Detecting stimuli (6)

Question 1

Describe three characteristics of a receptor?

Answer 1

• specific to one kind of stimulus
• some are cells some are proteins on cell surface membranes
• when they are in their resting state they are polarised

Question 2

What are the receptors in the human eye?

Answer 2

Photoreceptors - rods and cones

Question 3

What is the structure and fucntion of rod cells =?

Answer 3

• monochromatic vision
• contain a light sensitive pigment called rhodopsin which consists of two chemicals: retinal and opsin
• has a synapse with a bipolar neurone (one which connects photoreceptors to the optic nerve)

Question 4

What happens to the rods in dim light?

Answer 4

• sodium ions are pumped out of the cell by active transport
• sodium ions diffuse back into the cell through open sodium ion channels (cation channels)
• the inside of the rod is slightly more negative than the outside so it is depolarised (depolarised means there is not much difference in charge across the membrane)
• this triggers the release of neurotransmitters from the rod’s synaptic knob
• these neurotransmitters inhibit the bipolar neurone so it can’t fire an action potential to the optic nerve and subsequently the brain.

Question 5

What happens to the rods in bright light?

Answer 5

• light energy causes rhodpsin to break down into retinal and opsin - this is called bleaching
• the bleaching causes sodium ion channels to close
• Na+ is actively transported out of the cell but can’t diffuse back
• this build up of Na+ outside the membrane, making the outside much more negative
• this makes the membrane hyperpolarised
• this hyperpolarisation causes the rod cell to stop releasing neurotransmitters which inhibit the bipolar neurone
• this means the bipolar neurone depolarises and if the change in potential difference reaches the threshold an action potential is transmitted to the brain via the optic nerve