Retina- Vosko Flashcards
acuity vs sensitivity
more sensitive– less detail
more acuity– more detail
the job of the retina vs. the brain
The job of the retina is to break an image down into its component parts.
It’s the job of the brain to reconstruct it
Bipolar cells:
Segregation Point
1) “On” or “Off”
2) Connected to Rods or Cones
rod and cone segregation piont
color
sensitivity
acuity
retinal ganglion cells
Point of convergence where properties are coded by bipolar cells
(“on” bipolar cells synapse with
“on” retinal ganglion cells)
Dark current in receptor cells
is tonic.
Light causes hyperpolarization metabotropically
Hyperpolarization achieved by closing Na+ channels
when light hits, glutamate stops.
Rods and cones
contain photopigments with responses to light of particular wavelengths
Each photoreceptor cell has sensitivity to light of certain wavelengths, based on photopigment
Color discrimination works by comparing the response of two or more cones to the same light (except with S Cones)
*If there are two or more cone opsins, can perceive colors between the relative contribution of each
Primate retinas are organized specifically to have
increased acuity in photopic conditions
Convergence is stronger with rods, makes them more sensitive but less acute
This ratio of cones:bipolar cells is 1:1 in the fovea, where there are no rods, makes them more acute but less sensitive
cones are key for
acuity because there is less convergence.
the problem of cones
not many of them in relation to the rods.
to overcome it? must amp up the stimulus– need lots of light. This is why we can’t see the details in low light.
rods do what?
increase visual sensitivity to low-light conditions using both vertical AND horizontal integration
photopic vs scotopic
photopic environment- lots of light
scotopic- dark
mesopic- moonlight
what cells are contacted in what kind of light?
Horizontal cells are contacted in scotopic light
Both amacrine and horizontal cells are contacted in mesopic light
ribbon synapses
are specialized in neurons that release neurotransmitter tonically AND without Action Potentials (APs)
Orthogonal, planar rows to presynaptic membrane
Graded release of NT
Found in photoreceptors and bipolar cells
Release large amounts of NT
Quickly replenished
ON bipolar cells
¾ of all bipolar cells hyperpolarize to glutamate synapse with ON ganglion cells all rods, some cones Rod bipolar cells do not directly contact ganglion cells mGluRs (mGluR6)
ALL ROD BIPOLAR CELLS ARE ON BIPOLAR CELLS
OFF bipolar cells
depolarize to glutamate
synapse with OFF ganglion cells
some cones
(AMPA/kainate)
How can bipolar cells either hyperpolarize or depolarize to glutamate, an excitatory neurotransmitter?
mGluR6 is specifically found postsynaptically on ON bipolar cell dendrites
AMPA/Kainate channels are found on OFF
Recording APs from retinal ganglion cells (RGC’s) show
that they have a center-surround receptive field
Bipolar cells either hyperpolarize or depolarize to light–>
RGCs share the same response as the bipolar cell type that innervates them
Horizontal cells are hyperpolarized by light and accentuate contrasts
Different types of bipolar cells also encode
the duration of a light stimulus
amacrine cells
further modify the responses of RGCs
30 different types
Release GABA or glycine onto bipolar cells and RGCs
Also contribute to the “surround” response of the center-surround RGC
Co-release other neuropeptides
Necessary to detect motion in the retina
wide field amacrine cells
subtract background noise from moving objects
Starburst amacrine cells
respond to signals only in light movement across dendritic fields in a particular direction
retinal ganglion cells
are the final common pathway of the decomposed image as it will be carried into the brain via the optic nerve
melanopsin
a circadian photoreceptor?
sits in retinal ganglion cells (these ones respond to light themselves! transmit a non-image signal to other parts of the brain for circadian rhythm entrainment, levels arousal, etc.)
melanopsin, a vitamin-A based opsin photopigment
similar to insect photopigments
from frog melanophores
melanopsin localized to RGC layer of rodents and primates
2 functionally distinct light detection systems in mammalian retina
image-forming and circadian.
masking
pupillary light reflex
phase shifting
