Visual System Flashcards
What is the purpose of any sensory system?
To recognise and localise 3 types of stimuli:
- Food
- Mate
- Predator (danger)
Which brain areas are involved in visual processing? (2)
- Lateral geniculate nucleus (LGN)
- Primary visual cortex
What are the 2 main pathways involved in visual processing in the cortex?
- Ventral “what” stream
- Dorsal “where” stream
Where is the ventral stream located?
Starts in the primary visual cortex and extends into the inferior temporal lobe (round the side of the brain)
Where is the dorsal stream located?
Starts in the primary visual cortex and extends into the posterior parietal lobe (over the back of the brain)
What is the ventral stream responsible for?
Information about object identity
What is the dorsal stream responsible for?
Information about spatial location, speed, movement
What is the fovea? (2)
- Area of the retina with the highest visual acuity
- Contains cones
What are Muller cells?
Transparent cells present in the retina which guide light through the retinal tissue
What are the layers of the retina?
- Photoreceptors (at the wall of the eye)
- Outer plexiform layer
- Bipolar cells
- Inner plexiform layer
- Ganglion cells
What is in the outer nuclear layer?
Photoreceptors
What is in the inner nuclear layer?
Bipolar cells
What is in the outer plexiform layer? (2)
- Horizontal cells
- Between photoreceptors and bipolar cells
What is in the inner plexiform layer? (2)
- Amacrine cells
- Between bipolar cells and ganglion cells
Which cells form the optic nerve?
Ganglion cells
What is the purpose of horizontal and amacrine cells? (2)
- Modification of signalling between the other cells
- Mainly inhibitory signalling via GABA
What neurotransmitter do photoreceptors release?
Glutamate
What are the 2 types of photoreceptors?
Rods and cones
Where does phototransduction take place in photoreceptors?
In the membranes of the disks in the outer segment
What is the response of photoreceptors to light?
Hyperpolarisation
What kind of synapse does a photoreceptor have?
Ribbon synapse
What happens when photoreceptors are exposed to light? (3)
- Light activates rhodopsin which activates a G protein coupled receptor (transducin)
- Transducin activates PDE which cleaves cGMP into GMP
- Channels close = hyperpolarisation = reduced glutamate release
What is PDE?
- Phosphodiesterase
- Catalyses the hydrolysis of cGMP into GMP
What happens in photoreceptors in the dark? (3)
- There are high levels of cGMP which binds to channels and allows the influx of cations
- This causes depolarisation
- Constant release of glutamate
What are the 2 types of bipolar cells?
ON and OFF bipolar cells
What is the response of ON bipolar cells to light?
Depolarisation
What is the response of OFF bipolar cells to light?
Hyperpolarisation
Why do ON bipolar cells depolarise in response to light?
They have mGluR instead of AMPA receptors
Which receptors do OFF bipolar cells have?
AMPA receptors
Which proteins may be involved in the mechanism of ON bipolar cells? (2)
- TRPM1 which is expressed in ON but not OFF cells
- Nyctalopin which is required for the response of ON cells
Where in the inner plexiform layer do OFF bipolar cells project into?
The section which is closer to the bipolar cell layer
Where in the inner plexiform layer do ON bipolar cells project into?
The section which is closer to the ganglion cell layer
What are the 2 types of ganglion cells?
ON and OFF ganglion cells
Where in the inner plexiform layer do OFF ganglion cells project into?
Where the OFF bipolar cells project (closer to the bipolar cell layer)
Where in the inner plexiform layer do ON ganglion cells project into?
Where the ON bipolar cells project (closer to the ganglion cell layer)
What is a receptive field?
The area of the retina which activates a visual neuron when illuminated
What is the organisation of the receptive field of bipolar cells?
Centre-surround organisation
Why do you record opposite signals when shining light on the centre and periphery of a bipolar cell receptive field? (2)
- In the centre of the receptive field, photoreceptors synapse directly onto the bipolar cell
- In the periphery of the receptive field, photoreceptors connect to the bipolar cell via a horizontal cell which reverses the signal
What happens to the membrane potential of an OFF bipolar cell if you shine light at the centre of its receptive field?
Hyperpolarises
What happens to the membrane potential of an OFF bipolar cell if you shine light at the periphery of its receptive field?
Depolarises
What happens to the membrane potential of an ON bipolar cell if you shine light at the centre of its receptive field?
Depolarises
What happens to the membrane potential of an ON bipolar cell if you shine light at the periphery of its receptive field?
Hyperpolarises
What is the organisation of the receptive field of ganglion cells?
Centre-surround
Which cells in the retina fire action potentials?
Ganglion cells
What happens to the rate of action potential firing from an ON ganglion cell if you shine light at the centre of its receptive field?
Action potential firing rate increases
What happens to the rate of action potential firing from an ON ganglion cell if you shine light at the periphery of its receptive field?
Action potential firing rate decreases
What happens to the rate of action potential firing from an OFF ganglion cell if you shine light at the centre of its receptive field?
Action potential firing rate decreases
What happens to the rate of action potential firing from an OFF ganglion cell if you shine light at the periphery of its receptive field?
Action potential firing rate increases
What happens when you illuminate the entire receptive field of a ganglion cell (diffuse light)?
- Ganglion cell is not activated
- Ganglion cells are designed to respond to differences in illumination within the receptive field
What are the 2 major classes of ganglion cells?
- Parvocellular
- Magnocellular
What is the difference between magnocellular and parvocellular ganglion cells? (2)
- Magnocellular cells have a much larger dendritic tree than parvocellular cells
- Parvocellular cells process information about colour/shape whereas magnocellular cells process information about motion
Why do the receptive fields of ganglion cells have centre-surround organisation?
Inhibitory feedback from amacrine cells
Why do the receptive fields of bipolar cells have centre-surround organisation?
Inhibitory feedback from horizontal cells
Why might ganglion cells have an asymmetric dendritic tree?
Allows for detection of motion in a particular direction
What is adaptation?
A change in the response to the same stimulus
What is sypHy? (3)
- pH inside a synaptic vesicle is low
- Vesicle release releases neurotransmitter and H+
- pH dependent fluorescence to detect synaptic activity
What is iGluSnfr?
Fluorescence in the presence of glutamate
Where does adaptation occur?
Starts in the retina and then downstream
What are the types of adaptation?
- Sensitisation (facilitation)
- Depression
What is the difference between sensitisation (facilitation) and depression?
In depression, the amplitude of the response gradually decreases but in sensitisation, the response amplitude starts off small and gradually increases
How do the mechanisms of depression and sensitisation (facilitation) differ? (2)
- Depression happens downstream of Ca2+ activity because Ca2+ levels remain constant throughout
- Sensitisation happens upstream of Ca2+ activity because Ca2+ levels increase as the amplitude of the response increases
What is the mechanism of facilitation?
Negative feedback from amacrine cells
What is the mechanism of depression?
Depletion of the readily releasable pool (RRP)
