Medical Neuroscience: Biochemistry Of Vision

Front 1

What effect does a convex lens have on light?

Back 1

causes rays to converge on a focal point, in the case of the eye, on the fovea

Front 2

What must happen in order for us to see a particular color?

Back 2

-The color we see is due to the wavelengths that are not absorbed by an object, and thus are reflected off of the object
-We must have cones sensitive to that wavelength in order to see it

Front 3

What is the range of the visual spectrum that humans can generally see?

Back 3

from about 370nm to 750nm

Front 4

What functions are rods specialized for?

Back 4

-highly sensitive to light, so good for night vision
-do not respond well to moving stimuli because have slow response time

Front 5

What functions are cones specialized for?

Back 5

-are able to respond quickly, so good for moving stimuli
-are used more ofetn in daytime vision, for high illumination levels
-can detect color
-less sensitive than rods
-5 million in each eye, most dense in fovea
-three classes for different colors of light
-

Front 6

What are the three types of cones?

Back 6

-Red
-Blue
-Green

Front 7

Describe the photoreceptors of the retina.

Back 7

The outer segments contain stacks of membranes, where light absorbing molecules are found

Front 8

What is phototransduction?

Back 8

Light is transduced into a receptor potential by the photopigment in the rods and cones

Front 9

What does photopigment consist of?

Back 9

-retinal - vit A derivative
-opsin - glycoprotein

Front 10

What are the four types of opsin?

Back 10

-scotopsin - in rods
-in cones, 3 types: blue, green, and red opsin

Front 11

Retinal + Scotpsin =

Back 11

Rhodopsin - found in rods

Front 12

retinal + (blue, green, or red) Opsin =

Back 12

(blue, green, or red) photopigment

Front 13

How does phototransduction occur?

Back 13

-In the dark (at rest), the photoreceptor is somewhat depolarized
-In the presence of light, the photoreceptor hyperpolarizes
-The receptor potential in a photoreceptor is hyperpolarizing (-70mV)
-This occurs through the closing of Na+ channels that are open in the dark

Front 14

How is cGMP involved in phototransduction?

Back 14

-Na channels in photoreceptors are cGMP gated, open in the presence of cGMP
-cGMP is present inthe dark, so channels are open, and is depolarized at rest
-light causes reduction in the intracellular concentration od cGMP, causing channels to close
-This leads to hyperpolarization

Front 15

WHat happens when light hits retinal?

Back 15

-retinal absorbs light when it is detected by photopigment
-retinal undergoes a conformational change, which alters the structure of the opsin molecule
-This activates G-protein (transducin)
-Transducin activates a phosphodiesterase that reduces cGMP concentration

Front 16

Briefly describe the steps in the process of phototransduction.

Back 16

1. light activates photopigment
2. Altered photopigment activates transducin
3. Transducin activates phosphodiesterase
4. Phosphodiesterase reduces cGMP concentration
5. Decreased cGMP concentration leads to cGMP gated NA channel closing, and therefore hyperpolarization occurs
6. hyperpolarization leads to a decrease in glutamate release

Front 17

The phototransduction pathway causes an _____________ of the signal

Back 17

amplification

Front 18

In the phototransduction pathway, bipolar cells are _________ by glutamate

Back 18

inhibited

Front 19

With light stimulation what happens to glutamate release?

Back 19

-photoreceptors hyperpolarize and decrease glutamate release
--SInce bipolar cells are inhibited by glutamate, a reduction leads to depolarization

Front 20

Depolarization of the bipolar cells leads to ____________ of the ganglion cells.

Back 20

activation