Dark current

Question 1

What happens to photoreceptors when they are not being stimulated (in dark)

Answer 1

They are constantly being depolarised

Question 2

What do we call the depolarisation of photoreceptors

Answer 2

Dark current

Question 3

What happens when photoreceptors are stimulated (in light)

Answer 3

They hyperpolarise

Question 4

How often do photoreceptors send signals to the bipolar cells

Answer 4

Continuously

Question 5

What is found in the rod or cone shaped part of the photoreceptors

Answer 5

Flattened vesicles with rhodopsin or iodopsin in the membrane

Question 6

What type of synapse do photoreceptors have with the bipolar cell

Answer 6

Inhibitory synapse which inhibits bipolar cell from firing

Question 7

What happens when vesicles of neurotransmitter are released into synapse

Answer 7

The bipolar cell is inhibited

Question 8

What type of pump do Photoreceptors have

Answer 8

Sodium-potassium pump (3 sodium out and 2 potassium in )

Question 9

What does the sodium potassium pump on photoreceptors do

Answer 9

Creates a negative membrane potential and makes sodium ion concentration outside the cell high

Question 10

What do photoreceptors have that is unique to most neurones

Answer 10

A non-specific cation (anything that is positively charged) channel

Question 11

What mostly goes through the cation channel

Answer 11

Sodium ions due to the high concentration built up outside the cell due to the sodium potassium pump

Question 12

What are the properties of the cation channel in the dark

Answer 12

The channel will be open and sodium will diffuse down the electrochemical gradient

Question 13

What happens to the polarisation of the cell during the dark

Answer 13

The cell depolarises from -70mv to -40mv (dark current)

Question 14

What causes the constant state of depolarisation in the photoreceptor

Answer 14

Sodium potassium pump pumping sodium ions out and cation channel allowing sodium ions in (dark current)

Question 15

What does the constant state of depolarisation cause

Answer 15

Causes vesicles to constantly be released

Question 16

What happens when the neurotransmitter is released from the photoreceptor

Answer 16

It binds to the receptors on the bipolar cell and inhibits the bipolar cell from firing

Question 17

What will happen when light is absorbed by the photoreceptor

Answer 17

Cation channel will be closed, so no sodium ions enter the cell and sodium is being removed by sodium potassium pump causing cell to be hyperpolarised. This means no vesicles will be released so neurotransmitter in the synapse goes down and their will be less inhibition of the bipolar cell it can send an action potential to the brain so the brain knows their is light.

Question 18

What is rhodopsin

Answer 18

Protein pigment in rod cells that absorbs light

Question 19

What happens to rhodopsin when it absorbs light

Answer 19

It is broken down into opsin and retinal

Question 20

What is bound to the non-specific cation channel

Answer 20

Cyclic nucleotide cGMP

Question 21

What happens to the cGMP when rhodopsin is broken down in the presence of light

Answer 21

cGMP is hydrolysed

Question 22

What does the hydrolysation of the cGMP cause

Answer 22

Causes the cation channel to close as structure of cGMP has changed to GMP

Question 23

What does the closed cation channel stop

Answer 23

Stops the constant flow of sodium ions into the rod cell

Question 24

What is the result of the cation being blocked

Answer 24

membrane hyperpolarises sodium ions are still being pumped out the sodium potassium pump but not aloud back in via cation

Question 25

What does the hyperpolarisation cause

Answer 25

Vesicles are no longer released from the photoreceptor as voltage gated calcium ion channels are not opened. The bipolar cell is no longer inhibited.

Question 26

What is the result of the bipolar cell not being inhibited

Answer 26

It undergoes an action potential and signals are sent via the optic nerve to the brain