Vision

Question 1

Light enters the eye through

Answer 1

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

Question 2

Iris muscles control

Answer 2

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.

Question 3

The optic nerve comprises

Answer 3

the axons of retinal ganglion cells.

Question 4

Cornea and lens are responsible for

Answer 4

focusing the image on the retina

Question 5

Optic disk .

Answer 5

– 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

Question 6

Fovea

Answer 6

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

Question 7

Light entering the eye is

Answer 7

refracted to form a focused image at the retina

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

Question 8

Refraction

Answer 8

= change in direction of light waves.

Question 9

The major focusing part of the eye is actually the

Answer 9

cornea – lens provides additional “accommodation”.

Question 10

focal point

Answer 10

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

Question 11

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

Answer 11

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.

Question 12

Accommodation

Answer 12

supports image formation in the normal eye (Emmetropia)

Question 13

Far light source focused on retina

Answer 13

Lens flattens, pulled by ciliary muscles

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

Question 14

myopia (Short or nearsightedness)

Answer 14

• Eyeball too long or lens too strong

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

Question 15

Myopia is corrected with a …… lens

Answer 15

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

Question 16

the retina.

Answer 16

• 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.

Question 17

where are the photo receptors located?

Answer 17

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

Question 18

which cells in retina are spiking and non-spiking

Answer 18

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).

Question 19

Rods

Answer 19

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

Single range of wavelength sensitivity

Question 20

Cons

Answer 20

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.

Question 21

Phototransduction

Answer 21

is the conversion of light into electrical signals

Question 22

Photoreceptors are non-spiking neurons. (Why?)

Answer 22

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.

Question 23

How Light activates a phototransduction cascade?

Answer 23

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.

Question 24

Effect of light on the membrane potential ?

Answer 24

light leads to membrane potential becoming more negative

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

Question 25

Relative cone activation accounts for

Answer 25

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.

Question 26

High absorption means that

Answer 26

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.