ch. 50 vision Flashcards
what do most invertebrates have
light-detecting organ
simplest light-detecting organ
pair of ocelli called eyespots located near the head of planarians
what do eyespots of planarians do
allow planarians to move away from light and seek shaded locations (negative phototaxis)
what have ommatidia
insects, crustaceans, polychaete worms
compounds eyes
consist of several thousand light detectors called ommatidia
what are compound eyes effective at
detecting movement
color vision of insects
excellent - some can see ultraviolet range
where are single-lens eyes found
some jellies and polychaete worms, spiders, and many molluscs, vertebrates
how to single-lens eyes work
camera-like principle - iris changes diameter of the pupil to control how much light enters
choroid
thin, pigmented layer of the eye
what is just inside the choroid
retina with neurons and receptors
lens
transparent disk of protein
what is in front of the lens
clear and watery aqueous humor
what is behind the lens
jellylike vitreous humor
path of light through eyeball
- cornea
- aqueous humor
- pupil - controlled by iris constricted/dilating
- lens (living tissue)
- vitreous humor
- retina (photoreceptors, bipolar cells, ganglion cells)
- optic nerve
what are the ciliary muscle connected to
the lens by suspensory ligaments - holds lens in place
myopia
focused in front of the retina
hyperopia
focused behind the retina
fovea
center of visual field compact with tons of cones and contains no rods
what is most of the retina composed of
rods
2 types of photoreceptors in retina
- rods
- cones
where do the neurons of the retina relay visual information to
the optic nerve and brain
optic disk
place in retina where optic nerve attaches to eye, lacks photoreceptors and forms a blind spot
rods
more sensitive to light but don’t distinguish colors
cones
provide color vision
what do vertebrate visual pigments consist of
retinal bound to opsin
retinal
light-absorbing pigment
opsin
membrane protein
ex. of vertebrate visual pigment
rhodopsin
what does an absorption of light cause
change of shape in retinal
glutamate in light
no glutamate released
glutamate in dark
glutamate released as neurotransmitter from rods and cones to neurons called bipolar clues
when light hits retinal, what happens to its shape
changes from cis isomer (bent) to trans isomer (straight)
signal transduction in eyes
- light changes cis-retinal to trans-retinal
- trans-retinal activates rhodopsin
- G protein activated - transducin activated
- hydrolysis of cyclic GMP to GMP by phosphodiesterase
- sodium channels close
- cell hyperpolarized and rate of glutamate neurotransmitter release slows
what does cGMP do in the dark
bind to sodium ion channels and keeps them open
what happens when cGMP breaks down
Na+ channels close
- hyper polarizes cell
how is the signal transduction pathway usually shut off
enzymes convert retinal back to cis form
bleaching
- light strikes cis-retinal and turns it to trans
- bleaching occurs
- ATP to ADP used by enzyme converts trans back to cis
where does the processing of visual information begin
retina
what does the decrease in glutamate do
change membrane potential of bipolar cells
lateral inhibition
rod/cone stimulates horizontal cell, horizontal cell then habits more distant photoreceptors and bipolar cells
- regions receiving light appear lighter, dark surroundings appear darker
what does a single ganglion receive information from
array of rods and cone
- defines receptive field
what does a smaller receptive field typically result in
a sharper image
optic chiasm
where optic nerves meet near center of base of cerebral cortex
where are sensations from the left visual field of both eyes transmitted
to right side of brain and vise versa
what vertebrates have good color vision
fish, amphibians, reptiles, birds
what comprises the minority of mammals that can see color well
humans and primates
mammals with nocturnal vision
have high proportion of rods in the retina and probably see a pastel world during the day
what is the perception of color based off of in humans
3 types of cones
3 types of cones
red, green, and blue - different visual pigments (photopsins)
when are photopsins formed
when rental binds to 3 distinct opsin proteins
what does abnormal color vision result from
mutations in the genes for one or more photopsin proteins
what did researchers studying color vision in squirrel monkeys do
make breakthrough in gene therapy
- males are red-green color-blind
- when researchers introduced missing gene in virus, full color vision apparent
focusing
changing shape of the lens
near vision/accommodation
- ciliary muscles contract
- suspensory ligaments relax
lens becomes thicker and rounder
distance vision
- ciliary muscles relax
- suspensory ligaments pull against lens
- lens becomes flatter
