Retina Physiology - UAB School of Optometry

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RETINAL PHYSIOLOGY
SOME basic rules
1) Photoreceptors hyperpolarize in response to the light coming on.
2) Depolarization leads to an increase in neurotransmitter release
3) Ganglion cells produce action potentials
on G cells spike at light onset
off G cells spike when the light is off
4) Photoreceptors, bipolar cells, horizontal cells convey information through graded
electrical potentials that are proportional to the stimulus they receive.
I. On and Off Pathways
Signals about increasing and decreasing light intensity are conveyed by separate ON and
OFF pathways.
Cones have synapses with both ON and OFF bipolar cells
ON = depolarization of bipolar cells to light
OFF = hyperpolarization of bipolar cells to light
A. OFF pathway
1. Dark
(a) Cone is depolarized/ Glutamate release is high
(b) Glutamate binds to receptors on OFF bipolar cells, opens ion channel
(c) OFF bipolar cell is depolarized
(d) OFF ganglion cell is depolarized
2. Light
a)
b)
c)
d)
Cone is hyperpolarized/ Glutamate release is reduced
OFF bipolar cell ion channels close
OFF bipolar cell is hyperpolarized
OFF ganglion cell is hyperpolarized (action potential frequency
declines)
B. ON pathway
1. Dark
a) Cone is depolarized/ Glutamate release is high
b) Glutamic acid binding at receptors of ON bipolar cell activates G
protein
c) G protein mediates closing of ON bipolar cell ion channels
d) ON bipolar cell is hyperpolarized
e) ON ganglion cell is hyperpolarized
2. Light
a) Cone is hyperpolarized/ Glutamate release is reduced
b) G protein is deactivated in absence of glutamic acid binding
c) ON bipolar cell ion channels open
d) ON bipolar cell is depolarized
e) ON ganglion cell is depolarized(spike, action potential freq. Increases)
f)
VS 112 Ocular Anatomy
UAB School of Optometry
Page 1 of 2
II. Rod Signals
A. Rod (on) bipolar cell: rod signals go to a single type of bipolar cell. The rod
bipolar cells do not make synaptic contact with ganglion cells directly. Instead,
rod bipolar cells go to AII amacrine cells whose output is onto the terminal end of
cone bipolar cells.
B. To leave the retina, rod signals must ride “piggyback” on the cone pathways.
C. This results in a high level of signal convergence in rods’ signaling pathway.
The flow of information is rod → rod bipolar cell → AII amacrine cell → cone
bipolar cell → ganglion cell
III. Receptive Field YOU NEED TO UNDERSTAND/DEFINE RECEPTIVE
FIELD AND RETINAL TILING
The retinal field of a neuron is the region of visual field within which a stimulus will
affect the neurons behavior.
A. Photoreceptor RF vs. ganglion cell RF
Photoreceptors -> Bipolar cells -> ganglion cells have increasing receptive
fields due to signal convergence.
B. Difference due to convergence in signaling pathways
A. Tiling (Pg. 654) = an array of discrete elements that covers a surface without gaps or
overlap (like your bathroom floor tiles).
One student description:
“Because the retina can not sketch in detail all of the visual input at one time, the
visual system responds by detailed sketching of only a small part at a time. This is done
at the fovea. The tiles, or retinal circuits are very small in/near the fovea and larger away
from the fovea, but, the retinal image is sampled over the entire retina. This is done by
arranging the vertical pathway neurons in a tile-like fashion (tiling). This allows for a
continuous, singular image of the visual world. “
The highest density of tiles is seen in the regular tiling by hexagons of the cone
inner segments of the fovea. The Muller cell cytoplasm serves as grout between the tiles.
The tiling size (density) varies across the retina because of the changing density and size
of the tiles—the rods and cones, but it is complete. The same is true of other nerve cell
types. For example there are tilings of midget and diffuse bipolar cells, as well as blue
cone bipolar and rod bipolar cells. The same is true of amacrine and ganglion cells as
well. Every type of physiological or anatomical subtype tiles the entire retina.
VS 112 Ocular Anatomy
UAB School of Optometry
Page 2 of 2
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