Visual Systems Flashcards
What 3 types of stimuli does the visual system localise?
Food
Predators
Mates
What are the ways in which the visual system processes?
There is a positive feedforward, negative feedback and negative feedforwards
What are visual hemifields? What sides of the brain do they supply?
These are areas in the eye which detect light
Each eye has a left and right hemifield
Each hemifield supplies the opposite side of the brain with information
So The left hemifields (so the left side of both eyes retinas) supplies the right primary visual cortex and the right hemifields supply the left visual cortex.
Where is information from the retina and photoreceptors sent before the left and right visual cortices?
It is first sent to the LGN
The lateral Geniculate nucleus
This pre processes visual information and these neurones are very deep in the brain
Information THEN goes to the visual cortex.
Once visual information has reached the primary cortex, this information is send along two different streams to other regions of the brain.
What are these streams and what regions of the brain is the information sent to? And what is the difference between these two streams??
The ventral stream is used by the body to process object identity.
The ventral stream sends information to the inferior temporal cortex which helps with object recognition.
The dorsal stream is used for spatial recognition ( identifying where in space an object is)
This stream sends information from the primary cortex to the posterior parietal cortex.
What does the pupil and lens of the eye do?
What is special about the fovea? In comparison to the rest of the retina?
And where do you find most rod cells in the eye?
The pupil regulates the amount of light that falls in the retina
The lens focuses images onto the fovea
The fovea is the part of the retina with the highest visual acuity
The rest of the retina (non-fovea) contains rods and has a smaller visual acuity
You have more rods in the non fovea part of the retina THAN in the fovea!
Features of the retina? And what are muller cells?
The retina is a thick and not that transparent tissue
In the retina you have muller cells (on the outer most surface of the retina)
These act as light guides and help to transmit light through tissue.
The retina has 3 layers of feedforward neurones
What are these neurones called?
What do these neurones do?
These neurones are called photoreceptors, bipolar cells and ganglion cells.
These neurones send information in sequence to the primary visual cortex via the optic nerve
Information is gathered in photoreceptors, this then goes to bipolar cells and then to ganglion cells. This then sends information to the optic nerve to go to the brain
What are the two inhibitory neurones - negative feedback neurones in the brain?
These are the horizontal cells and amacrine cells. These are neurones which inhibit signals being sent
Note in some cases horizontal cells can help with information sent, they still do inhibit, but they inhibit to a lesser degree when connected to multiple photoreceptors
What are the two layers of synapsing in the retina?
The outer plexiform layer:
- this contains synapses between photoreceptors, bipolar cells and horizontal cells
The inner plexiform layer?
- contains synapses between the amacrine cells, ganglion and bipolar cells
What are the two different types of photoreceptors?
What happens when you shine a light on them?
What is the difference between the two types of photoreceptors?
The two different types of photorecptors located on the retina are rods and cones
When a light is shone on them they hyperpolarise, which causes the membrane potential to drop
Difference? Cones are activated by bright light
Rods are activated by dim light
What part of the cones and rods experience phototransduction?
The outer segments of these photoreceptors
How does cyclic GMP affect phototransduction?
What channels does it usually keep open when they are in high amounts
And how does a reduction in GMP allow for signalling
Cyclic GMP (not AMP) activates channels on photoreceptors
Normally when there is no light, these channels remain activated and open, as there is alot of CGMP in the cytoplasm
These channels help the photoreceptors to hyperpolarise when they shut. This happens when the amount of cGMP is reduced. When hyperpolarised these cells can send signals to bipolar cells.
CGMP is reduced by the enzyme phosphodiesterase.
How is phosphodiesterase activated?
Remember this is what breaks down cGMP in the photoreceptors of the retina, helping channels in these photoreceptors to close and allowing for hyperpolarisation
When light is shone on photoreceptors
The rhodopsin pigments (a pigment in rod cells) found here change their pigment conformation.
These then cause G protein coupled receptors on the photoreceptors membrane to be activated
The G alpha sub units the dissociate from the G protein and activates the phosphodiesterase
Thus The phosphodiesterase is activated when you shine a light on a photoreceptor.
What does the phosphodiesterase do found in the cytoplasm of photoreceptors?
These decrease the amount of cGMP
This then causes the channels to close
When these channels are open they move sodium and calcium out / into the cell. This allows for depolarisation of the membrane. HOWEVER note signals from photoreceptors arent sent when the photoreceptors are polairsed. Instead they are sent when the cells hyperpolaise
Thus when the channels close the membrane is allowed to hyper polarise, which sends a signal to bipolar cells
What happens when photoreceptors hyper polarise in terms of glutamate release? What cells detect glutamate release?
When photoreceptors have light shone on them
They activate and hyperpolarise.
glutamate is detected from bipolar and horizontal cells
When photoreceptors are hyper polarised they release less glutamate.
In the dark usually photoreceptors release alot of glutamate which excites bipolar cell. When there is light, these bipolar cells become less excited and
This LACK of excitement sends a signal
What are the two types of bipolar cell?
Note each of these cells has a receptive field - this is the area on the neurone which is stimulated by light.
On centre bipolar cells:
- these have their receptive fields in the centre of them
- they depolarise when light stimulates them (via photoreceptors)
OFF centre cells?
- these hyperpolarise when stimulated by light
- their receptive fields are OFF so away from the centre of these cells
What is important to understand about bipolar cells and how they have receptive fields?
At the synaptic junction, bipolar cells have lots of dendrites which from a circle
Stimulation via photoreceptors can either go to the centre of the circle or away from the centre.
Depending on the type of bipolar cell, stimulation can either cause the bipolar cell to hyperpolarise or depolarise.
Why do off centre cells hyperpolarise?
Simple
As they receive less glutamate when photoreceptors are stimulated the bipolar cells channels dont stay open, they shut instead. Causing hyper polarisation
AND REMEMBER there are two types of bipolar cells!!!!
There is the the off and on centre bipolar cells. A photoreceptor might connect to and on or off centre bipolar cell
Why do ON centre bipolar cells depolarise? Remember they have their receptive fields in the centre of the cell.
This is because the ON centre cells don’t produce a lot of inotropic receptors such as AMPa which respond to glutamate.
Instead they have metabotropic glutamate receptors. These are coupled to G proteins
Remember when photoreceptors become active, they release less glutamate, however they do produce a very SMALL amount
This small amount detected by the on centre cells causes a signalling cascade as there are the g proteins located on the on centre receptive field.
This cascade causes more channels to open (in regards to the on centre cells) when the reduced but small amount of glutamate hits the centre of these on cells. The cells thus open, not close, causing sodium to move into the bipolar cells and for an action potential to occur.
What is the difference between on and off centre cells?
The on centre cells have metabotropic (inhibitory) ion channels
This means they have g proteins receptors in their membranes. Even if stimulated by a little glutamate this causes a signalling cascade and causes channels to further open for depolarisation to occur.
The OFF centre cells have ionotropic excitatory cells. These ionotropic channels shut when they receive too little stimulation i.e. from lack of gluatmates = this leads to HYPERPOLARISATION
What happens if you stimulate different parts of an individual neurones in retina in ref to their dendrites or transduction zones?
Stimulating different parts of an individual neurone causes difference responses
And remember different parts of the dendritic tree at synpatic junctions at the outer and inner plexiform cortexes and at the photoreceptors transduction zones
So what is the receptive field of a neurone?
Its the part of the neurone which gets stimulated!!
How to see if a bipolar cell is an off or on type bipolar cell?
Use two different light stimuli on them
Use a spot of light (which is a bean in the centre of a circle)
Or an annular (which is a ring of light with the centre blacked out)
If the bipolar cells are ON centre cells, the you see a response of depolarisation when these cells have a SPOT shone on them shining at their centre (remember depolarisation is due to them having g protein coupled receptors which cause a cascade leading to opening of channels for depolarisation)
Whereas if they were off centre cells then there is no response from a spot of light test.
