Photoreceptors Flashcards
where are the photoreceptors located?
in the retina
what is the conversion of light to electrical signals called?
phototransduction
What are the two main photoreceptors in the retina?
cones and rods
does our eye contain more cones or rods
rods
T/F. photoreceptors release action potentials?
false they respond to stimuli with graded membrane potentials
basic structure of cones and rods
their membrane folds into disk like layers which contain visual pigments
the inner segment contains the nucleus and organelles for protein synthesis
in the basal layer the synapse release glutamate
which direction do both cones and rods point to
the back of the eye
how do photoreceptors detect light
they use membrane bound visual pigments.
when light hits these pigments they change shape, causing a chemical cascade that hyperpolarizes the cell reducing glutamate release
why are photoreceptors more active in the dark?
because there is less light hitting the pigments, depolarizing the cells triggering more release of glutamate
why is glutamate release important in the eye?
because it helps transfer visual information
what is the pigment found in rods?
rhodopsin
how many pigments are in cones
3
how are photoreceptors distributed in the eye
not uniformly, instead they are packed in the macula a central disk and also packed tightly in the fovea
fovea
the fovea is used for detailed vision
are there photoreceptors in the blind spot?
no, this is where axons carrying visual information exit the eyeball to form the optic nerve
What type of light are cones used for?
bright light because they are less sensitive than rods
they distinguish colours but don’t operate in dim conditions
what type of light are rods used for?
dim light, in sunlight they are bleached out, meaning that their rhodopsin is broken down
does the fovea contain an even amount of cones and rods?
no fovea contains almost only cones
Where is the rod presence concentrated?
in the peripheral retina
what are the three layers of neurons in the retina
photoreceptors synapse on the bipolar cells and those synapse on the ganglion cells
where is convergence strongest in the eye? Where is it weakest?
strongest in the peripheral retina and weakest in the fovea
receptive field
every neuron in the visual system has a receptive/visual field. which is the region of the retina where the light affects the cell’s activity- the set of photoreceptors which affect the cell
what type of receptive field do bipolar cells have?
center-rounded fields with round center region and doughnut shaped surround
what does it mean that bipolar cell receptive field can be on/off center
on center means the cells are excited by light in the center of field and inhibited by light in the surround.
off center means cells are inhibited by light in the center and excited by light in the surround
what do on center cells respond the most to? off-center
on-center cells respond most when light fills their center and the surround is dark.
off center respond best when a dark spot fills their center and the surround light
what do bipolar cells react the strongest to?
when there is a contrast in light
Do bipolar cells fire action potentials?
no, they respond with graded membrane potentials
what is the major difference between ganglion cells and bipolar cells and photoreceptors
ganglion cells can fire action potentials
how are ganglion and bipolar cells similar
ganglion cells can also be off/on centred meaning it responds well to contrast
what does this emphasis on contrast explain
the chevreul illusion where left right hand bands seem darker than the middle even though it is the same color throughout
where are ganglion cells most sensitive
in the periphery but poor at reporting spatial detail
where are ganglion cells less senstive
near the fovea but they have better spatial resolution
Magnocellular ganglion cells
provide information that is used by brain to infer movement- 10% of ganglion cells are M cells
Parvocellular ganglion cells
provide information that is used to infer form and fine detail like texture. 70% of ganglion cells are P cells
melanopsin ganglion cells
these are photoreceptors that have their own visual pigment- melanopsin- they project to suprachiasmatic nucleus a center for circadian rhythms
how does visual information move>
ganglion cells> retina> brain
optic chiasm
the point where the visual information crosses to the other side.
Information from the nasal half of each retina cross the optic chiasm
T/F. both nasal and temporal fibers cross the chiasm.
false only nasal fibers cross
Why do fibers cross
because the right side projects to the left side and vise versa
where does information move after crossing the chiasm?
goes from chiasm > thalamus> cortex
what are the nerve bundles emerging from the chiasm called? Where do they end
the optic tracts. they end in the 2 lateral geniculate nuclei (LGN) in the thalamus
where does LGN project
to the primary visual cortex (V1) via optic radiations
where is the V1 located
in the occipital lobe
how are visual areas in the brain organized? what does that mean?
retinotopically, meaning that neurons close to each other get information from close together parts of retina
why does the fovea get a lot of space?
because it has many photoreceptors, bipolars, and ganglion cells, carrying lot of information
fill in the blank.
a large proportion of visual cells in the brain receive and process data from ______ photoreceptors.
foveal
