Vision I Flashcards
What is light?
-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
What are the different parts of the eye?
-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
What are the parts of the retina?
-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)
What are the characteristics of the fovea?
-2 mm in diameter (pit)
-no optic fibers
-high concentration of cones
-most acute vision (details)
How does refraction/focus work?
-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
How close to the eye is too close?
-6.5cm from the eye
-Presbyopia –> hardened lens, insufficient accommodation
What are 2 common kinds of eye conditions and what is the lens accommodation?
-Myopia: short-sighted; eye-ball too long –> concave lens
-Hyperopia: far-sighted; eye-ball too short –> convex lens
What is the Laminar Organization of the Retina?
-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
What are 3 important points to consider about the organization of the retina?
- Photoreceptors are the only light-sensitive cells in the retina
- Ganglion cells are the only output from the retina
- Ganglion cells are the only retinal cells to fire action potentials
What are the characteristics of outer segments of photoreceptors?
-contains a stack of membranous disks
-light-sensitive photopigments in those disk membranes absorb light
-this triggers changes in the photoreceptor membrane potential
What are some characteristics of cones?
-do most of the work under daytime lighting
-3 types of cones each with different photopigment
What are some characteristics of rods?
-more disks –> x1000 more sensitive to light
-only one type of photopigment
What is the photoreceptor distribution across the retina?
-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
What is the difference between central and peripheral retina?
-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
How do rods react to light?
-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
Who does most of the work in daylight conditions?
-under daylight conditions cones do the work (fovea)
–3 different photopigments (instead of rhodopsin): red, green and blue (RGB)
What happens when cones are inactive?
-we cannot see color when cones are inactive:
–inability to distinguish wavelength
–poor color discrimination in our periphery
How long does dark adaptation take?
-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)
What are the 4 steps of dark adaptation?
-pupillary dilation (2-8mm)
-rhodopsin regeneration
-functional circuitry of the retina
-sensitivity to light increases x1,000,000
How does adaptation work and how could we reduce the delay?
-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)
What are the characteristics of Central Retina?
-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)
What are the characteristics of Peripheral Retina?
-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
What are the characteristics of Duplex Retina?
-contains two co-existing systems specialized for either light or dark conditions
What are the beginning processes of phototransduction?
-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)
What is the process that stimulates transducin?
-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)
What does transducin activate?
-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)
What is the difference of phototransduction under dark or light conditions?
-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)
What is a brief summary of phototransduction?
-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)
What is phototransduction in cones like?
-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
What is Optogenetics?
-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
