PSC1002/L25 The Eye

Question 1

What is the function of the cornea?

Answer 1

Refraction of light to bring into focus

Question 2

What is the function of the lens?

Answer 2

Accommodation
Adjusting reflective properties of eye to focus over wide range of distances

Question 3

What is the function of the iris?

Answer 3

Pigment determines eye colour
Muscles adjust pupil diameter according to light levels and emotional signals

Question 4

What is the function of the pupil?

Answer 4

Opening in centre of iris allows light entry

Question 5

What kind of cells exist in the retina? (3)

Answer 5

Photoreceptors
Horizontal cells
Bipolar cells
Amacrine cells
Ganglion cells
Nerve fibres

Question 6

At which point is light focussed at the back of the eye?

Answer 6

Retina

Question 7

How is an image projected onto the retina?

Answer 7

Inverted
Left/right reversed

Question 8

How wide is the visual field of each eye?

Answer 8

150 degrees

Question 9

How does the lens adjust as an object gets closer? (2)

Answer 9

Becomes more rounded to shorten the focal length
Refracts light more

Question 10

Which muscles contract to round the lens more?

Answer 10

Ciliary muscles

Question 11

Which structures are pulled tight to flatten the lens?

Answer 11

Zonulas

Question 12

Describe myopia (short-sightedness). (3)

Answer 12

Too much focal power for its length
Light rays converge in front of retina
Image at retina is blurred

Question 13

Describe hyperopia (long-sightedness). (3)

Answer 13

Too little focal power for its length
Light rays converge behind retina
Image at retina is blurred

Question 14

What is the function of the foveal pit? (2)

Answer 14

Upper layers of retina cells pushed aside to allow light direct access to photoreceptors
Region of highest acuity

Question 15

Describe the fovea. (3)

Answer 15

Pit in centre of macula
Central part of visual field
Area of highest acuity (highest image resolution) with highest density of cone receptors, colour vision

Question 16

Describe the blind spot (optic disc). (3)

Answer 16

Region where nerve fibres and blood vessels leave the eye
Blind in this region
Brain ‘fills in’ missing area

Question 17

What kind of cells make up the retina? (3)

Answer 17

Pigment epithelium
Photoreceptors - rods & cones
Other connecting cells

Question 18

Describe pigment epithelium of the retina. (3)

Answer 18

Cells at back of retina
Photoreceptors embedded
Contain melanin black pigment to absorb stray light

Question 19

Briefly describe the 2 types of photoreceptors in the retina.

Answer 19

Rods (scoptic vision - low light level)
Cones (photopic vision - high light levels)
R:C 20:1

Question 20

Describe rod photoreceptors. (3)

Answer 20

Most sensitive to light
Sensitive to brightness
Only detect contrast
Function at low light levels
Bleached at high light levels

Question 21

Describe cone photoreceptors. (3)

Answer 21

3 types sensitive to long (red), medium (green) and short (blue) waves
Enable colour vision
Responsible for highest acuity
Work only at high light levels

Question 22

Explain dark adaptation. (3)

Answer 22

Cones are not sensitive
Rods bleached
Visual sensitivity increases as rods recover
Colour loss as switch from colour vision to contrast only vision

Question 23

How are cones distributed? (2)

Answer 23

Larger and bigger further away from retina
Allows highest acuity at fovea

Question 24

What photopigment is present in rods and cones?

Answer 24

Rhodopsin
3 different pigments in cones

Question 25

Describe rhosdopsin. (4)

Answer 25

Opsin + 11 CIS-retinal
In dark, opsin and retinal bound together
On light exposure, retinal molecule changes shape
Causes split from opsin
‘Bleaching’

Question 26

Describe how darkness leads to release of neurotransmitter. (5)

Answer 26

cGMP levels high keep Na+ channels open
Na+ current into rod higher than K+ current out
Rod depolarised to -40mV
Transmitter release
Constant release of NTM in dark

Question 27

Describe how light leads to hyperpolarisation of rods. (6)

Answer 27

Light bleaching of rhodopsin activates G-protein transducin
Decreased cGMP
Na+ channels close
K+ continues to flow out
Rod hyperpolarises to -70mV
Less transmitter release