visual system Flashcards
Explain positive feedforward
Presynaptic neuron activates post synaptic neuron
Explain negative feedback
Postsynaptic neuron inhibits pre-synaptic neuron
Explain negative feedforward
Neurons activates neuron downstream
What does the right hemifield activate?
Through the optic chiasm, visual signals from right hemifields of both eyes are sent to left hemisphere of the brain activating it.
What does the left hemifield activate?
Through the optic chiasm, visual signals from left hemifield of both eyes are sent to right hemisphere of the brain activating it.
What is the main function of the retina?
Image acquisition
What is the lateral geniculate nucleus and what does it do?
A thalamic nucleus that relays information from the retina to the primary visual cortex
Gives rise to axons that project to the primary visual cortex
It pre-processes the visual information
What are the two main visual pathways in the cortex?
Ventral and dorsal
Describe the ventral stream? Where does it terminate?
Object feature – ‘what’
Process information of object identity
Terminates at inferior temporal
Describe the dorsal stream? Where does it terminate?
Spatial location ‘where’
Important for understanding information of location, speed, direction of an object
Terminates at posterior parietal
Explains the function of the the pupils
What’s the function of the lens?
pupil - Regulate amount of light that falls on the retina
lens -Focuses image on the fovea, the area with highest visual acuity
Describe Muller cells and Why do we need Muller cells?
They serve as light guides
Transmit light through the tissue
Because tissue is very thick, and light needs to travel through entire tissue as the photoreceptors are located deeper in the eye.
what is the basic layout of the retina?
Consists of 3 layers of neurons and 2 layers of synapses
State the feedforward neurons
Photoreceptors, bipolar cells and ganglion cells
State the feedback neurons
Horizontal cells and amacrine cells
State the two layers of synapses
Inner and outer plexiform layers.
What is the basic system of retinal information processing?
Information about light flows from photoreceptors to bipolar cells to ganglion cells, which project axons out of the eye in the optic nerve.
Horizontal cells and amacrine cells modify the responses of bipolar cells and ganglion cells via lateral connections.
Describe the inner plexiform
Between the ganglion cell layer and the inner nuclear layer.
Contains synaptic contacts between bipolar cells, amacrine cells, and ganglion cells.
Describe the outer plexiform
Between the outer and inner nuclear layers.
Where photoreceptors make synaptic contacts with the bipolar and horizontal cells.
Describe rods
Active at dim light
Has long cylindrical outer segment, containing many disks.
The greater number of disks and higher photopigment concentration in rods makes them 100 times more sensitive to light that cones
Describe cones
Active at bright light
Have a short, tapering outer segment with few membranous disks.
Where does phototransduction happen?
and
How do photoreceptors respond to light flashes?
Where does phototransduction happen?
In the outer segment in rods and cones
How do photoreceptors respond to light flashes? By hyperpolarisation (decrease in membrane potential)
What is cGMP a form of and why is it important
Form of GMP with a covalent bond.
Important of signalling in cells
Describe phototransduction
¥ Rhodopsin is activated by light – leading at a conformational change
¥ It stimulates the G-protein, transducin, to become transducin GTP
¥ The α subunit activates the enzyme phosphodiesterase (PDE)
¥ PDE reduces cGMP levels, closing Na+ channels
Leading to hyperpolarisation
What does light to do cGMP levels?
Light decreases cGMP levels, closing the channels and preventing Na+ influx, hyperpolarising photoreceptors
What happens when the retina is activated by light?
Photoreceptors are hyperpolarised and release less glutamate.
What happens to bipolar and horizontal cells when there is less release of glutamate?
Every time a light is shone, there is less release in glutamate, and this leads to less activation of bipolar and horizontal cells
what are on and off centre cells ? What happens to On and Off cells when light intensity increases?
they are bipolar cells = ON cells depolarise and OFF cells hyperpolarise
What receptor do OFF cells express?
Ionotropic glutamate receptor (excitatory)
What receptor do ON cells express?
Metabotropic glutamate receptor (inhibitory)
What is a receptive field?
An area in the retina (or space) which when illuminated activates a visual neuron.
Anything that stimulates a neuron
What do many visual neurons have?
Centre-surround organisation of the receptive field – so illumination of the centre and the surround leads to responses in opposite polarities.
Explain centre surround organisation
If light shone on in centre of ON (centre of receptive field) = depolarisation
If light shone on away from centre of ON cell= hyper polarisation
The same neuron can hyperpolarise and depolarise in response to light depending where you shine the light
occurs in Bipolar and ganglion cells
Describe what happens when OFF centre bipolar cell is stimulated directly from photoreceptor?
Ones that activate directly are in centre of receptor field. – hyperpolarisation of bipolar cells
Describe what happens when OFF centre bipolar cell is stimulated indirectly by horizontal cells?
Photoreceptors that stimulate bipolar cell via horizontal cells have inhibitory effect. Because Horizontal cells usually inhibitory.
When stimulated, less inhibition coming from horizontal cells leading to depolarisation.
