Week 4 Vision Objective 2

Question 1

photosensitive pigments rods and cones

Answer 1

rods, rhodopsin

cones, three colors (these are transmembrane conjugated proteins)

Question 2

Major functional segments of rods and cones: outer segment

Answer 2

Outer segment contains light sensitive photochemicals
Inner segment contains organelles, especially mitochondria
nucleus
synaptic body: connects with horizontal and bipolar cells

Question 3

light sensitive pigment proteins in the rods

Answer 3

1. scotopsin

2. carotenoid pigment “retinal”

Question 4

rhodopsin =

Answer 4

scotopsin + retinal

Question 5

describe the chemical reaction that occurs in rhodopsin exposed to light

Answer 5

11-cis retinal is converted to trans retinal and dissociates from scotopsin

Question 6

Rhodopsin + light —>

Answer 6

scotopsin + all-trans-retinal

Question 7

The intermediate in the rhodopsin/light reaction is called

Answer 7

metarhodopsin II, which excites electrical changes in the rods

Question 8

rhodopsin-retinal visual cycle

rhodopsin + light —> 1

1 —> 2

2 —> 3

3 —> 4

Answer 8

rhodopsin + light —> bathorhodpsin (nsec) —> lumirhodopsin —> metarhodopsin I (msec) —> metarhodopsin II (sec)

metahordopsin II = activated rhodopsin, excites electircal changes in the rods, and the rods transmit visual image into CNS system

Question 9

metarhodopsin II —>

Answer 9

all-trans-retinal

Question 10

all trans retinal + isomerase —>

Answer 10

II cis retinal

Question 11

all trans retinal + _______ —-> _______

Answer 11

all trans retinol +isomerase —> II cis retinol II- cis-retinal +scotopsin (rhodopsin restored)

II cis retinal + scotopsin —> rhodopsin

Question 12

all-trans-retinol =

Answer 12

vitamin A

Question 13

In flow in rods (3 parts): light excitation

Answer 13

excitation increases negative intrarod membane potential (hyperpolarization)

Question 14

In flow in rods (3 parts): rod membrane

Answer 14

rhodopsin decompensation decreases rod membrane conductance for sodium ions in outer segment

Question 15

in flow in rods (3 parts): last part

Answer 15

hyperpolarization of entire rod membrane

Question 16

cGMP’s levels and role in conductance

Answer 16

in the outer segment of the rod, the membrane is leaky to sodium ions that flow through cGMP gated channels

cGMP is open in the dark, permitting positive sodium to run back into the rod = reduced electronegativity in the dark
when rhodopsin in the outer segment is exposed to light, it is activated and begins to decompose, and cGMP channels are closed: outer membrane’s conductivity for sodium is reduced dramatically

Question 17

the negativity of the rod, how it is achieved

Answer 17

the middle segment membrane constantly pumps sodium from the inside to the outside

potassium leaky channels allow K to diffuse out of cell

thus there is a negative potential on the inside

Question 18

ion flow in the outer segment in the dark and in the light

Answer 18

in the dark, sodium leaks into the cell through cGMP channels and reduces the electronegativity (around -40 mV)

1. rhodopsin decomposes under light
2. retinal portion becomes activated
3. activated retinal stimulates transducin
4. transducin activated cGMP phosphodiesterase
5. cGMP P causes cGMP—> 5’GMP
6. Decreases in cGMP causes Na-channels to close
7. Photoreceptor becomes hyperpolarized (super positive duh)

Question 19

Color vision

Answer 19

similar to rods, but use opsins instead of scotopsins (like rods)

retinal portion is same as rod
cones are blue, green, and red sensitive