[14.4] (2/2) receptors working together in the eye

Question 1

how do rod and cone cells act as transducers?

Answer 1

conserve light energy into electrical energy of a nerve impulse

Question 2

what pigments are found in the membrane of rod and cone cells?

Answer 2

• rods = rhodopsin
• cones = iodopsin (different types)

Question 3

how is a generator potential produced in rod and cone cells?

Answer 3

• pigments absorb light and break down into smaller molecules
• pump proteins and other proteins embedded in the membrane open to let in Na+ by facilitated diffusion / active transport
• this establishes a GP which can lead to an AP in the sensory neurone

Question 4

what is light sensitivity?

Answer 4

ability to establish generator potential

Question 5

what is visual acuity?

Answer 5

ability to give high resolution images in the brain

Question 6

describe the distribution and number of rod cells

Answer 6

• more rods than cones
• more at periphery of retina
• absent at fovea

Question 7

why do rods have high light sensitity?

Answer 7

• 3 rods are connected to a single biploar cell, which is connected to 1 SN
• a threshold value has to be exceeded before a GP is created
• there is much greater chance that threshold value will be exceeded
• this is due to summation

Question 8

why do rods have low visual acuity?

Answer 8

• light received by rods sharing the same neurone will only generate a single impulse travelling to the brain regardless of how many of the neurones are stimulated
• brain cannot distinguish between separate sources of light

Question 9

what type of images do rods produce?

Answer 9

• black and white only
• they cannot distinguish different wavelengths of light

Question 10

what are the 3 basic types of cones?

Answer 10

• red absorbing cones
• green absorbing cones
• blue absorbing cones

> other colours are mixtures of cones reflecting each wavelength

Question 11

describe the distribution and number of cone cells

Answer 11

• fewer cones than rods
• fewer at periphery of retina
• concentrated at fovea

Question 12

why do cones have low light sensitivity?

Answer 12

• cones are found at fovea, where light is focused by lens
• there is highest intensity of light, so cones don’t need to be light sensitive

Question 13

why do cones only respond to high light intensity?

Answer 13

• cones are connected to their own seprate bipolar cell connected to a sensory neurone
• stimulation of a number of cone cells cannot be combined to help exceed threshold value and so create a GP

Question 14

why do cones have high visual acuity?

Answer 14

• each cone is connected to its own single bipolar cell
• if 2 adjacent cones are stimulated, brain receives 2 separate impulses
• brain can distinguish between 2 separate sources of light that stimlated the 2 cones