Lecture 20: Vision 2 Flashcards
What two parts of the eye are crucial for focusing light onto the plane of the retina?
The cornea and the lens
The retina is part of the ________
brain
What type of cells are furthest from incoming light located at the back of the retina?
light sensitive cells (photoreceptors)
What does light travel through before reaching the photoreceptors in the retina?
ganglion cells and interneurons
What are the three types of interneurons in the retina?
bipolar cells
horizontal cells
amacrine cells
Describe bipolar cells
these have two connections connecting photoreceptors and retinal ganglion cells
Describe horizontal cells
their dendrites run horizontally across the top of the photoreceptor
these are needed to mediate the input between the photoreceptor and the bipolar cells
Describe amacrine cells
their dendrites are touching the bipolar cells and the retinal ganglion cells and they help to mediate input and modulate the input between bipolar cells and the retinal ganglion cells
Do interneurons fire action potentials?
no, they communicate by graded changes in the membrane potential (local change in excitability) and then there is a local release of neurotransmitter into the synapse to influence the excitability of neighbouring cells
Do retinal ganglion cells fire action potentials?
yes, they fire action potentials to the brain
What are the two types of photoreceptors called?
rods and cones
Which of the photoreceptors functions in low light?
Rods
Which of the photoreceptors is responsible for day and twilight vision?
cones
Which of the photoreceptors is not colour sensitive?
rods
Which of the photoreceptors is responsible for night vision?
rods
Which of the photoreceptors requires relatively high light levels?
cones
Are there more cones or rods per retina?
rods
Which photoreceptor has three types and what are these three types?
cones
these are ones that are sensitive to red light, ones that are sensitive to green light and ones that are sensitive to blue light
Why are photoreceptors sensitive to light?
because they have photopigment
Where is the photopigment located on a rod?
it sits on the outer segment of the rod formed into discs of membrane to increase the surface area hugely
Why are rods sensitive to low light?
because due to the discs of membrane, the surface area of the photopigment is very large
Where is the photopigment located on a cone?
it sits on the outer segment of the cone formed into small invaginations of the membrane
Why are cones sensitive to bright light?
because the invaginations of the membrane mean that there is only a small surface area of photopigment
The presence of photopigments makes photoreceptors what?
light sensitive
What are the two components of photopigment?
- a membrane spanning protein called an “opsin”
2. a chromophore called retinal
Retinal is a derivative of what?
vitamin A
What does a deficiency in retinal mean?
night blindness
Rods and cones differ in what opsin they have. What opsin do rods have?
rhodopsin
Rods and cones differ in what opsin they have. What opsin do cones have?
they have either an S (blue), M (green) or L (red) photopsin
Cones that are sensitive to blue light have what photopsin?
short wave length photopsin
Cones that are sensitive to green light have what photopsin?
medium wavelength photopsin
Cones that are sensitive to red light have what photopsin?
long wavelength photopsin
What is phototransduction?
the transfer of light into a chemical signal and neurotransmitter release
Describe the process of phototransduction in the dark
In the absence of light, retinal is non-activated and in its inactive 11-cis isoform. The concentration of intracellular cyclic GMP is very high and this causes the cyclic GMP gated channels to open. This causes an influx of Na+ ions into the cell which causes Ca2+ channels to open and the Ca2+ influx causes neurotransmitter (normally glutamate) to be released into bipolar cells.
In what isoform is retinal during phototransduction in the dark?
its inactive 11-cis isoform
Describe the process of phototransduction in the light
Due to the presence of light energy (photons), retinal is converted from its inactive form into the active all-trans isoform. This activates a G-protein called transducin which activates cyclic GMP phosphodiesterase to break down cyclic GMP. Because there is less cGMP, the cGMP-gated channels close and so no Na+ comes in. The photoreceptor is hyperpolarised, there is no Ca2+ released and so there is less glutamate release onto the bipolar cells
What is the one step that allows all vision?
the conversion of retinal into its active all-trans isoform
Light of specific wavelength is _______ from coloured objects
reflected
Perception of colour is created by what?
the relative activation of 3 cone types
Under normal conditions, how many colours can humans perceive?
1 million
Why can we see the colour yellow?
because of the relative activation of red and green cones at different levels
Why can we see the colour turquoise?
due to the relative activation of all cones at different levels
What are some diseases that can cause colour blindness? Why is this?
- glaucoma
- diabetes
- Alzheimer’s and Parkinson’s disease
this is because they call damage the optic nerve or the retina
How does glaucoma damage the retina?
because it causes an increase in pressure in the aqueous humour
How does diabetes damage the retina?
due to a build up of sugar on the retina
Colour blindness can also be inherited. What causes it genetically?
mutation of the gene encoding for the production of M and L opsins (red and green sensitivity) on the X chromosome (for red/green colour blindness)
Because there are more males than females with red/green colour blindness, what do we know about the mutation?
it is a recessive mutation on the X chromosome
At night, intracellular cGMP levels are high BECAUSE cGMP-gated channels are open
both statements are correct and not causally related
What is retinal processing?
this is how the activation of photoreceptors signals are transferred up to the retinal ganglion cells
Briefly describe retinal processing
Light goes through to the retinal ganglion cells to the back of the retina to be picked up by photoreceptors passing through interneurons. This changes the neurotransmitter release on the bipolar cells and horizontal cells (this can be excitatory or inhibitory). The bipolar cells activate (eg.) 2 ganglion cells, the horizontal cells can interact with the interneurons which can affect amacrine cells and ganglion cells which means that the ganglion cells are receiving information from a huge network
In the process of retinal processing, information from ________ and _________ is combined, so that the output from ganglion cells depends critically on the _______ and _____ pattern of light stimulation on the retina
photoreceptors
interneurons
spatial
temporal
The inputs to each ganglion cell arise from neighbouring photoreceptors in a circumscribed area of the retina, the cell’s what?
receptive field
What is meant by spatial pattern of light stimulation on the retina?
it is in regards to _______ and ______ orientation to allow us to direct certain _______
it is in regards to space and visual orientation to allow us to direct certain objects
What is meant by temporal pattern of light stimulation on the retina?
this is with regards to time because over time we have differential pattern of light falling onto our retina and retinal ganglion cells
The output of the retinal ganglion cells depends on what?
The pattern of ________ and the movement of ______ that falls on the _________
the pattern of light and the movement of light that falls on the retina
What shape are ganglion receptive fields?
they are roughly circular
Ganglion receptive fields are divided into how many parts?
2
What are the different parts of a ganglion receptive fields?
there is a circular central zone (centre)
an annulus around the centre called the surround
Why are ganglion receptive fields roughly circular?
because the interneurons and the photoreceptors fall into a circular motion behind it
Ganglion cells respond optimally to what? Why is this?
differential illumination of the centre and the surround because they are contrast detectors
The ganglion cell receptive fields come in how any main types?
2
What are the main types of ganglion cell receptive fields called?
on-centre field and off centre field
What is an on-centre receptive field activated by? What is the response?
light falling on the centre there is an increase in action potential firing
What causes a decrease in action potential firing of an on-centre receptive field?
lights falling on the surround
What is an off-centre receptive field activated by? What is the response?
light falling on the surround there is an increase in action potential firing
What causes a decrease in action potential firing of an off-centre receptive field?
lights falling on the centre
What causes no change in the rate of action potential firing for both an on-centre and off-centre receptive field?
when light falls on both the centre and the surround
As well as there being two different types of receptive fields, how else do receptive fields differ?
they are different sizes
What is the issue with large receptive fields?
if two rays of light fall on two parts of the same receptive field, we can’t discriminate between them
Whereabouts is there the highest acuity? Why?
in the fovea because there are the smallest receptive fields there
Are there more rods or cones in the fovea?
cones, no rods
Where is the acuity lower? Why?
further from the fovea because there are larger receptive fields
What does a small receptive field mean?
there are a low convergence onto ganglion cells ie. behind each ganglion there are less interneurons and rods and cones
If there is high convergence ganglion cells, there are what?
large receptive fields and low acuity and the rods and cones are in the periphery
If there is low convergence ganglion cells, there are what?
small receptive fields and high acuity
What makes up the retinal ganglion cell’s receptive field?
the retinal ganglion cells receive input from a number of photoreceptors and interneurons and this is known as the ganglion’s receptive field
