Systems Lecture 4 Flashcards
how many photoreceptors in retina
100 million
how many photoreceptors in afferent axons
1 million - convergence from retina to axon
binocular vision is what
two images- allows us to have depth perception
general flow of visual information
exceptions?
retina –> lateral geniculate nucleus –> area 17/ V1/ striate cortex –> 2 dozen other cortical areas
although some connections bypass thalamus (LGN) - although these ones do not help us see things
how light info goes through cell types in retina
photoreceptors (outside of retina; closest to brain) –(horizontal cells) –> bipolar – (amacrine) –> ganglion
photoreceptors to bipolar cells is what type of communication
chemical
ganglion cells communicate how
electrically - action potentials
cell type to cell type; antagonistic vs same activation
photoreceptor –> bipolar (antagnositic)
bipolar –> ganglion (same)
what is the purpose of lateral cell communication - horizontal cell and amacrine cells
they are “gate keepers” cam influence the degree of inhibitory or excitatory synapses by modulating release of neurotransmitters in the synaptic cleft
source of output from retina?
other communication in eye?
ganglion cells only - electrical communication through action potentials
- others hyperpolarize of depolarize depending on light - chemical communication - neurotransmitter release proportional to hyperpolarization and depolarization
lining in back of eye ____ is important for what
melanin - deflecting light - limits light scattering in back of retina
rods:
more sensitive for light - responsible for night vision (scoptic vision) - no colour - short- more rods than cones - lots of convergence in order to detect any light
cones:
longer than rods; less of them than rods; 3 photopigments (short, medium long responsive to different wavelengths); daytime vision (photopic vision)
- high acuity in central field bc there are many cones
fovea:
best acuity if light hits here; full of cones; no rods; pit in fovea to help reduce light defraction to photoreceptors/ cones and/or to minimize interference of light passing through other cells
mesopic
scoptic
photic
light conditions and corresponding photoreceptors
mesopic- both rods and cones; low lght
scoptic - night vision - rods
photic - light conditions- cones
high level of rods where on retina
periphery - why in dark you see more in your peripheral vision
spot with no photoreceptors in eye
blind spot- where optic nerve exits eye
rods or cones more convergence
rods - need to be very light sensitive - only need small amount of light to activate ganglionic cells
ON center bipolar cell receptor? how does this respond to the presence of light
metabotropic -
light–> cone, hyperpolarizes, no glutamate - they are no open and are excited
- called ON center because light turns them on - rapid firing
OFF center bipolar cell receptor? response to light?
ionotropic glutamate receptors (glutamate gated)- light –> photoreceptors –> hyperpolarize –> no glutamate–> cant open receptor –> inhibited
OFF bc light turns them off - reduced firing
in response to light, photoreceptors
light- hyperpolarize - no glutamate released
dark - depolarize - glutamate released
why is the receptive field surround antagonistic to receptive field center?
surround - bilateral connection by horizontal cells to bipolar cells - photoreceptor –> horizontal (same activation) horizontal –> bipolar (same activation)
inhibition of bipolar cell activation reduces firing, no firing only occurs…
in response to cancellation from surround when center is on - has to be equally covered
when dark completely coves the receptive field - surround and center - is the response cancelled?
no bc surround has more real-estate
P type ganglionic cells
smaller than M type
have a more prolonged response to light - sustained action potential firing as long as stimulus is present
90% prevalent; slower I optic nerve than M type
M type ganglioniccells
larger than P type cells 5% prevalent
fast transient response to light - sensitive to low light; faster in optic nerve than P type
where are M and P cells locate in visual system
function?
within on and off center surround environments; ganglion cells; communicate to bipolar cells - net output of convergence dictates whether cells become excited or inhibited
retinofugal projection
flees eye
afferent projection - goes towards CNS
three components:
1) optic nerve
2) optic chiasm
3) optic tract
decussation means
crossing over of info
types of visual fields
temporal and nasal hemifields - nasal is the part closest to nose, temporal is part closest to outside; for both right and left eye
if cut optic tract what would happen
lose half of vision on opposite side
left tract - right half vision
right tract - left half vision
if cut optic nerve what would happen
lose 1/4 vision, only vision that isn’t covered by another part of the eyes
cut optic chiasm what would happen
lose outside vision - sill have middle vision- no vision can cross over to be processed by correct part of brain
visual pathway generally
retina–> LGN –> optic radiation –> primary visual cortex (area 17)
what happens if you cut and damafe one side of thalaus and cover one part of eye
link to fear and stress gone - cat could see snake with one working eye and be scared. Cover that one up and use other working eye to see snake - now not scared
superior colliculus function
important for eye movement
-
____ % of info that comes from eye goes to superior colliculus through _____
10% via retinal tectum projection
Lateral Geniculate Nucleus
located in thalamus
- 6 curved layers (geniculatus) going from bottom up
- first two layers are magnocellular - stain heavily with nissel staining- parvo cells are III to VI (alternating and starting at layer III with ipsilateral)- input comes from retinal ganglion cells - from contralateral visual field (not contralateral eye); not true that left visual field goes to left eye and right visual field goes to right eye –> crossing over
- output to striate cortex
