Insect eyes (and some other)

Question 1

What type of eyes do insects have?

Answer 1

Compound eyes, although they do have simple eyes during development or as ambient light sampling devices called ocelli.

Question 2

What does the coexistence of rhabdomeric and ciliary photoreceptors in insects etc speak to?

Answer 2

We have rhabdomeric too (ipRGCs) so both R and C photoreceptors existed in the common ancestor of vertebrates and invertebrates.

Question 3

What is the most popular type of eye?

Answer 3

Compound. 95% of animals are beetles.

Question 4

What type of photoreceptor is a rod? Why? What kind of eye does it exist in?

Answer 4

Ciliary, because it has a modified cilium. Exists in simple eyes.

Question 5

What type of photoreceptor does a compound eye have? Why?

Answer 5

They have series of microvili called rhabdomeres. They look different and respond to light differently.

Question 6

Compare rod/cone to rhabdomere in terms of size. Give a number.

Answer 6

Rod/cone is large. Rhabdomere discs are really small. Microvili volume is small. 100 nm. You can pack them tight.

Question 7

What are the components of a microvilus? How do these respond to light? Relate it to size of receptor.

Answer 7

Opsins, chromophores, channel (not CNG, Gq activated). Light strikes chromophore. Changes opsin concentration. Opens channel. Not a lot of amplification since vili are tight.

Question 8

Compare ciliary to rhabdomeric in terms of photon capture range.

Answer 8

Ciliary is split into rod/cone which can respond to different amount of photons. Rhabdomeric must respond to both a few photons and also thousands of photons.

Question 9

How is rhabdomeric phototransduction different?

Answer 9

Instead of closing CNG channels, it opens a different type of cation channel. Depolarizes instead of hyperpolarizing. cGMP->GMP replaced by PIP2->IP3. Arrestin-beta instead of alpha.

Question 10

How do compound eyes focus light on photoreceptors? What is this called?

Answer 10

Ommatidion/facets bring light rays to focus on 8-9 photoreceptors.

Question 11

How many facets does a drosophilia have? How many degrees FoV? What resolution does this translate to?

Answer 11

700 facets, 180 fov, <1 kilopixel.

Question 12

What are the types of facet/photoreceptor alignments?

Answer 12

Apposition (photoreceptors separated per facet), Superposition (separate receptors from facet which allows several facets to function as a single light collecting device), Neural Superposition (fuses input from receptors, but restricts fusion to receptors along same optical axis)

Question 13

Explain apposition eyes.

Answer 13

Central receiving element gets good image, rest bad. If not straight ahead, you see bad.

Question 14

Explain superposition eyes.

Answer 14

Increase the collecting surface. Individual receptors gather over several degrees of RF. Rhabdomeres fuse to be called a rhabdom.

Question 15

Explain neural superposition eyes.

Answer 15

Allow photons to hit from separate angles. Each hits separate facet&separate receptor. Requires “neural calculus”, make receptors talk to a single target in a ganglion. Maintain acuity, increase sensitivity.

Question 16

What dies having little cytoplasm per cilia in rhabdomeric receptors achieve?

Answer 16

Change Ca concentration massively by adding 1 Ca ion. Increases concentration by 100 nanomole.

Question 17

What’s a rhabdom? What type of eye uses it? What type of response?

Answer 17

Single collecting element caused by fusion of rhabdomeres in a single ommatidium. Both apposition and superposition eyes use it. All or nothing response.

Question 18

How do you calculate interreceptor difference in rhabdoms?

Answer 18

There is one rhabdomere that captures photons from a common source. Distance between these rhabdomeres over rhabdoms = distance btw facet centers.

Question 19

In apposition eyes, what resolution can the eyes discriminate?

Answer 19

Photons can’t travel between facets. If an object is far enough to be captured by different rhabdoms then it will go to different receptors. This is the minimum resolution you can discriminate.

Question 20

What do superposition eyes do in terms of acuity and sensitivity vs apposition eyes?

Answer 20

Ommatidia can collect light from many lenses, increases sensitivity by increasing collecting surface of rhabdom. However, RF per rhabdom will be much larger, acuity is way lower. They can see in dim light though.

Question 21

How do neural superposition eyes compare to apposition and superposition in terms of sensitivity and acuity?

Answer 21

Ommatidia collect from large regions, but each photoreceptor has its own optical axis. They send to a common neuron so they get sensitivity like superposition and acuity like apposition.

Question 22

How are photoreceptors arranged in ommatidia?

Answer 22

6 central receptors on the outside. 7&8 on top of each other in the middle (different per animal).

Question 23

What types of opsins to photoreceptors in one ommatidia express? How does this compare to humans in terms of color?

Answer 23

R1-6 express Rh1, which is achromatic. Like rods. Central receptors (R7-8) express Rh3/4-5/6, they provide some sort of color vision.

Question 24

Explain reisomerization of all-trans to 11cis in drosophilia. (3-OH-11-cis-retinal in some)

Answer 24

They have a mechanism to reisomerize alltrans-11cis but this isn’t enough. They need PDH which is driven by isomerase. Wihout that they are blind.

Question 25

Make the analogy between light cycle of vertebrates and invertebrates. Compare cones/rods/drosophilia.

Answer 25

Cons through Muller cells, rods through RP, droso through PDH.

Question 26

Explain rhabdomeric phototransduction.

Answer 26

Rh goes to Rh*, this activates a Gq that turns on PLC. This causes hydrolysis of PIP2 to IP3 and DAG. DAG opens TRP and TRPL channels.

Question 27

What cation channel do humans have? What do bugs have?

Answer 27

CNG channel in humans, TRP and TRPL channels in bug. This channel prefers calcium by a factor of 50-1.

Question 28

Explain the arrangement of the fly visual system.

Answer 28

Photoreceptor axons enter dendrites of second order neurons arranged by layer and column. R1-6 to lamina, 7-8 to medulla.

Question 29

How do flies figure out speed and direction of stimuli?

Answer 29

Cells in medulla and lobula collect across columns, sampling across visual field. This allows them to respond to objects moving in a specific direction. They have horizontally and vertically sensitive cells. Separate circuitry for v/h.

Question 30

How are droso thought to achieve directional selectivity?

Answer 30

Neural superposition. R1-6 terminate on lamina L1-5 (5 gets indirect thru amacrine). L1 and 2 are necessary. L1 computes direction of bight (ON) edges and L2 dark (OFF) edges. -L1-, -L2=L3, -L3=L4, L4-, -Am-L5-

Question 31

How do flies escape?

Answer 31

At Lobula plate, it takes 100ms for directionally sensitive cells to respond. Phototransduction is 50ms. The parts of brain get direct electric contacs, not even bothering with synapses. within 100ms these motor neurons send messages to escape muscles.

Question 32

How do monarch/viceroy butterflies distinguish each other when birds can't?

Answer 32

R2-3 sees 500nm, R8 sees 470nm R9 short 435 nm. They send axons to different places. R8 to a col in medulla. R2-3 thru lamina and lamina axons go to a column in medulla (2-3 at same place). Cells in medulla can code for difference btw wavelenth and luminance, thus they are color tuned. Monarch and viceroy have mutex pigments. Butterflies have opsins that allow them to distionguish this range better.