Physiology-Visual Physiology Flashcards
What cells are found in the peripheral retina?
Retinal Pigmented Epithelium -> Photoreceptors -> Bipolar Cells -> Ganglion Cells
Why isn’t resolution great in the peripheral retina?
There are a lot of cell layers that light needs to pass through before it hits rods and cones.
Why can you see better at the fovea than at the peripheral retina?
At the fovea, the layers above the rods and cones have been pulled back to allow for a more direct path for light to travel.
Why do night vision goggles project green?
In low light settings, rods are the main functioning cells. They function most optimally at the blue-green wavelength of 496nm, despite not being able to see differences in color.
Why is it best to kind of look away from an object when trying to visualize it in low light settings?
Optic disc and fovea do not have any rods, which are the main operators in low light settings.
Why is visual acuity not optimal in rods?
You have multiple rod photoreceptors that converge onto a single bipolar cell and ganglion. Thus any stimulus is not overly specific.
Why are rods so sensitive to light?
Multiple photoreceptors can receive small amounts of light and synapse on the same bipolar neuron to cause an action potential, where one single photoreceptor would not have had sufficient stimulus to do so.
What different types of cones do you have?
Blue (short), green (medium) and red (long) receptors.
Where are the cones located?
All cones in the fovea. Some cones in the peripheral retina.
Why can’t you read notes with your peripheral vision?
Cones are what give you your visual acuity because they have one cone per bipolar neuron and ganglion. The cones are for the most part concentrated in the fovea.
At what illumination are rods the only photoceptors that get stimulated?
Scotopic range (Dark room)
At what illumination are rods the main photoreceptor with few highly sensitive cones?
Mesopic range (Moonlight)
At what illumination are rods overloaded with light so you rely on cones only?
Photopic (Sunlight)
A photon comes through your pupil and hits a rod. How does signal transduction proceed from here?
Cis -> Trans -> Activation of photopigment -> Transducin activation (G-protein) -> Activation of GMP phosphodiesterase -> cGMP -> 5’-GMP -> cGMP-gated Na+ channel closes -> hyperpolarization -> less neurotransmitter release at cell base (may cause a depolarization or hyperpolarization of the bipolar cell depending on the cell type)
cGMP Na+ channel at night
Dark keeps it sensitized from depolarization.
On-center vs. off-center pathways
On-center pathway: photoreceptor is dumping inhibitory neurotransmitter on the bipolar cell and firing rate is low. Off-center pathway: photoreceptor is dumbing excitatory neurotransmitter on the bipolar cell and firing rate is high.
What pathway is firing at a basal rate in the dark? What happens when light comes in?
Cones are depolarized and releasing inhibitory neurotransmitter on the bipolar cell. The bipolar cell is hyperpolarized and releases minimal excitatory neurotransmitter on the ganglion cell. When light excites the cone, it hyperpolarizes, less inhibitory neurotransmitter is released, bipolar cells release more neurotransmitter and ganglion cells depolarize and increase firing rate (below).
What is the purpose of surround inhibition?
Surround inhibition causes ganglion to fire at different rates as the rods and cones move from dark to bright areas. If they were uniformly made to fire when it was dark or bright, firing patterns would be the same. With surround inhibition, the pattern varies and allows you to interpret boundary changes.
