Chapter 9 The Eye

Question 1

Corneas feature

Answer 1

sufficient refractive power, measured in diopters, to focus light on the retina at the back of the eye

Question 2

Accommodation by the lens; (a) To focus the eye on a distant point
(b) Near objects

Answer 2

a) relatively little refraction is required. The ciliary muscle relaxes, stretching the zonule fibers and flattening the lens
b) require greater refraction provided by a more spherical lens. This is achieved by contracting the ciliary muscle so there is less tension in the zonule fibers.

Question 3

The pupillary light reflex

Answer 3

connections between the retina and neurons in the brain stem that control the muscles that constrict the pupils. An interesting property of this reflex is that it is consensual; shining a light into only one eye causes the constriction of the pupils of both eyes.

Question 4

The visual field for one eye

Answer 4

The visual field is the total amount of space that can be viewed by the retina when the eye is fixated straight ahead.

Question 5

Visual acuity

Answer 5

The ability of the eye to distinguish two points near each other. Acuity depends on several factors but especially on the spacing of photoreceptors in the retina and the precision of the eye’s refraction.

Question 6

Visual angle

Answer 6

Distances across the retina can be expressed as degrees of visual angle.

Question 7

Miosis and what causes

Answer 7

Constacted pupils

-parasymphetic nervous

Question 8

Myoryasis an what causes

Answer 8

Dialeted pupils

-Sympathetic nervous

Question 9

The pathaway of retinal information

Answer 9

light flows from the photoreceptors to bipolar cells to ganglion cells (The layers are seemingly inside-out.), which project axons out of the eye in the optic nerve.
-Horizontal cells and amacrine cells modify the responses of bipolar cells and ganglion cells via lateral connections.

Question 10

Rod photoreceptors

Answer 10

• in dark
• about 92 million rods in retina, peripheral retina is sensitive to low levels of light
• same photopigment
• none in the fovea(?)
• peak sensitivity 500nm
• The greater number of disks and higher photopigment concentration in rods makes them over 1000 times more sensitive to light than cones.

Question 11

Cone photoreceptors

Answer 11

• 5 million in fovea, where a high spatial acuity
• some in the peripheral retina
• three differenttypes of cones, different photopigments(three opsins that give the photopigments different spectral sensitivities, blue, green, red)
• resposible of seeing colors
• central vision blind in scotopic(vision of the eye under low-light levels) light levels

Question 12

Phototranduction in rods

Answer 12

• When dark:
• sodium channels open by second messenger cGMP
• light absorbs to the stacked disks’ membrane in rods and in the rhodopsin(photopigment) it changes the confirmation of retinal->rhodopsin consists opsin and retinal. Opsin is the G-protein-coupled channel and retinal its agonist. ->rhodopsin stimulates a G-protein called transducin -> activates the effector enzyme PDE which breaks down the cGMP.-> Na+ channels close->hyperpolarization->no action potential-> Ca2+ cahnnels close and no more glutamate is released in synapse cleft->stimulation of bipolarcells (depending if Off or On cells)

Question 13

dark current

Answer 13

The movement of positive charge across the membrane, which occurs in the dark

Question 14

How do the sodium channels stay open in the dark?

Answer 14

Sodium channels are stimulated to open—are gated—by an intracellular second messenger called cyclic guanosine monophosphate, or cGMP. cGMP is produced in the photoreceptor by the enzyme guanylyl cyclase, keeping the Na + channels open.

Question 15

What is Rhodopsin photopigment formed of

Answer 15

• The receptor protein is called opsin(seven transmembrane alpha helices)
• The prebound agonist is called retinal, a derivative of vitamin A

Question 16

Phototranduction in cones

Answer 16

• really similar to rods

- photopigments require more energy to become bleached(valkaista)

Question 17

Young– Helmholtz trichromacy theory

Answer 17

According to the theory, the brain assigns colors based on a comparative readout of the three cone types. When all types of cones are equally active, as in broad-spectrum light, we perceive “white.”

Question 18

dark adaptation

Answer 18

• It involves the regeneration of unbleached rhodopsin and an adjustment of the functional circuitry of the retina so that information from more rods is available to each ganglion cell.
• The constriction of the pupil helps a bit in reducing the light.

Question 19

light adaptation

Answer 19

Ca2+ enters a cone through the same cGMP-gated channels as Na+; it inhibits the synthesis of cGMP.

Question 20

Which cells fire action potentials?

Answer 20

Only ganglion cells fi re action potentials; all other cells in the retina (except some amacrine cells) respond to stimulation with graded changes in membrane potential

Question 21

In which kind of two types of retinal neurons are the photoreceptors synaptic contact with?

Answer 21

bipolar cells and horizontal cells. bipolar cells create the direct pathway from photoreceptors to ganglion cells; horizontal cells feed information laterally in the outer plexiform layer to influence the activity of neighboring bipolar cells and photoreceptors.

Question 22

The receptive field

Answer 22

• consists of the locations that increase or decrease the ganglion cell’s firing rate, center and surround
• Light anywhere else on the retina, outside this receptive field, would have no effect on firing rate.

Question 23

Bipolar cells and receptive fields can be categorized into two classes:

Answer 23

ON and OFF, based on the response to the glutamate released by photoreceptors. -The circuitry that gives rise to bipolar receptive fields consists of direct input from photoreceptors and indirect photoreceptor input relayed by horizontal cells

Question 24

OFF bipolar cells

Answer 24

Light shined onto a cone will hyperpolarize some bipolar cells because light effectively turns them off. (ionotropic glutamate receptors)

Question 25

ON bipolar cells

Answer 25

Light shined onto a cone will depolarize bipolar cells
-An ON-center bipolar cell is depolarized by light in the receptive field center via the direct pathway(G-protein-coupled (metabotropic) receptors and respond to glutamate by hyperpolarizing. )

Question 26

the receptive field center

Answer 26

a circular area of retina providing direct photoreceptor input

Question 27

the receptive field surround

Answer 27

a surrounding area of retina providing input via horizontal cells, the intervening horizontal cell, the effect of light on the surround photoreceptors is always opposite the effect of light on the center

Question 28

ON-center(off-surround)ganglion cell

Answer 28

will be depolarized and respond with a barrage of action potentials when a small spot of light is projected onto the center of its receptive field. If it is projected on the surround, it will be hyperpolarized (horizontal cells makes the hyperpolarization even stronger). There will be burst of nerve impulses after the stimulation=post-inhibitory rebound.

Question 29

post-inhibitory rebound

Answer 29

There will be burst of nerve impulses after the stimulation.

Question 30

An OFF-center(on-surround)

Answer 30

cell will fire fewer action potentials when a small spot of light is projected to the center of its receptive field; when projected on the surround, it will depolarize (The horizontal cells inhibit the release of glutamate on the synapse).

• not particularly responsive to changes in illumination
• ganglion cells are mainly responsive to differences in illumination

Question 31

M-type ganglion cells

Answer 31

• 5% of ganglion cell population
• larger receptive fields
• they conduct action potentials more rapidly in the optic nerve, -are more sensitive to low-contrast stimuli.
• responses are not color-specific
• The lack of color opponency in M- cells is accounted for by the fact that both the center and the surround of the receptive field receive input from more than one type of cone. M-cell receptive fields are therefore denoted as simply either ON center/OFF surround or OFF center/ON surround.

Question 32

P-type ganglion cells

Answer 32

• smaller than M-type
• 90% of the ganglion cell population
• sensitive to differences in the wavelength of light.
• color-sensitive neurons are called color-opponent cells, reflecting the fact that the response to one color in the receptive field center is canceled by showing another color in the receptive field surround.

Question 33

nonM-nonP

Answer 33

• 5%,are less well characterized

Question 34

Tasks of the M and P ganglion cells:

Answer 34

sends information to the brain about three different spatial comparisons: light versus dark, red versus green, and blue versus yellow.

Question 35

Responses to a light-dark edge crossing an OFF-center ganglion cell receptive field?

Answer 35

The whole receptive field lighted=very little action potential
part of the surround dark= no action potential
The center and almost all of the surround=big action potential
The whole field dark=very little action potential

Question 36

Color opponency in ganglion cell, color-opponent center-surround receptive field of a P-type ganglion cell, who does it response?

Answer 36

• No light: very little action potential
• Red light in the center, no light surroun: A strong response is elicited by red light to the receptive field center, which receives input from long-wavelength-sensitive (red) cones. -Extending the red light into the surround inhibits the response because the green cones providing input to the surround are also somewhat sensitive to long wavelengths of light.
• Even stronger inhibition is produced by green light in the receptive field surround, which optimally drives the green cones.

Question 37

What would be the response to white light on the entire receptive field?

Answer 37

Because white light contains all visible wavelengths (including both red and green), both center and surround would be equally activated, canceling each other, and the cell would not respond to the light.

Question 38

Why is diffuse blue light a strong stimulus here, whereas diffuse red light was a much weaker stimulus in RG(red-green) cells?

Answer 38

in the RG cell, red light is absorbed by the “green” photoreceptors, but in the BY neuron, very little blue light is absorbed by the “red” and “green” photoreceptors that make up the receptive field surround.

Question 39

intrinsically photosensitive retinal ganglion cells (ipRGCs)

Answer 39

-a small percentage of retinal ganglion cells transduce light.
-use melanopsin as a photopigment
-they are photoreceptors
-Unlike the hyperpolarization that light causes in rods and cones, ipRGCs depolarize to light.
-these photosensitive neurons send axons out of the eye without additional neurons and synaptic connections.
-large dendritic fields; really sensitive to light
Note! Humans who are totally blind also appear to unconsciously synchronize their behavior to daily changes in sunlight

Question 40

Parallel processing

Answer 40

different visual attributes are processed simultaneously using distinct pathways. E.g. depth, ON-center and OFF-center ganglion cells different receptive fields and response properties.

Question 41

Information from 97 million photoreceptors is funneled into

Answer 41

1 million ganglion cells.

Question 42

On and off ganglion cells redpond to and don’t respond to

Answer 42

Responds on differences in illumination in the reseptive fields
Doesn’t respond on changes of illumination