Dark current Flashcards
What happens to photoreceptors when they are not being stimulated (in dark)
They are constantly being depolarised
What do we call the depolarisation of photoreceptors
Dark current
What happens when photoreceptors are stimulated (in light)
They hyperpolarise
How often do photoreceptors send signals to the bipolar cells
Continuously
What is found in the rod or cone shaped part of the photoreceptors
Flattened vesicles with rhodopsin or iodopsin in the membrane
What type of synapse do photoreceptors have with the bipolar cell
Inhibitory synapse which inhibits bipolar cell from firing
What happens when vesicles of neurotransmitter are released into synapse
The bipolar cell is inhibited
What type of pump do Photoreceptors have
Sodium-potassium pump (3 sodium out and 2 potassium in )
What does the sodium potassium pump on photoreceptors do
Creates a negative membrane potential and makes sodium ion concentration outside the cell high
What do photoreceptors have that is unique to most neurones
A non-specific cation (anything that is positively charged) channel
What mostly goes through the cation channel
Sodium ions due to the high concentration built up outside the cell due to the sodium potassium pump
What are the properties of the cation channel in the dark
The channel will be open and sodium will diffuse down the electrochemical gradient
What happens to the polarisation of the cell during the dark
The cell depolarises from -70mv to -40mv (dark current)
What causes the constant state of depolarisation in the photoreceptor
Sodium potassium pump pumping sodium ions out and cation channel allowing sodium ions in (dark current)
What does the constant state of depolarisation cause
Causes vesicles to constantly be released
What happens when the neurotransmitter is released from the photoreceptor
It binds to the receptors on the bipolar cell and inhibits the bipolar cell from firing
What will happen when light is absorbed by the photoreceptor
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.
What is rhodopsin
Protein pigment in rod cells that absorbs light
What happens to rhodopsin when it absorbs light
It is broken down into opsin and retinal
What is bound to the non-specific cation channel
Cyclic nucleotide cGMP
What happens to the cGMP when rhodopsin is broken down in the presence of light
cGMP is hydrolysed
What does the hydrolysation of the cGMP cause
Causes the cation channel to close as structure of cGMP has changed to GMP
What does the closed cation channel stop
Stops the constant flow of sodium ions into the rod cell
What is the result of the cation being blocked
membrane hyperpolarises sodium ions are still being pumped out the sodium potassium pump but not aloud back in via cation
What does the hyperpolarisation cause
Vesicles are no longer released from the photoreceptor as voltage gated calcium ion channels are not opened. The bipolar cell is no longer inhibited.
What is the result of the bipolar cell not being inhibited
It undergoes an action potential and signals are sent via the optic nerve to the brain
