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Flashcards in Vision Deck (26)
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1
Q

Light enters the eye through

A

pupil, which is not a hole it is a transparent membrane

2
Q

Iris muscles control

A

pupil diameter
- dilator pupillae
- sphincter pupillae
. Muscles in the iris help to dilate or contract the pupil, controlling the amount of light entering the eye to improve image formation.

3
Q

The optic nerve comprises

A

the axons of retinal ganglion cells.

4
Q

Cornea and lens are responsible for

A

focusing the image on the retina

5
Q

Optic disk .

A

– point of entry/exit for optic nerve and blood supply. Since there are no photoreceptors – blind spot. We don’t normally notice this blind spot for two reasons:
• 1 – we have two eyes, and the blind spot in one eye, corresponds to a region with photoreceptors in the other eye
• 2 – we perceptually “fill-in” or interpolate the visual scene around the blind spot

6
Q

Fovea

A

– point of highest acuity, where photoreceptors are most densely packed.

7
Q

Light entering the eye is

A

refracted to form a focused image at the retina

Light entering the eye must be focused on the retina for clear vision.

8
Q

Refraction

A

= change in direction of light waves.

9
Q

The major focusing part of the eye is actually the

A

cornea – lens provides additional “accommodation”.

10
Q

focal point

A

The point at which the light waves focus or converge is called the focal point.

11
Q

• To focus on a nearby object requires a——— lens

A

rounded lens => contract ciliary muscles, which loosens the zonule fibers, causing the lens to become more spherical.
•For a close source, light rays are diverging – need stronger refraction.

12
Q

Accommodation

A

supports image formation in the normal eye (Emmetropia)

13
Q

Far light source focused on retina

A

Lens flattens, pulled by ciliary muscles

light rays are parallel – don’t need to refract light rays as much.

14
Q

myopia (Short or nearsightedness)

A
  • Eyeball too long or lens too strong

* myopic eyeballs are noticeably longer than normal. Conveys increased risk of retinal detachment.

15
Q

Myopia is corrected with a …… lens

A

concave lens, which diverges light rays before they reach the eye.

16
Q

the retina.

A
  • Thin layer of neurons around the perimeter of the globe – the retina.
  • This contains the photoreceptors, which convert light into electrical energy, and the ganglion cells, which carry the electrical signals out of the eye.
17
Q

where are the photo receptors located?

A

Photoreceptors are actually furthest from the light source – light actually has to pass through the network of retinal cells before reaching the photoreceptors.

18
Q

which cells in retina are spiking and non-spiking

A

Ganglion cells are the output cells of the retina – they are spiking neurons. (Photoreceptors and bipolar cells are non-spiking, so they only show graded changes in membrane potential).

19
Q

Rods

A

High light sensitivity (i.e. active in low light levels)

Single range of wavelength sensitivity

20
Q

Cons

A

Low light sensitivity (i.e. active in high light levels)
Three types of cone with different range of wavelength sensitivity => mediate colour vision
•Normal vision relies almost entirely on cones.

21
Q

Phototransduction

A

is the conversion of light into electrical signals

22
Q

Photoreceptors are non-spiking neurons. (Why?)

A

In darkness, rod membrane potential is around -30 mV

Na+ channels are normally kept open by the presence of cGMP, which is continually produced in the photoreceptor.

23
Q

How Light activates a phototransduction cascade?

A

light sensitive protein (photopigment – rhodopsin in rods) undergoes a conformational change, activates a G-protein, which activates an enzyme that reduces cGMP levels( This activates a G-protein cascade that ultimately converts cGMP to GMP.) This closes the sodium channel, and hyperpolarises the cell.

24
Q

Effect of light on the membrane potential ?

A

light leads to membrane potential becoming more negative

Light off = more glutamate; light on = less glutamate.

25
Q

Relative cone activation accounts for

A

colour perception
• Individual cones are not sufficient to produce colour vision – we need 2 or more.
• We can distinguish different colours based on the relative activation of the cones.
• Important – colour perception is based on relative activation of 3 different cone classes.
• It’s not that red stimuli just activate red cones and blue stimuli just activate blue cones – it’s always the relative activity that matters. This is a general principle in sensory encoding.

26
Q

High absorption means that

A

light of that wavelength produces a large response in that cone. So Blue cones are best activated by short wavelengths. Another way to think about it – how easily does a photon with a certain wavelength get absorbed by the opsin protein.