lecture 14 relaying visual info from eye to brain Flashcards
what do photoreceptors do in response to light
hyperpolarize and release less glutamate
what do on bipolar cells do in response to light
depolarize and release more glutamate
what do off bipolar cells do in response to light
hyperpolarize and release less glutamate
what do photoreceptors do in response to dark (and what do the on and off bipolar cells do)
depolarize and release more glutamate (on cells inverse the signal so they release less, and off cells do the same as photoreceptors so they release more too)
what neurotransmitter is released by photoreceptors and how is the the response different between bipolar cells despite the same neurotransmitter
glutamate
on cells have mglur6
(inhibitory, metabotropic)
off cells have ampa kainate (excitatory, ionotropic)
what cells do bipolar cells synapse onto
retinal ganglion cells (rgc)
retinal ganglion cells
output neurons in retina
only cells in retina that fire true action potentials (have to send electric signal from eye to brain)
have two types; on and off
what is the optic nerve made of
axons of retinal ganglion cells
action potentials
large enough membrane potential change that can be recorded without sticking electrode in
how did stephen kuffler learn about recording responses of retinal ganglion cells
experimental animal is anesthetized and positioned facing a screen
electrode positioned near a retinal ganglion cell to record action potentials (lots of spontaneous activity)
shine spots or other shapes on screen and record frequency of action potentials
what is this
spontaneous background activity of retinal ganglion cells, basically baseline
region or characteristics of the
sensory space that elicits the greatest activity (action
potentials or largest graded potentials in the retina) from either a sensory cell or neuron within the CNS. For neurons in the retina, receptive filled properties include
the area of the visual field (location in space) where light (or dark) would hit the neuron
receptive field
neurons that are locating near to each other
have receptive fields that are near each other (retinotopic map)
receptive field of a specific neuron
sometimes includes other aspects of the stimulus like color, movement, etc
receptive fields tend to
enlarge and increase in complexity as information passes through the brain
neurons in the eye respond to-
a relatively basic stimulus: light on and off
receptive field of retinal ganglion cells
dim light: background (spontaneous) activity
bright spot: response
another bright spot nearby: response
distant bright spot: no response, not part of receptive field
what happens if we enlarge stimulus
bright spot of light: response
bigger bright spot: more response
huge bright spot: firing rate back to spontaneous baseline
spot near receptive field (right outside, right above, right below)
small inhibition
bright annulus (donut)
big inhibition, neuron stops firing for some time
what does this experiment with the lights tell us
receptive field not just a single location of light
consists of center and surround parts of receptive field
on and off response of visual neurons
both bipolar cells and rgcs have center surround type receptive fields
if you cover entire receptive field with uniform light or dark
go back to baseline- they’re balanced
rgc’s will remain near resting firing rate
bipolar cells will not change membrane potential (graded response)
lateral pathway
responsible for surround
horizontal cells at synapse between photoreceptors and bipolar cells
amacrine cells at synapse between bipolar cells and rgcs
direct pathway (the center)
photoreceptors to bipolar cells to retinal ganglion cells to optic nerve
horizontal and amacrine cells
both inhibitory
release gaba
linked via gap junctions forming a network
can also help adjust entire retinal circuit response to changing levels of illumination
—a page of paper will look white inside or illuminated by a flashlight and white when out in bright sun even though number of photons is different
visual systems cares about…
relative light intensity, not absolute
lateral inhibition
neurons are inhibited when neighbors are active
helps with contrast
each photoreceptor and interneuron can be part of a center for one rgc and a surround for another rgc
what does center surround organization do
enhances sensitivity to edges and contrast (which is why we can interpret one line drawings, visual field is used to finding edges)
parallel output pathways from the retina
retina doesnt send point by point intensity and color information to brain
high processed- breaks up visual world in many different features
around 30 different types of rgcs
-color center/surround (red center, green surround for example)
-prefer moving stimuli
-large or small receptive fields, etc
m type retinal ganglion cells (magnocellular)
larger
color insensitive receptive fields
respond to stimulation with a burst of action potentials
often fire rebound action potentials
(cell fires after being inhibited for a while)
p type retinal ganglion cells (parvocellular)
smaller and smaller receptive fields
large majority of rgcs
often color specific receptive fields
respond to prolonged stimulation with sustained action potential rate
retina
the innermost layer of the eye and is Itself a layered structure that contains the visual
sensory neurons, circuitry for the initial processing of visual information, as well as neurons that
transmit that information to the brain. But the neurons of the retina do not simply detect light and pass
that message on, a great deal of processing and filtering occurs in the retina, before the visual
information reaches the brain
Photoreceptor
visual sensory cell that
converts light into electrical signals. Located at the innermost layer of the retina. Photoreceptors are depolarized in the dark and hyperpolarize when activated by light
Bipolar cell
excitatory neuron in the retina
that transmits information from the
photoreceptors to the retinal ganglion cells and amacrine cells. Bipolar cells do not fire action potentials, but have a graded release of neurotransmitter
Horizontal cell
inhibitory neuron in the retina whose actions influence the signals that are transmitted from the photoreceptors to the bipolar cells. They make inhibitory, GABAergic
synapses on the synaptic terminals of photoreceptors
Amacrine cell
inhibitory/modulatory neuron whose actions influence the signals that are transmitted from the bipolar cells to the retinal ganglion cells
Retinal ganglion cell (RGC)
Output cell of the retinal that transmits information from the eye to the brain. Only type of neuron in the retina that fires action potentials. There are both “on” and “off” that can be further divided into many different subtypes of RGCs based on specific receptive field
properties (color, size of receptive field, etc
Center/surround
In the retina, both bipolar cells and
RGCs have center surround receptive fields. For an “ON cell”, that means that the cell is excited by center stimulation (i.e. light/“on”), inhibited by light in the surround, and would be more optimally stimulated by dark in the surround. These receptive fields are
very useful because they enable cells to detect more than the brightness of a spot of light, but rather detect contrast (edges or boundaries between light and dark)
In the visual system, center/surround receptive fields are first found in the bipolar cells of the retina. The center of the
receptive field is formed by the direct synapses between
photoreceptors and the bipolar cell while the surround is caused
by the inhibitory actions of horizontal cells being activated by
the surrounding photoreceptors
OFF bipolar cell
bipolar cell that is inactivated by light (activated by dark). Their dendrites contain excitatory
ionotropic (AMPA type) glutamate receptors. Since photoreceptors release more glutamate in the dark, “OFF” cells also release more neurotransmitter in the dark and less neurotransmitter in the light
ON bipolar cell -
bipolar cell that is active by light. Their dendrites contains inhibitory mGluR6 type glutamate
receptors, thus they invert the signal the photoreceptors send.
Since photoreceptors release less glutamate in the light, “ON”
cells invert that signal and release more NT in the light
ON RGC
retinal ganglion cell that fires more action potentials when light is in the center. Receives excitatory
synaptic input from ON-bipolar cells via AMPA type glutamate
receptors
OFF RGC
retinal ganglion cell that fires fewer action potentials when light is in the center. Receives excitatory
synaptic input from OFF-bipolar cells via AMPA type
glutamate receptors
Illustrate how the major cell types of the retina are connected to each other and the function of
each
Explain how shining light on either the center or the surround of an on-center (or off-center)
ganglion cell’s receptive field differentially affects the cell’s activity and how that response may
differ from an on or off-center bipolar cell.
Define receptive field and explain why a center-surround receptive field enhances sensitivity to
contrast and edges
the science u
phenomenon
reduction
synthesis
two important lessons from the retina
hierarchical processing
parallel processing
hierarchical processing
the receptive field properties get more complicated as the signal moves from photoreceptors to retinal ganglion cells
parallel processing
streams of information related to properties of the visual scene are dealt with simultaneously by different circuits
properties of visual world that visual system encodes
shape
color
position
movement
what doesnt the visual system encode
we sense relative not absolute intensities- how bright is it compared to its surrounding, even if theyre the same one will look darker/brighter
movements faster than 30fps per second
movements slower than minute hand on a watch
wavelength that are infrared or lower wavelengths (microwave)
wavelengths that are uv or shorter
lights brighter than some saturating photon flux
light that is dimmer than some very low photon flux
vertical depth disparity
retinofugal projection
pathway of optic nerve to brain called retinofugal
has 5 parts before reaching visual cortex
r and l optic nerves cross at
optic chiasm
two sides of retina
temporal (close to temple)
nasal (closer to nose)
visual field
right visual field comprised of right nasal and left temporal
left visual field comprised of left nasal and right temporal
temporal part
does not leave the side of visual field it’s on
optic chiasm
site of a decussation (crossing) so that the L visual field from both eyes projects to the R cortical hemisphere and R field to left hemisphere. the nasal part of retina crosses to the contralateral side, the temporal part remains on ipsilateral side
after the chiasm, the right and left optic nerves are called
optic tracts, and contain ipsilateral temporal and contralateral nasal rgc axons
lateral geniculate nucleus (lgn)
nucleus of thalamus dealing with vision
thalamus takes in sensory info, but lgn is focused solely on vision
is in dorso lateral part of thalamus
left lgn is right visual field
right lgn is left visual field
has 6 layers, each layer filled with cells
retinal axons that stay ipsilateral (from temporal side of retina) project to 3 layers of lgn (2,3,5)
contralateral (nasal lateral) projects to other layers, (1,4,6)
optic radiations
r and l optic radiations exit thalamus to primary visual cortex (area 17, v1, or striate cortex) of r and l occipital lobes of cerebral cortex
nissl stain
stains material with nucleic acid in it
ventralmost layers 1 and 2
contain larger neurons (magnocellular) for motion
dorsalmost small cells
smaller cells, parvocellular, for shape
ventral to each layer of lgn
numerous tiny neurons, koniocellular, for color
three functional streams in lgn
magnocellular
parvocellular
koniocellular
magnocellular
layers 1 and 2 of lgn
larger neurons
motion
m type rgc
parvocellular
layers 3, 4, 5, 6
smaller neurons
shape
p type rgc
konoiocellular
ventral to each layer of lgn
numerous tiny neurons
color
non m non p rgc
m type, p type, nonm-nonp
retinal ganglion cells thought to innervate 3 lgn cell types respectively
parallel processing
rather than doing each separately, retina is doing all three at once
cortiofugal pathway
retina is not main source of synaptic input to lgn
80% of excitatory synapses in lgn are from primary visual cortex
