Vision

Question 1

Name 4 different eye types

Answer 1

Pinhole, compound, camera and mirror

Question 2

How is the image formed in a pinhole eye?

Answer 2

Pinhole emits light onto the underlying retina (directly). The small pupil creates a dim image on the retina.

Question 3

How is the image formed in a compound eye?

Answer 3

Composed by identical units called ommatidia (composed by 8 photoreceptors), each consisting of a lens element, that focuses light incident from a narrow region of space onto the rhambdom.

Question 4

How is the image formed in a camera eye?

Answer 4

Light is focused onto the retina by a single optical system comprising a lens and, in some cases, an overlying cornea.

Question 5

How is the image formed in a mirror eye?

Answer 5

Light weakly focused by the cornea passes through the retina and is then reflected from a hemispherical concave mirror lining the back of the eye.

Question 6

What are the camera-type eye ocular cells called?

Answer 6

Cilliary cells; rods and cones

Question 7

What is the compound eye ocular cells called?

Answer 7

Rhabdomeric cells

Question 8

How is light intensity related to the release of neurotransmitters in the two types of ocular cells?

Answer 8

Cilliary: inhibits NT release
Rhabdomeric: stimulates NT release

Question 9

What could explain the emergence of cilliary cells?

Answer 9

They can be packed more densely, and they use less ATP

Question 10

Which type of cilliary cell is thought to have evolved first?

Answer 10

Cones

Question 11

What is the function of rods and cones?

Answer 11

Rods: spatial resolution, more light sensitive
Cones: colour vision

Question 12

How many different opsins can the cones express, and how do they differ?

Answer 12

Three: OPN1SW (short waves=blue), OPN1MW (midlength waves=green), OPN1LW (long waves=red)

Question 13

What is the difference between the retinal molecules used in cameara-type eyes and in compound eyes?

Answer 13

Camera: 11-cis-retinal
Compound: 11-cis-3-hydroxy-retinal

Question 14

What does the retinal molecules bind to?

Answer 14

Opsin

Question 15

Describe the signaling pathway in cilliary photoreceptors

Answer 15

Photon stimulates the conversion of 11-cis-retinal bound to the opsin to all-trans-retinal –> release of retinal, and activation of the opsin receptor. Opsin is coupled to a Gt protein, and by activation the G_alpha is released from G-beta-gamma. G-alpha activates PDE –> reduction of cGMP –> inhibition of CNG Na+ channel –> inhibition of Ca^2+ influx (and AP) –> inhibition of glutamine release

Question 16

How is the signal inhibited in cilliary cells?

Answer 16

GRK1 phosphorylates opsin –> inactivation of opsin –> increase of cGMP –> increase of Ca^2+ –> depolarization of Vm –> release of glutamine

Question 17

Describe the signaling pathway in rhabdomeric cells.

Answer 17

Photon stimulates the conversion of 11-cis-3-hydroxy-retinal bound to the opsin to all-trans-3-hydroxy-retinal –> activation of the opsin receptor –> G-alpha dissociates from G-beta-gamma –> G-alpha activates PLC –> PLC converts PIP_2 into IP3 + DAG –> DAG lipase converts DAG into 2-MAG + FA –> MAG lipase converts 2-MAG into PUFA + glycerol –> PUFA gates TRP & TRPL –> Ca^2+ and Na^+ influx –> cell depolarizes –> histamine release

Question 18

How is the signal inhibited in rhabdomeric cells?

Answer 18

1) Another photon converts all-trans-3-hydroxy-retinal to 11-cis-3-hydroxy-retinal –> inactivation of opsin –> inhibition of signal
2) Arrestin inhibits the Ca^2+ channels

Question 19

Why do camera type eyes in general support much higher spatial resolution than compound eyes of insects?

Answer 19

The pupils are bigger in camera eyes –> the light is bend more by small pupils = all directional info is lost, and less brightness. Waves are bend by edges.

Question 20

Where in the animal kingdom are camera type eyes found?

Answer 20

Vertebrates, cephalopods, some arachnids, some jelly fish, a plant (algae, dinoflagellate), some gastropods.

Question 21

Accommodation is the ability of an eye to change focal distance. The vertebrate eye does this by change the shape of the lens or by moving the lens. Compound eyes cannot accommodate – why do they not need this?

Answer 21

They only sample light from one area in space, so they don’t need to accommodate.
In small pupils the angle of which the light hits are pretty much the same all over.

Question 22

What does the dynamic range of an eye refer to and how is it controlled in the human eye?

Answer 22

The number of photons the rods and cones can absorb depending on the brightness of a habitat.
It’s controlled by the number of active opsins in the retina (called adaptation), by controlling the process of conversion all-trans-retinal to 11-cis-retinal.

Question 23

Explain the term flicker fusion frequency. Give examples of animals (other than those from the lecture) you would expect to have a high or low fff respectively.

Answer 23

How many photons are absorbed/perceived per time. The frequency at which a photoreceptor no longer sees the light as flickering, but as a constant.
Insects would be expected to have high fff. Predators.
Deepsea fish would be expected to have low fff.

Question 24

What must all eyes do?

Answer 24

Sense the direction of light

Question 25

True or false: the longer the distance between the retina and the lens the better the spatial resolution?

Answer 25

True

Question 26

True or false: the smaller the size of the photoreceptor the better?

Answer 26

True (min. of 1 microm)

Question 27

True or false: the larger the angle the more diffraction?

Answer 27

True

Question 28

What is the most important part for focusing of the camera-type eye of animals living above water?

Answer 28

The cornea, which is filled with saltwater

Question 29

What is the lens involved in?

Answer 29

Accomodation: making sure the resolution matches the distance by contracting or extending the lens. Near/long sigthness is caused by a flaw in this mechanism.

Question 30

What is the fovea?

Answer 30

Cones lined in the back of the eye directly in line with the lens (in the centre of the optical axis of the eye) not covered by any other cells –> no interference. It is packed densely with cone cells.