vision objectives - michael Flashcards
no objectives
2) Stages of visual perception
a) Light collection: light is collected by specialized neurons called photoreceptors
b) Transduction: photoreceptors convert light to an electrical signal
c) Neural processing – retina: some information extracted before electrical signals are sent through the optic nerve to the lateral geniculate nucleus
d) Neural processing – visual cortex: various cortical areas extract additional information
2) Stages of visual perception
a) Light collection: light is collected by specialized neurons called photoreceptors
b) Transduction: photoreceptors convert light to an electrical signal
c) Neural processing – retina: some information extracted before electrical signals are sent through the optic nerve to the lateral geniculate nucleus
d) Neural processing – visual cortex: various cortical areas extract additional information
a) Light collection:
light is collected by specialized neurons called photoreceptors
b) Transduction:
photoreceptors convert light to an electrical signal
c) Neural processing – retina
some information extracted before electrical signals are sent through the optic nerve to the lateral geniculate nucleus
d) Neural processing – visual cortex
various cortical areas extract additional information
3) Compound versus refractive eyes
a) Compound eye: large number of collectors funnel light onto sheet of receptors
i) Found in insects
ii) Sensitive to movement at any distance
iii) Operates over a wide range of wavelengths (can see UV light)
iv) Sensitive to polarized light
b) Refractive eye: image is formed on retina by refraction through lens
i) Found in vertebrates
ii) Maximal resolving power
c) To have a compound eye with the same resolving power as your eye, you would need a compound eye about 3 feet in diameter (probably not important)
a) Compound eye:
large number of collectors funnel light onto sheet of receptors
i) Found in insects
ii) Sensitive to movement at any distance
iii) Operates over a wide range of wavelengths (can see UV light)
iv) Sensitive to polarized light
b) Refractive eye:
image is formed on retina by refraction through lens
i) Found in vertebrates
ii) Maximal resolving power
4) General structure of the eye:
a) Outer layer: sclera (outer white layer) and cornea (clear, major refractive element of the eye); limbus is the junction of the cornea and sclera
b) Choroid layer: consists of the pigment epithelium and blood vessels supplying oxygen and nutrients to the retinal cells
c) Retina: photoreceptor cells responsible for detection and processing of light signals
a) Outer layer:
sclera (outer white layer) and cornea (clear, major refractive element of the eye); limbus is the junction of the cornea and sclera
b) Choroid layer:
consists of the pigment epithelium and blood vessels supplying oxygen and nutrients to the retinal cells
c) Retina
photoreceptor cells responsible for detection and processing of light signals
5) Passage of light through the eye
a) Cornea
b) Anterior chamber: after passing through the cornea, the light passes through the aqueous humor (fluid in the anterior chamber).
i) Aqueous humor is produced by ciliary epithelium.
ii) Aqueous humor drains to the venous circulation via the canal of Schlemm.
(1) Blockage of this canal is a cause of glaucoma, which is optic nerve damage and loss of peripheral vision
c) Iris and pupil: pupil is the aperture, pupil size is controlled by iris.
d) Lens: adjusted to allow for focus; changing the refractive epower of the lens is accommodation
i) Degree of tension in the lens is controlled by ciliary muscles (parasympathetic nervous system)
ii) Cataracts are caused by opaque spots in the lens
iii) Lens is made of lens fibers: long thin cells filled with long, fibrous, water-soluble proteins called crystallins
e) Retina: light next passes through vitreous humor then hits the retina. Light is either absorbed by photoreceptors or passes through and hits the pigmented epithelium lining the back of the eye
i) Epithelium composed of cells with melanin (black pigment); this prevents the light from reflecting back and distorting the image
f) Fovea: the part of the retina on which light is usually focused: has more cones than rods, has high density of photoreceptors, less distortion than in other regions
g) Optic disk/”blind spot”: a “gap” in the retina where the optic nerve leaves the eyeball (no photoreceptors)
h) Tapetum: a layer of guanine crystals in some nocturnal animals that reflects the light off the back of the eye and gives the retina a second chance to capture the light
a) Retinal detachment:
happens at the junction between the pigmented epithlium and the photoreceptor layer; photoreceptors move out of the right focal plane and become estranged from their blood supply
b) Macula degeneration
most common visual deficit in the elders (6 million cases in US), involves loss of pigmented epithelial cells and eventually photoreceptors. “Wet” (tissue degradation) and “dry” (deposits of protein and lipid aggregates called “drusen”) forms
c) Retinitis pigmentosa
photoreceptor degeneration
order of cells in retina
photoreceptor, bipolar, horizontal, amacrine, ganglion cells
7) Photoreceptors
a) Synaptic terminal: for transmitter release
b) Inner segment: nucleus and cell machinery
c) Outer segment: considered modified cilia, filled with membranous disks containing photopigments.
(1) Disks detect light
d) Rods are more sensitive than cones in detecting light (more photopigment, longer cells)
e) Rods mediate night vision (lower threshold for excitation; loss of rods results in night blindness)
f) Cones have better temporal and spatial resolution (less convergence in the cone system)
g) Rod vision is achromatic; cones are responsible for color vision
8) Physiology of photoreceptor cells
a) Reponse to light is always hyperpolarization
b) In the dark, at rest, there are cation-selective channels open that let Na and Ca in; these channels bind cGMP
i) This creates a steady inward current of cations called “dark current” and keeps the membrane potential ~-40mV instead of -70 or -80mV
c) In response to light, the cation-selective channels close, the membrane hyperpolarizes, less Ca enters the cell, and less transmitter (glutamate) is released
d) Light is absorbed by the protein rhodopsin, which isomerizes and binds to transducin (a G protein)
e) Transducin binds GTP and activates phosphodiesterase, which hydrolyzes cGMP resulting in less cation channels remaining open
f) Rhodopsin kinase phosphorylates rhodopsin and promotes its association with the protein arrestin. This stops the hydrolyzation of cGMP.
i) cGMP inhibits rhodopsin kinase
g) Vitamin A is the precursor of retinal, which along with opsin make up rhodopsin; vitamin A deficiency leads to night blindness
h) Guanylate cyclase is inhibited by calcium. In response to a period of sustained light (walking from a dark to a light room), hyperpolarization of the cell leads to decreased intracellular Ca, which will increase guanylate cyclase activity, increased intracellular cGMP, increased binding of cGMP to cation-selective channels, and adaptation to the new balance that is reached by the new amount of guanylate cyclase activity.
Color vision
a) Different opsins have different optimal wavelengths at which particular opsins maximally absorb light
b) Dichromats express two different opsins (humas are trichromats)
c) The blue opsin is on chromosome 7 while the green and red opsins are on chromosome X. This explains why green and red variations are X-linked while blue variations are autosomal
d) Protanopia – red color blind
e) Deuteranopia – green color blind
f) Tritanopia – blue color blind
