T or F?
Hyperpolarization of photoreceptor (PR) cells by light results in decreased release of glutamate.
Light stimulation of opsin pigment in PR cells leads to opening or closure of Na+ channels?
Closure of Na+ channels
Light hyperpolarizes and dark depolarizes
Are OFF cells excited or inhibited by glutamate?
ON bipolar cells become ______polarized by glutamate, resulting in ___creased glutamate release
ON bipolar cells become ______polarized by LESS glutamate, resulting in ___creased glutamate release
ON ganglion cells are activated in the _________ (presence/absence) of light in the center of the retinal field.
OFF ganglion cells are _________ (activated/inhibited) in the presence of light in the center of the retinal field.
With darkness in the center of the RF, photoreceptors release more/less glutamate?
In darkness, OFF bipolar cells respond to increased glutamate by releasing ______ (more/less) glutamate. This is _________ (excitatory/inhibitory) to the OFF ganglion cell.
In the LIGHT, ___ bipolar cells are inhibited and ___ bipolar cells are excited.
OFF - inhibited
ON - excited
In the DARK, ___ bipolar cells are inhibited and ___ bipolar cells are excited.
ON - inhibited
OFF - excited
Is there an active or a passive response to darkness in the retina?
What is a receptive field?
Circular area of retina organized into concentric rings. Inner receptors (receptive field center) activate ganglion cells one way and surrounding region (receptive field surround) activates ganglions in the opposite way.
What is the difference between ON-center retinal ganglion cells and OFF-center retinal ganglion cells?
ON-center= respond to light spots on a dark background OFF-center= respond to dark spots on a light background
What is the difference between a direct and an indirect pathway of signal transmission in the retina?
Direct = PR cell (central) --> Bipolar cell Indirect = PR cell (peripheral) --> Horizontal cell --> Bipolar cell
Light shown onto receptive field surround will do what to ON-bipolar and ON-ganglion cells?
What do horizontal cells do?
Reverse the activity of the receptors in the center. Lateral inhibition.
What is the dual effect of shining light onto the surround receptive field?
- Light in the surround area stimulates horizontal cells to release glutamate to activate OFF cells
- Light activates horizontal cells that inhibit the ON cells that normally respond to light
Why is there little activity of ganglion cells in a receptive field whose center and surround are both fully exposed to light?
There is no contrast.
The center will react to light (ON). The surround will inhibit this response (activate OFF, inhibit ON). The result is little activity.
(same thing but opposite for all dark)
How do you maximize firing of an ON-center OFF-surround ganglion cell receptive field?
Expose as much as possible of the center to light. Expose as much as possible of the surround to dark (without causing dark over the center).
Center ON cells will fire with light. Surround OFF cells will be inhibited and ON cells will be more activated (by horizontal cells).
What do center-surround fields enable ganglion cells to do?
Encode contrast. Rate of activity indicates amount of contrast between surround and center part of its RF.
What kind of visual feature enhances retinal ganglion cell recognition of contrast?
How do ganglion cells code color?
Reciprocal excitation-inhibition by various cones with different pigments differentiates colors. Ganglion cells are stimulated by one color and inhibited by others.
i.e. Red ON center, Green OFF surround form a red/green ganglion cell RF.
Rods are primarily found in the peripheral part of retina and code for color.
Rods are in periphery but do not code for color
Cones are only found in the fovea
Cones are primarily found in the fovea but there is a thin layer of cones spread out over the rest of the retina.
Why does shining a light over an entire ON-center OFF-surround receptive field result in low activity/firing of ganglion cells?
The ON-center cells will be activated by light. But the OFF-surround cells will reverse that because the horizontal cells will cause OFF cells to be activated and ON cells to be inhibited. This negates the ON cell firing from the center.