visual physiology Flashcards
what change light into electrical potentials? two types?
photoreceptors; rods and cones
where in the eye are photoreceptors located?
outermost layer of the neural retina
rods
detect low light (good light sensitivity) but bad visual acuity
no color discrimination
cones
good visual acuity and color vision, but bad sensitivity to light
ANATOMY: Both rods and cones have…
outer segment with stack of either membranous discs (rods) or infoldings (cones) which contain the photopigments that respond to light
discs in outer segment?
infoldings in outer segment?
rods
cones
Rods vs Cones which has more per retina? photopigment? distribution? color discrimination? acuity? wiring to ganglion cells? temporal resolution? light sensitivity?
rods –>almost 20x more
rods: rhodopsin, peripheral to fovea, no color discrimination, low acuity, highly convergent, poor resolution, high sensitivity
cones: 3 types of cone opsins, concentrated central in fovea, yes color discrimination, high acuity, less convergent, good resolution, low sensitivity to light
vision at…
low levels of illumination?
intermediate levels?
high levels?
scotopic vision
mesopic vision
photopic vision
scotopic vision
at low levels of light, rods only function
visual acuity is poor and no color perception
mesopic vision
middle levels of light, vision uses both rod and cone systems
improved acuity and poor color perception
photopic vision
high levels of light, vision is mostly just cone system
rod system is saturated and cant respond after that point
BEST visual acuity and color perception
photopic vision
high levels of light, vision is mostly just cone system
rod system is saturated and cant respond after that point
BEST visual acuity and color perception
photoreceptors are unusual in that…
their stimulus, light, produces hyperpolarization
photoreceptor mechanism (in the dark)
- cGMP gated Na channel is open and depolarizes receptor(rod or cone)
- receptor releases NT on bipolar cell
photoeceptor mechanism (in light)
- light causes cGMP Na channel to close
- Na/K atpase and K current hyperpolarize the receptor
- NT release is decreased
- more light = more hyperpolarization = less NT
flow of info:
roads and cones make direct synaptic contact with?
and in turn those make direct synaptic contact with?
bipolar cells
ganglion cells
bipolar cell –> ganglion cell is always _____?
excitatory
bipolar cell –> ganglion cell is always _____?
excitatory
multiple ___ can converge on a single bipolar cell; but only one/few ____ can converge on a single bipolar cell
rods
cones
greater _____ in the rod system allows greater light sensitivity but less _____
convergence
visual acuity
info pathway to brain
photoreceptors –> bipolar cells –> ganglion cells –> optic tract fibers synapse in the lateral geniculate body in the brain or the superior colliculus
On-center response to light
- light hyperpolarizes the photoreceptor
- decreases inhibitory NT released onto bipolar cell
- depolarization of the bipolar cell and increased excitatory NT released onto ganglion cell
- excited ganglion cell causes increased action potential firing rate
off-surround response to light
- light hyperpolarizes the off-surround receptor which hyperpolarizes adjacent horizontal cell causing less inhibitory NT onto on-center cone
- depolarization of the on-center receptor
- more inhibitory NT onto bipolar cell
- bipolar cell hyperpolarizes and releases less NT onto ganglion cell
- hyperpolarization of ganglion cell –> decreased firing rate
off-surround response to light
- light hyperpolarizes the off-surround receptor which hyperpolarizes adjacent horizontal cell causing less inhibitory NT onto on-center cone
- depolarization of the on-center receptor
- more inhibitory NT onto bipolar cell
- bipolar cell hyperpolarizes and releases less NT onto ganglion cell
- hyperpolarization of ganglion cell –> decreased firing rate
what is the function of the on-center/off-surround system?
when is it useful?
to sharpen boundaries between areas of light and dark by making the black adjacent to the border appear blacker, and white whiter
- –enhances visual acuity
- –ex: when reading b/w paper or reading in different light levels
does lateral inhibition in the oncenter.off surround system emohasize difference in illumination or the absolute illumination level?
the difference between two areas!
illuminate the on center?
off surround?
all in light? all in dark?
increeased ganglion firing
decreased ganglion firing
firing is the same – only changes in transition areas to recognize edges
3 cone types?
blue green and red
each type of cone on its own is________
color blind - cant tell difference between change in color or light intensity
which cone is least sensitive to light?
blue
which cone is intermediate in sensitivity?
green
cone that is most sensitive to light?
red
blue cone is most sensitive to___
wavelengths that allow us to see blue to violet end of spectrum
red and green cones are found throughout ____
the fovea, including center where fine detail is best
fine detail is best in the…
central fovea
blue cones are found___
in the fovea but not in the central part so they do not help with fine detail
central foveal vision is ____
dichromatic (red and green cones only)
absence of blue cones in central fovea solves what problem
chromatic abberration — where a blurred image is made due to the fact that the lens refracts blue light more than red and green
red-green color blindness
a single cone type is missing or non functional
cannot distinguish between red and green
–recessive, sex linked trait on x chromosome
more common in males
