Chapter 50

Question 0

Where is rhodopsin found?

Answer 0

Rods

Question 1

Give the layers of the cells from the anterior portion of the retina to the posterior portion

Answer 1

ganglion cells –> amacrine –> bipolar –> horizontal –> photoreceptors (rod/cones)

Question 2

What is the fxn of rhodopsin?

Answer 2

When light energy gets to the rhodopsin, it causes its decomposition to scotopsin and
retinal. This breakdown causes the transmission of the action potential to the
optic nerve. There are a series of decomposition steps, but the important one to know is that metarhodopsin II is the activated rhodopsin and is when the action potential is sent down

Question 3

Night blindness cna be caused by a deficiency in what vitamin?

Answer 3

vitamin A

Question 4

What is the mechanism of rod excitation and phototransduction?

Answer 4

When a rod is exposed to light, it hyperpolarizes to be stimulated (this is the
opposite of almost every other sensory receptor). A hyperpolarization occursbecause when rhodopsin decomposes, it decreases the rod membrane
conductance for sodium ions in the outer segment of the rod, however the
sodium pump is still functioning, pushing all the sodium out of the cell. The
greater the amount of light energy, the more negative the rod becomes.

Question 5

What are the 5 steps to phototransduction?

Answer 5

1. Light photon converts rhodopsin to metarhodopsin II (activated
   rhodopsin)
2. Activated rhodopsin activates transducin
3. Transducin activates phophodiesterase
4. Phosphodiesterase hydrolyzes cGMP (the chemical that holds sodium
   channels open in the dark) and causes a decrease in sodium conduction
5. Rhodopsin kinase inactivates rhodopsin and entire cascade reverses

Question 6

What is light adaption?

Answer 6

If a person is in bright light for many hours, large portions of the photochemicals in both the rods and the cones will have been reduced.
Much of the retinal of both the rods and the cones will have been converted to vitamin A. The sensitivity of the eye to light is correspondingly reduced

Question 7

What is dark adaptation?

Answer 7

If a person remains in darkness for many hours, the rods and cones have relatively high levels of light sensitive pigments. Furthermore,
vitamin A is converted back into retinal to give more light sensitive pigments.
The sensitivity of the eye to light is correspondingly increased. Cones adapt
much more quickly than rods, but do not achieve anywhere near the same
amount of sensitivity change as the rods do in darkness.

Question 8

What are the 3 spectral sensitivites to each cone?

Answer 8

Different cones are sensitive to different colors of light (red, blue and green). The spectral sensitivities of the 3 types of cones in humans have proved to be
essentially the same as the light absorption curves for the 3 types of pigment
found in the cones.

Question 9

What is color blindness?

Answer 9

Color blindness is when a singlegroup of color receptive cones is missing from the eye

Question 10

What are the roles of the rods/cones?

Answer 10

transmit signals to the outer plexiform layer where theysynapse with bipolar cells and horizontal cells

Question 11

What are the roles of the horizontal cells?

Answer 11

transmit signals horizontally in the outer plexiform layer fromthe rods and cones to the bipolar cells

Question 12

What are the roles of bipolar cells?

Answer 12

transmit signals vertically from the rods, cones and horizontal
cells to the inner plexiform layer, where they synapse with ganglion cells andamacrine cells

Question 13

What is the role of amacrine cells?

Answer 13

transmit signals in 2 directions, either directly from bipolar cells to ganglion cells or horizontally within the inner plexiform layer fromaxons of the bipolar cells to dendrites of the ganglion cells or to other
amacrine cells.

Question 14

What are the roles of ganglion cells?

Answer 14

transmit output signals from the retina through the optic nerve into the brain

Question 15

What are the roles of the interplexiform cells?

Answer 15

transmit signals in a retrograde direction from the innerplexiform layer to the outer plexiform layer, and are inhibitory to control
lateral spread of visual signals by the horizontal cells in the outer plexiform
layer.

Question 16

Horizontal cells allow for what type of vision?

Answer 16

high visual accuracy in transmitting contrastborders in the visual image

Question 17

Why can amacrine cells help tract predators?

Answer 17

Amacrine cells are interneurons that help analyze visual signals before they ever leave the retina. Directional Sensitive are a type of amacrine cell that respond to movement of a spot across the retina in a specific direction

Question 18

What are “W” ganglion cells?

Answer 18

40% of all ganglion cells. Small and transmit signals in their optic nerve fibers at a slow velocity. Receive excitation from rods, transmitted by way of
small bipolar cells and amacrine cells. Broad fields in the peripheral retina
because the dendrites of the ganglion cells spread widely in the inner plexiform
layer, receiving signals from broad areas. Especially sensitive for detecting
directional movement in the field of vision, and important for crude rod vision
under dark conditions.

Question 19

What are the roles of the “X” ganglion cells?

Answer 19

55% of ganglion cells. Medium diameter and transmit signals at a
moderate speed. Have small fields because their dendrites do not spread widely in the retina. Their signals represent discrete retinal locations. These cells
transmit fine details of the visual image. They are responsible for all color vision

Question 20

What are the roles of the “y” ganglion cells?

Answer 20

5% of ganglion cells. Large diameter and fastest transmission speeds.
Broad dendritic fields, so that signals are picked up by these cells from
widespread retinal areas. Respond to rapid changes in the visual image- either
rapid movement or rapid change in light intensity- sending bursts of signals for
only small fractions of a second. These cells most likely tell the CNS when a newvisual event occurs anywhere in the visual field, but without specifying with
great accuracy the location of the event, other than to give appropriate clues
that make the eyes move toward the area.