Photoreceptors

Question 1

What are the three regions of the retina?

Answer 1

• The periphery
• Fovea
• Blindspot

Question 2

What receptors do each region of the retina contain?

Answer 2

Periphery = mostly rod cells
Fovea = only cone cells
Blind spot = none

Question 3

How do cone and rod receptor cells stimulate an action potential?

Answer 3

• contain pigment which absorbs light and is broken down
• this stimulates the opening of Na+ channels and Na+ diffuses in causing depolarisation
• if the threshold is passed, an action potential is generated

Question 4

What are the names of the pigments in rod and cone cells?

Answer 4

Rod cells = rhodopsin
Cone cells = iodopsin

Question 5

Describe the structure of a retina.

Answer 5

• rod and cone cells are connected to bipolar neurones
• when rod and cone cells are stimulated, the bipolar neurone is depolarised
• so impulses move along the bipolar neurone, along the optic nerve and towards the brain

Question 6

Compare the properties of rod and cone cells.

Answer 6

• rod cells are more numerous in the retina than cone cells
• rod cells are spread across the retina but not in the fovea whereas, cone cells are more numerous in the fovea
• there is only one type of rod receptor with a pigment whereas, there are three types of cone receptors with different forms of the pigment (red, green and blue)
  -rod cells are unable to detect colour, cone cells can
• rod cells are sensitive in low light intensities, cone cells are only sensitive to high light intensities
• rod cells have low visual acuity, cone cells have high visual accuity

Question 7

Why are cone cells not stimulated in dim light intensities, but rod cells are?

Answer 7

• dim light stimulates the cone cells to release ACh and it diffuses across the post synaptic membrane
• each cone is connected to one bipolar neurone so the small amount of ACh released does not build up
• very little ACh binds to receptors on post synaptic membrane so, few Na+ gated channels open so very little Na+ diffuses in
• this produces a weak generator potential so the threshold is not passed and an action potential is not generated so no impulses reach the brain so we can’t see
• however, several rod cells are connected to one bipolar neurone so enough ACh builds up so the threshold inside the post synaptic knob is passed and an action potential is generated

Question 8

Why do rod cells have poor visual acuity, but cone cells have high?

Answer 8

• several rid cells are connected to one bipolar neurone whereas, one cone cell is connected to one bipolar neurone
• so when the threshold is reached inside the bipolar neurone connected to the rod cells, only one set of impulses are sent to the brain so the person can only see one image, so have a low visual acuity
• however with each cone cell being attached to its own bipolar neurone, multiple sets of impulses are sent to the brain so the person can see multiple images and have a higher visual acuity