Chapter 11: Image Processing by the Retina

Question 1

There are five basic classes of neurons in the retina:

Answer 1

photoreceptors, bipolar

cells, ganglion cells, horizontal cells, and amacrine cells

Question 2

There are five basic classes of neurons in the retina:

Answer 2

photoreceptors, bipolar

cells, ganglion cells, horizontal cells, and amacrine cells

Question 3

The retina contains two types of photoreceptors:

Answer 3

rods and cones

Question 4

Functional Ranges for Rods and Cones

Answer 4

Cons: Scotopic (no color vision)

Rods: Mesopic (Moonlight), Photopic (indoor lighting, sunlight)

Question 5

Phototransduction:

Answer 5

Absorption of light by the photopigment in the outer segment of the photoreceptors
initiates a cascade of events that changes the membrane potential of
the receptor, and therefore the amount of neurotransmitter released by the photoreceptor
terminals

Question 6

The processes of horizontal cells enables….

Answer 6

lateral interactions between photoreceptors and bipolar cells that are thought to maintain the visual system’s sensitivity to contrast, over a wide range of light intensities, or luminance.

Question 7

The processes of amacrine cells are……

Answer 7

postsynaptic to bipolar cell terminals and presynaptic to the dendrites of ganglion cells.

Question 8

Shining light on a photoreceptor, either a rod or a cone, leads to membrane hyperpolarization rather than depolarization

Answer 8

In the dark, the receptor is in a depolarized state, with a membrane potential of roughly -40 mV (including those portions of the cell that release transmitters).

Question 9

The selective response of on- and off-center bipolar cells to light incrementsand decrements is explained by the fact that they express different types of glutamate receptors

Answer 9

Off-center bipolar cells have ionotropic receptors (AMPA and kainate) that cause the cells to depolarize in response to glutamate released from photoreceptor terminals. In contrast, on-center bipolar cells express a G-protein-coupled metabotropic glutamate receptor (mGluR6).
When bound to glutamate, these receptors activate an intracellular cascade that closes cGMP-gated Na+ channels, reducing inward current and hyperpolarizingthe cell. Thus, glutamate has opposite effects on these two classes of cells, depolarizing off-center bipolar cells and hyperpolarizing on-center cells.

Question 10

Photoreceptor synapses with off-center bipolar cells are called sign-conserving.

Answer 10

since thesign of the change in membrane potential of the bipolar cell (depolarization or hyperpolarization) is the same as that in the photoreceptor.

Question 11

On-Center and Off-Center Cells are specialized

to detect different Images

Answer 11

In order to understand the response of on- and off-center bipolar cells to changes in light intensity, recall that photoreceptors hyperpolarize in response to light increments, DECREASING their release of neurotransmitter

Question 12

In order to understand the response of on- and off-center bipolar cells to changes in light intensity, recall that photoreceptors hyperpolarize in response to light increments, decreasing their release of neurotransmitter are freed from the hyperpolarizing influence of the photoreceptor’s transmitter, and they depolarize

Answer 12

In contrast, for off-center cells, the reduction in glutamate represents the withdrawal of a depolarizing influence, and these cells hyperpolarize. Decrements in light intensity naturally have the opposite effect on these
two classes of bipolar cells, hyperpolarizing on-center cells and depolarizing
off-center ones

Question 13

In order to understand the response of on- and off-center bipolar ceUs to changes in light intensity, recall that photoreceptors hyperpolarize in response to light increments, decreasing their release of neurotransmitter are freed from the hyperpolarizing influence of the photoreceptor’s transmitter,
and they depolarize

Answer 13

In contrast, for off-center cells, the reduction in glutamate represents the withdrawal of a depolarizing influence, and these cells hyperpolarize. Decrements in light intensity naturally have the opposite effect on these
two classes of bipolar cells, hyperpolarizing on-center cells and depolarizing
off-center ones

Question 14

Dark Surround Activates Center R cell

Answer 14

In a dark surround, outer R releases Glutamate.

Glutamate activates horizontal cells which releases GABA to hyperpolarize the center R cell.

When hyperpolarized the center R cell is more responsive to light.

Question 15

Bright Surround Inhibits Center R cell

Answer 15

In a bright surround, outer R stop releasing Glu.

Horizontal cells no longer releases GABA, resulting in depolarization of the center R cell.

When depolarized the center R cell is less responsive to light.