PHYL 2041 — Visual System 1
Eye anatomy and optics
What are the main functions of the cornea, iris, pupil, lens, and retina?
What is the function of the retinal pigment epithelium (RPE)?
What are the functions of the choroid and sclera?
What are the aqueous humor and vitreous humor, and where are they located?
Why is the retina considered part of the CNS?
Which structure provides approximately 75% of the eye’s refractive power?
Why can’t the cornea adjust its curvature during accommodation?
What is accommodation?
What happens to the lens when you look at a nearby object?
What happens to the lens when you look at a distant object?
Which branch of the autonomic nervous system controls accommodation?
Which autonomic pathway constricts the pupil?
Which autonomic pathway dilates the pupil?
What causes myopia, and where is light focused?
What causes hyperopia, and where is light focused?
What causes presbyopia?
2. Retina anatomy
What are the main layers of the retina, from the photoreceptor side to the ganglion cell side?
What is the function of the photoreceptor outer segment?
What is the function of the photoreceptor inner segment?
Where are photoreceptor cell bodies located?
What happens in the outer plexiform layer?
What happens in the inner plexiform layer?
Where are ganglion cell bodies located?
What forms the optic nerve?
In which direction does light travel through the retina compared with the direction of neural information?
What is the role of the RPE in supporting photoreceptors?
3. Rods and cones
What are the main functions of rods?
What are the main functions of cones?
Which photoreceptor type is more sensitive to light?
Which photoreceptor type is most useful in dim light?
Which photoreceptor type supports color vision?
Which photoreceptor type is most important for high-acuity central vision?
Which photoreceptor type is more abundant in most vertebrate retinas?
Why are rods useful for detecting very dim light?
Why are cones better for detailed vision?
What happens to rod and cone performance as light levels change?
4. Phototransduction
What is phototransduction?
What two components make up rhodopsin?
What is the chromophore in rhodopsin called?
What happens to 11-cis-retinal when it absorbs a photon?
What happens to rhodopsin after the chromophore changes shape?
What is transducin, and what activates it?
What enzyme does activated transducin activate?
What does phosphodiesterase (PDE) do to cGMP?
What are cyclic nucleotide-gated (CNG) channels?
What happens to CNG channels when cGMP levels decrease?
What happens to the photoreceptor membrane potential in response to light?
Why is a photoreceptor relatively depolarized in darkness?
What happens to cGMP levels in darkness?
What happens to cGMP levels in light?
What happens to the photoreceptor’s membrane potential in darkness versus light?
Why is the photoreceptor response to light a graded potential rather than a typical action potential?
What does it mean when a photoreceptor response becomes saturated?
How does phototransduction amplify a weak light signal?
Why is rhodopsin abundance important for detecting light?
5. Neurotransmitter release from photoreceptors
What is the membrane potential of a photoreceptor in darkness?
What happens to voltage-gated calcium channels in darkness?
How does calcium entry affect neurotransmitter release?
Which neurotransmitter do photoreceptors release?
What happens to glutamate release when light hits a photoreceptor?
Why does light cause reduced neurotransmitter release?
What is tonic neurotransmitter release?
How does a change in photoreceptor voltage affect the next neuron in the pathway?
6. Retinoid cycle and outer-segment renewal
Why must the visual pigment chromophore be regenerated?
What is the overall purpose of the retinoid cycle?
Which retinal cell layer carries out much of the chromophore regeneration?
Why can’t photoreceptors simply keep using the same chromophore indefinitely?
Why must photoreceptor outer segments be continually renewed?
Approximately what percentage of an outer segment is replaced daily?
Where are new outer-segment discs formed?
What happens to old outer-segment tips?
What is the role of RPE cells in outer-segment renewal?
Why can photoreceptors renew their outer segments even though they cannot divide?
7. Color vision
What are the three types of human cones?
What do L, M, and S stand for?
How do the opsins in different cone types differ?
What does it mean for cones to have different absorption spectra?
How does the brain distinguish between different colors?
Why can the same wavelength stimulate more than one cone type?
How does the relative activity of different cone types contribute to color perception?
Why is color vision primarily associated with cones rather than rods?
8. Fovea, macula, and optic disc
What is the macula?
What is the fovea?
Where is the fovea located relative to the macula?
Why does the fovea have the highest visual acuity?
Why are cones densely packed in the fovea?
What happens to the inner retinal layers at the foveal pit?
How does moving inner retinal tissue away from the foveal center improve vision?
What is the optic disc?
Why does the optic disc create a blind spot?
Are photoreceptors present at the optic disc?
How does the distribution of rods and cones differ between the fovea and peripheral retina?
Why do common laboratory animals such as mice not have a human-like fovea?
9. Visual acuity and convergence
What is visual acuity?
What is convergence in a retinal circuit?
What happens when many rods converge onto one ganglion cell?
How does rod convergence affect sensitivity to dim light?
How does rod convergence affect receptive-field size?
Why does high convergence reduce the ability to distinguish two nearby points?
What happens to convergence in the foveal cone pathway?
Why does low convergence improve visual acuity?
Why is foveal vision less sensitive to very dim light than rod-dominated peripheral vision?
What is the relationship between receptive-field size and visual acuity?
10. Bipolar cells and parallel processing
What is the role of bipolar cells in the retina?
What is a ribbon synapse?
What are the two major types of bipolar cells that receive cone input?
What stimulus does an ON-bipolar cell respond to?
What stimulus does an OFF-bipolar cell respond to?
What is the difference between the ON and OFF pathways?
Why is having ON and OFF pathways an example of parallel processing?
What happens to an ON-bipolar cell when light increases?
What happens to an OFF-bipolar cell when light increases?
Why is the photoreceptor’s glutamate release important for determining bipolar-cell responses?
How do mGluR6 receptors contribute to ON-bipolar-cell signaling?
How do AMPA or kainate receptors contribute to OFF-bipolar-cell signaling?
Why can the same photoreceptor signal produce opposite responses in ON and OFF bipolar cells?
11. Horizontal cells and lateral inhibition
What are horizontal cells?
Where do horizontal cells receive and provide input?
What is lateral inhibition?
How does lateral inhibition improve visual contrast?
What is a receptive-field center and surround?
What happens when a medium-intensity spot is surrounded by a dark region?
What happens when the same spot is surrounded by a bright region?
Why can the same central stimulus produce different responses depending on its surroundings?
How does lateral inhibition help the visual system detect edges?
How can lateral inhibition contribute to visual illusions?
Why does the perceived brightness of an area depend partly on the brightness of its surroundings?
12. Apply your understanding
If PDE activity increases, what happens to cGMP, CNG channels, and photoreceptor membrane potential?
If light closes CNG channels, what happens to calcium entry and glutamate release?
If the RPE cannot properly remove old outer-segment material, what might happen to photoreceptor function?
If many photoreceptors converge onto one ganglion cell, what happens to sensitivity and acuity?
If foveal cones had the same convergence as peripheral rods, how might detailed vision change?
If the ON-bipolar-cell mGluR6 pathway were disrupted, which type of retinal response would be affected?
If a bright surround is added around a fixed central spot, how could horizontal-cell activity change the center’s response?
If the optic disc contains no photoreceptors, why don’t we normally notice a permanent black spot in our visual field?
Why can a photoreceptor detect a single photon but still produce a graded response?
Explain the complete pathway from a photon striking rhodopsin to reduced glutamate release.