Vision I

Question 1

What is light?

Answer 1

-light is electromagnetic radiation that is visible to our eyes
-electromagnetic radiation has a wavelength, frequency, and amplitude
-wavelength is the inverse of frequency and it ranges from 400-700nm

Question 2

What are the different parts of the eye?

Answer 2

-pupil allows light in
-iris gives eye colour
-sclera is the white of the eye
-cornea provides refraction and protects the eye (sclera)
-lens focuses light onto the retina
-retina is the light-sensitive layer of cells at the back of the eye
-optic nerve carries axons from the retina

Question 3

What are the parts of the retina?

Answer 3

-Optic disk: where blood vessels originate, and optic nerve exits (no photoreceptors; blind spot)
-Macula: no blood vessels (central vision; “large fovea”)
-Fovea: highest acuity, thinner retina (center of the retina - nasal vs temporal)

Question 4

What are the characteristics of the fovea?

Answer 4

-2 mm in diameter (pit)
-no optic fibers
-high concentration of cones
-most acute vision (details)

Question 5

How does refraction/focus work?

Answer 5

-eye collects light, focuses it on retina/fovea, for maximum acuity
-cornea provides strongest refraction power - air to water
-lens provides additional refraction, particularly for near objects –> accommodation
-lens is critical for objects closer than 9m

Question 6

How close to the eye is too close?

Answer 6

-6.5cm from the eye
-Presbyopia –> hardened lens, insufficient accommodation

Question 7

What are 2 common kinds of eye conditions and what is the lens accommodation?

Answer 7

-Myopia: short-sighted; eye-ball too long –> concave lens
-Hyperopia: far-sighted; eye-ball too short –> convex lens

Question 8

What is the Laminar Organization of the Retina?

Answer 8

-sclera
-choroid
-pigment epithelium
-photoreceptors: cones & rods (releases NTs but not AP)
-horizontal cells: input from photoreceptors and project to other photoreceptors and bipolar cells
-bipolar cells (no AP)
-amacrine cells: input from bipolar cells and project to retinal ganglion cells, bipolar cells, other amacrine cells
-retinal ganglion cells (RGCs): only output from the retina
–to optic nerve

Question 9

What are 3 important points to consider about the organization of the retina?

Answer 9

1. Photoreceptors are the only light-sensitive cells in the retina
2. Ganglion cells are the only output from the retina
3. Ganglion cells are the only retinal cells to fire action potentials

Question 10

What are the characteristics of outer segments of photoreceptors?

Answer 10

-contains a stack of membranous disks
-light-sensitive photopigments in those disk membranes absorb light
-this triggers changes in the photoreceptor membrane potential

Question 11

What are some characteristics of cones?

Answer 11

-do most of the work under daytime lighting
-3 types of cones each with different photopigment

Question 12

What are some characteristics of rods?

Answer 12

-more disks –> x1000 more sensitive to light
-only one type of photopigment

Question 13

What is the photoreceptor distribution across the retina?

Answer 13

-rods and cones are not uniformly distributed across retina
-97 million photoreceptors per retina:
–5 million cones;
–92 million rods (rods 20:1)
-1 million retinal ganglion cells

Question 14

What is the difference between central and peripheral retina?

Answer 14

-central retina: less convergence - few cones per ganglion cell; fovea is all cones; more acuity
-peripheral retina: more convergence - more rods per ganglion cell; mostly rods; more sensitive to light

Question 15

How do rods react to light?

Answer 15

-rods are highly sensitive to light:
–provide good light sensitivity in dark conditions
–saturated (non-functional) under daylight conditions
–fovea is blind during scotopic conditions

Question 16

Who does most of the work in daylight conditions?

Answer 16

-under daylight conditions cones do the work (fovea)
–3 different photopigments (instead of rhodopsin): red, green and blue (RGB)

Question 17

What happens when cones are inactive?

Answer 17

-we cannot see color when cones are inactive:
–inability to distinguish wavelength
–poor color discrimination in our periphery

Question 18

How long does dark adaptation take?

Answer 18

-going from photopic to scotopic conditions requires 20-25 mins of dark adaptation (how long it takes for the sun to set)
-reverse process is light adaptation (5-10mns)

Question 19

What are the 4 steps of dark adaptation?

Answer 19

-pupillary dilation (2-8mm)
-rhodopsin regeneration
-functional circuitry of the retina
-sensitivity to light increases x1,000,000

Question 20

How does adaptation work and how could we reduce the delay?

Answer 20

-adaptation means that the retina measures relative changes in light rather than the absolute light level
-eye-patches are a possible means to decrease the delay for light/dark adaptation (eye under eye patch is already adapted to dark but loses a bit of depth perception)

Question 21

What are the characteristics of Central Retina?

Answer 21

-active under photopic conditions (cones)
-lack of rods means that it is largely blind under scotopic conditions
-perceives color since it possesses most of our cones
-low convergence onto RGCs means high acuity (detail)

Question 22

What are the characteristics of Peripheral Retina?

Answer 22

-active under both photopic and scotopic conditions
-primary visual input under scotopic conditions
-primarily consists of rods and therefore has low color discrimination
-high convergence onto RGCs means low acuity, but high sensitivity

Question 23

What are the characteristics of Duplex Retina?

Answer 23

-contains two co-existing systems specialized for either light or dark conditions

Question 24

What are the beginning processes of phototransduction?

Answer 24

-photoreceptors convert electromagnetic radiation into neural signals
-their function is similar to post-synaptic GPCRs
-release neurotransmitter when depolarized (like other neurons)
-EXCEPT that light will decrease rather than increase second messenger activity
-this means that photoreceptors are hyperpolarized by light
*photoreceptors are dark detectors (release GABA when it’s dark)

Question 25

What is the process that stimulates transducin?

Answer 25

-rhodopsin is the photopigment in the disc membrane of rods
-light absorption causes a change in retinal conformation
-this conformation change activates the opsin (causes bleaching)
-opsin bleaching stimulates transducin (G-protein)

Question 26

What does transducin activate?

Answer 26

-transduction activates phosphodiesterase (PDE) which reduces cGMP concentration (normally high in the dark)
-Na+ ion channels close and cell hyperpolarizes
-this system works through signal amplification
-provides incredible sensitivity to small amount of light (single photon)

Question 27

What is the difference of phototransduction under dark or light conditions?

Answer 27

-under dark conditions: photoreceptor is depolarized (-30mV) –> dark current; glutamate is released
-under light stimulation: photoreceptor becomes hyperpolarized (-65mV); glutamate release decreases as a proportion of the amount of light (graded response)

Question 28

What is a brief summary of phototransduction?

Answer 28

-rods are depolarized in the dark and release NT
-light hyperpolarizes the rod and reduces NT release
-light activates rhodopsin which activates transducin
-transducin activates PDE which reduces cGMP
-this closes Na+ channels (hyperpolarizes cell & reduces glutamate)

Question 29

What is phototransduction in cones like?

Answer 29

-different opsins:
–red - long wavelength
–green - medium wavelength
–blue - short wavelength
-each type of cone has a preferred wavelength but is activated by a broad range of wavelengths
-color perception: contributions of blue, green, and red cones to retinal signal
-Young-Helmholtz trichromacy theory of color vision

Question 30

What is Optogenetics?

Answer 30

-optogenetics is a method for controlling a neuron’s activity using light and genetic engineering
-harnesses the properties of naturally occurring photoreceptors to selectively control neural activity with light (high temporal control (msec))
-DNA from a light-sensitive protein is inserted and expressed in a neuron