What 3 different types of stimuli is the visual system tuned to recognise? (and give an example of how)
- Food
- Predator
- Mate
Sensory neuron has large dendritic trees
What is an information bottleneck?
Cant process more than a certain amount of information
- Means the retina needs to decide what is worth sending to the brain
How is the visual system organised?
- Retina containing photoreceptors etc.
- Optic nerve carries visual info from the retina to the brain
- Axons of retinal ganglion cells synapse the LGN of the thalamus
- LGN acts as a relay station transmitting visual info to the primary visual cortex
How are the left and right visual field correlated to the brain?
Left visual field processed on right side of the brain
Right visual field processed on left side of brain
Once information enters the visual cortex what two main paths can it go down?
- Dorsal stream: ‘where’ pathway responsible for locating object and its spatial position e.g. is a moving object moving towards or away from us + its size
- Ventral stream: ‘what’ pathway responsible for coding for object identity
How is light focussed?
- Pupil regulate the amount of light that falls on retina
- Lens focuses image on fovea (largest number of photoreceptors and tightly packed with lots of cones for high visual acuity)
- Rest of retina has smaller acuity and primarily contains rods
- When something happens , the part of visual stimuli that is interesting to us is moved to be in line with the fovea so we can see it the most clearly
How does light hit photoreceptors when the retina isn’t transparent?
- Light goes through Muller cells
- Much more transparent
- Acts as a ‘light guide’ this allows a certain number of photoreceptors to be activated
Describe the layout of the Retina
the ganglion cell layer, closest to the vitreous humor, outputs info from retina to brain
inner plexiform layer - synapses, between ganglion (gabaergic) and amacrine/bipolar cells
inner nuclear layer - amacrine cells, horizontal cells involved in modulating info and bipolar cell bodies
outer plexiform layer - closer to back of the eye - synapses, between bipolar and photoreceptor cells (Glutaminergic)
outer nuclear layer - cell bodies of the photoreceptors
photoreceptor outer segments
pigmented epithelium - absorbs light the photoreceptors don’t to prevent reflection back to the retina, increasing visual acuity
*signal transduced at photoreceptors (GP), bipolar cells (GP) send signal onto ganglion cells (AP)
How do photoreceptors respond to light?
Hyperpolarisation (decrease in membrae potential)
Compare rods and cones
Rods -
High sensitivity
More disks, with higher concentration of photopigments in each disks, so 1000x more sensitive than cones (so at night - only rods used, so no colour seen)
Low resolution - many receptors to one bipolar neuron
Cones -
High resolution/visual acuity (and colour)
Fewer disks and lower photopigment concentrations
Not as easily saturated (less sensitive) so better at continuous response to stimuli
why in the peripheral retina are there many bipolar neurons synapsing eith one ganglion cell?
allows for high sensitivity (light levels are lower) but low acuity as it is unclear exactly where the light came from, seeing as there is multiple inputs
what makes photoreceptors so unusual (and how does it work)?
- depolarised at rest and hyperpolarised when activated
- ligand gated ion channel on the inside, ligand being cGMP
- when it’s dark cGMP keeps these channels open allowing influx of cations (mostly Na+), keeping the cell depolarised (open K+ channels allow K+ to leave so as not to depolarise too far)
- when its light cGMP levels drop, channels close, hyperpolarise the cell
What causes cGMP levels to drop?
- Photons change the retinal in the GPCR from cis to trans exchanging the GPCR’s GDP for GTP
- Alpha subunit activates phosphodiesterase, an enzyme that converts cGMP to GMP
- Light causes low cGMP levels and the LGICs close
How does hyperpolarisation lead to constant glutamate release?
- Hyperpolarisation spreads to synapse and the synapse for photoreceptor releases glutamate, sustains constant release
- So when you flash a light, downstream cells receive less glutamate
What are ON and OFF Bipolar cells?
Some bipolar cells hyperpolarise while others depolarise in response to light
OFF= hyperpolarise in response to light (excitatory ionotropic receptor)
ON= depolarise in response to light (inhibitory metabotropic GPCR)
how does light adaption work, specifically the role of calcium?
initial light = high hyperpolarisation
if it continues, photoreceptors gradually depolarise in order to make room for response if light intensity increases
the LGICs allow in calcium as it is a cation
calcium inhibits guanylyl cyclase, the enzyme making cGMP, so in the dark calcium comes in and prevents too much cGMP from overly depolarising the cell
in the light, when LGICs are closing, no calcium allows guanylyl cyclase to go crazy, opening those channels and allowing depolarisation even in the presence of light which initially causes hyperpolarisation
What receptors are used in ON and OFF Bipolar cells?
OFF - AMPA receptors joined with glutamate receptor, when glutamate binds the channel opens and neuron is depolarised signalling a DECREASE in light intensity (light OFF)
ON - Metabotropic Glutamate Receptors generating signalling cascade
What is TRPM1?
- Expressed in ON Bipolar cells
- Linked to the regulation of membrane potential
What is Nyctalopin?
- Expressed in ON cells but not OFF cells
- Proteoglycan required for light and glutamate responses in ON cells
What happens when there is KO experiments done with Nyctalopin and TRPM1?
- No current which shows TRPM1 is responsible for ion channel activation
No ON cell activity
So what is the purpose of ON Bipolar cells?
The respond to increases in light, so initially they are depolarised with light but they keep releasing glutamate until they are hyperpolarised
- This info is then sent to ganglion and amacrine cells
Where do ON and OFF cells send signals to?
OFF cells stratify in the **Inner Plexiform layer **
**ON **cells stratify in the outer plexiform layer (closer to photoreceptor cells)
What are the two types of receptive field for the retina?
**On-centre/off-surround: **
- Ganglion cells are excited by the stimulation of the centre (hyperpolarisation)
- Ganglion cells are inhibited by stimulation of the surround (depolarisaton)
Off-centre/ on-surround
- Ganglion cells are inhibited by stimulation of the centre (depolarisation)
- Ganglion cells are excited by the stimulation of the surround (hyperpolarisation)
How do the centre-surround arrangements created?
- Convergence of input from bipolar cells- each bp cells receives input from multiple cones/rods
- Lateral inhibition by horizontal cells- activation of one photoreceptor can inhibit the neighbour to enable higher contrast between light and dark regions (spatial resolution)
- Feedback from amacrine cells- Provide feedback inhibition to bipolar cells to help spatial properties of receptive field
- Convergence on to ganglion cells for centre-on/off fields formed through integration of excitatory and inhibitory inputs from bipolar cells and horizontal cells
