L4 - Visual System Flashcards
Visual system is tuned to recognise and localise which 3 stimuli?
Food
Predator
Mate
Morphology of a sensory neuron is largely defined by?
Its function
What are the 3 ways processing level is organised?
Positive feedforward
Negative feedback – horizontal cell back to photoreceptor
Negative feedforward
Which side of the brain does the right hemifield activate?
Left brain
Left hemifield activates right brain
What is the main function of the retina?
Image acquisition
Determines what is important for the brain to know
Where does the info from the eye go?
Goes to the LGN
Preprocess the visual information
Where does the info from the LGN go?
Goes to the primary cortex
Main processing of information
What are the two main visual pathways in the cortex?
Ventral
Dorsal
Ventral visual pathway
What
Object feature stream
Inferior temporal
Dorsal visual pathway
Where
Spatial location stream
Posterior parietal
What is the function of the pupil?
Regulates amount of light that falls on retina
What is the function of the lens?
Focuses image on the fovea
What is the function of the fovea?
Part of retina with highest visual acuity
Contains mainly cones
What is the function of the rest of the retina?
Smaller acuity
Contains mainly rods
Send signals to brain to help focus retina
What is the function of Muller cells?
Light travels through Muller cells
Helps guide the light through the tissue
What are the 3 feedforward neurons of the retina?
- Photoreceptors – pink
- Bipolar cells – blue
- Ganglion cells – purple
What are the 2 feedback neurons of the retina?
- Horizontal cells – yellow
2. Amacrine cells – orange
What is the function of amacrine cells?
Connect to ganglion and bipolar cells
Shapes way bipolar cells process info
What are the two layers of synapse in the retina?
Inner plexiform layer
Outer plexiform layer
What is the function f the inner plexiform layer?
Dense and complex structure
Contains synapses between bipolar, amacrine and ganglion
5-6 layers of synapses
Rods are active at?
Dim light
Cones are active at?
Bright light
How do photoreceptors respond to light flashes?
By hyperpolarisation
Where does phototransduction occur?
In the outer segment
Which two proteins are involved in phototransduction?
Cyclic GMP
GMP
Membrane of photoreceptor cells contains?
Channels that transmit Na and Ca
Phototransduction in the dark method
The channels open and the membrane depolarises
Large concentration of cyclic GMP in cytoplasm
Phototransduction in the light method
A signal cascade involving G proteins
G proteins activate phosphodiesterase
Converts cyclic GMP to GMP reducing amount of cyclic GMP
Channels close - hyperpolarisation of photoreceptor
Laminar retinal organisation
Bipolar and horizontal cells receive input from photoreceptors
Which bipolar cells hyperpolarise in response to light?
OFF cells
Which bipolar cells depolarise in response to light?
ON cells
What do photoreceptors release when depolarised by light?
Release glutamate
Sign inverting synapse in ON bipolar cells
On cells express metabotropic glutamate receptor – excitatory
- Acts through G-protein cascade
- Opens Na channels in membrane of bipolar cells
What do OFF bipolar cells express?
Ionotropic glutamate receptor - inhibitory
What do On bipolar cells express?
Metabotropic glutamate receptor – inhibitory
What is a receptive field?
An area in the retina which when illuminated activates a visual neuron
Centre-surround organisation of bipolar cell receptive field
Many visual neurons have center-surround organisation of the receptive field
- Illumination of the center leads to responses in opposite polarities
Activation of bipolar cells with spot and annulus of light leads to?
Responses with different polarities
Many photoreceptors converge?
To one bipolar cell
Mechanism of centre-surround organization of bipolar cells?
Some photoreceptors
- Activate directly
- Activate via horizontal cells
Photoreceptors that activate directly are found?
In the centre
Cause cell to hyperpolarise
Photoreceptors that activate via horizontal cells are found?
In the surround
Cause cell to depolarise - inhibitory
IPL contains which 3 synapses of bipolar cells?
Axons of ON bipolar cells
Dendrites of ON ganglion cells
Dendrites of ON amacrine cells
Morphology of ganglion cells dendritic trees
Ganglion cells have diverse but distinct morphology of the dendritic trees
- Size
- Symmetry
- Denseness
Ganglion cells receptive fields have?
Centre-surround organisation
Does illumination of the whole receptive field activate ganglion cells?
No
Ganglion cells respond to differences in?
Illumination that occur within the receptive field
What happens if you stimulate centre of ganglion cells?
Increase rate of spiking – on cell
Complete stop of spiking – off cell
What happens if you stimulate surround of ganglion cells?
Complete stop of spiking – on cell
Increase rate of spiking – off cell
What happens if you stimulate the whole receptive field of ganglion cells?
No difference in response
What are the two classes of ganglion cells?
Parvocellular (80%)
Magnocellular (10%)
Magnocellular characteristics
Large dendritic tree Transient response Fast conduction velocity High sensitivity Function - detect motion
Parvocellular characteristics
Small dendritic tree Sustained response Slow conduction velocity Low sensitivity Function – processing info about shape and colour
What is adaptation?
Decrease in activity
Often reasons for visual illusions
What is sensitisation?
Increase in activity
