eyeball is too long, so the light entering the eye converges in front of the retina.
What kind of lens is used to correct myopia?
A concave lens
What does it mean to be hyperopic?
It means that the person is farsighted.
When a person is hyperopic, what is wrong with the eye?
The eyeball is too short, so the light entering the eye converges behind the retina.
What kind of lens is used to correct hyperopia?
A convex lens
What is astigmatism?
When the lens or cornea is not spherical, causing refraction problems.
What is presbyopia?
When the lens gets stiff and is unable to accommodate for near vision (can’t be round enough)
What is cataracts?
It is when the lens changes colour and becomes more opaque, making it harder for light to get in.
Where does transduction occur?
It occurs at the photoreceptors at the back of the eye, closest to the retinal pigment epithelium.
What are the major types of photoreceptors? What is their purpose?
Rods and cones. Rods are active in low light conditions, while cones are active in high light conditions and provide colour vision.
Describe the order of major structures through which light passes in order to get to the retina.
Vitreous humour -> ganglion cells -> bipolar and amacrine cells -> horizontal cells -> rods and cones -> retinal pigment epithelium
What is the role of ganglion cells?
They receive many signals from photoreceptors and converge them into axons to the optic nerve.
Explain the purpose of the fovea centralis.
allow high vision acuity
Describe the 4 steps in the processing of photons after they’ve hit the retina.
- They go into the discs of cones or rods where the photon separates the chromophore from the opsin protein
- A G-protein cascade is started
- cGMP in to GMP
- The sodium channels are closed
Describe the structure of photoreceptors.
They have an inner segment and an outer segment with discs where the photons can be captured
Describe the state of photoreceptors in the dark.
In the dark, the sodium channels are open meaning that neurotransmitters are released by the photoreceptors to inhibit the transmission of an action potential by the ganglions
Describe what happens when a photoreceptor gets exposed to light conditions after being in the dark.
The sodium channels will close, meaning that the photoreceptors will stop inhibiting the ganglions
Compare rods and cones in terms of:
a) Sensitivity and day/night vision
b) Amount of opsin
c) Amplification
d) Response time
e) Type of light sensitivity
RODS
a) High sensitivity for night vision
b) More rhodopsin to capture more light
c) High amplification because there are not a lot of photons used in the process
d) Slow response
e) More sensitivity to scattered light
CONES
a) Low sensitivity (need more to be activated) for day vision
b)Less opsin
c) Low amplification (enough photons to activated everything)
d) Faster response
e) Most sensitive to direct axial rays
How do the rod and cone system compare in terms of acuity?
The rod system has a lower acuity because they are not in the Fovea and are highly convergent
How do the rod and cone systems compare in terms of opsins?
Cones have more kind of opsin which allows the perception of colour
How does dark adaptation work?
In bright light, the rods are inactivated and the cones are active. When it goes to dark, there is a temporary blindness where the rods need time to be reactivated and the cones are inactivated
How does light adaptation work?
In the dark, the cones are inactivated and the rods are active. When the lighting changes, the rods are initially saturated (all the opsin is separated from the chromophore) and cause a temporary blindness while the cones are slowly reactivating
Explain what happens chemically during phototransduction in rods when a photon hits an opsin molecule
The bond between the opsin protein and the chromophore it broken
Explain the cycle of opsin+chromophore in dark adaptation in rods.
When there is less light, there are more chromophore+opsin complexes created
Explain the cycle of opsin+chromophore in light adaptation in rods.
More bonds are broken and can become saturated (no more to break)
What cells in the retina have receptive fields?
Retinal ganglion cells
What are the two types of receptive fields on ganglion cells?
They have colour and brightness receptive fields
Retinal ganglion cells have […] receptive fields with…
Center-surround
inhibitory surround and excitatory centers (or vise-versa)
Retinal ganglion cells signal […] across their RF
the relative difference of light (contrast)
What happens to retinal ganglion cells if you shine light with a bright center and a dark surround?
on(excitatory)-center ganglion cells will excite the next cell while off(inhibitory)-center will inhibit
What happens to retinal ganglion cells if you expose the eye to a dark center with a bright surround?
on(excitatory)-center ganglions will inhibit the next cell while off(inhibitory)-center will excite the next
What determines the chromatic sensitivity of the photoreceptor?
The opsin molecule
Why is it harder for us to differentiate between colours at night?
Because the rod’s opsin molecules are not associate to a colour’s wave length like cone’s
Retinal ganglion cells differentiate between colours using […]
colour opponent receptive fields
What are the 2 types of colour-opponent receptive fields?
Red-green and blue-yellow
What is the cause of colour blindness?
If there’s a colour deficit, meaning that the opsin is not quite functionning or not quite sensible enough to be differentiated
Describe the flow of information from the visual field to the brain.
Things seen in a visual field go through the ipsilateral optic nerve. Then, the medial fibers will cross in the optic chiasm. Once they reach the optic tract, all fibers are from the contralateral visual field [right tract carries the left visual field.] Finally, they rach the visual cortex
Where is the visual cortex located?
In the occipital lobe
If someone has a lesion on their optic nerve on the left side, what is the consequence on their vision?
Loss of vison in the ipsilateral eye
If someone has a lesion on their optic tract on the left side, what is the consequence on their vision?
Loss of vision in the contralateral visual field
If someone has a lesion on the optic chiasm right at the center, what is the consequence on their vision?
Bilateral loss of the lateral visual hemifields
If someone has a lesion on the visual cortex on the left side, what is the consequence on their vision?
Loss of vision of the contralateral visual field
At the primary visual cortex, describe the receptive fields and the types of images recognized.
It starts with a small RF . The primary visual cortex can see simple image of features such as orientation of lines, without perception of what is seen
Once the visual information reaches the primary visual cortex, what are the possible pathways it can travel along?
It can go to the partietal visual cortex or the temporal visual cortex which can both provide understanding (perception) of what is seen
Describe the parietal visual stream in terms of the receptive fields and the type of information recognized.
Large RF
Recognizes spatial features and motion (where)
Describe the temporal visual stream in terms of the receptive fields and the type of information recognized.
Large RF
Recognizes complex images and features (what)
