1. The Human Visual System
[Biology 100, San Diego State University, Spring 2006]
VICENTE M. REYES, Ph.D.
Dept. of Pharmacology,
U.C. San Diego, La Jolla, CA

2. Learning Objectives:
To know the exterior and interior anatomy of the human eye and what the function of each
component is.
To appreciate the importance of the retina and its components as the “nerve center” of vision.
To understand why vitamin A is needed for vision.
To understand the difference between “straight-ahead” vision and “peripheral” vision, and know
which type of photoreceptor cells are largely responsible for each.
To know the difference between rod cells and cone cells and how they contribute to vision.
To understand the molecular basis of vision, and appreciate the fact that changes in molecular
structure (“conformational changes”, a recurrent theme in biochemistry) is the trigger for the
visual process.
To understand how color is perceived; to know what is “trichromatic vision”.
To appreciate the fact that the visual process is a “signal transduction” event and requires
G-proteins (transducin).
To understand why the human eye has a lot in common with a regular camera.
To know the nature of the different common eye disorders, including cataracts, color blindness,
astigmatism, macular degeneration, etc.
To understand why diabetes sometimes causes loss of vision.

3. gives the eye its “color” (e.g., blue eyes,
green eyes, hazel eyes, brown eyes, etc.)
the “white”
of the eye
1.) The part of the eye that adjusts the size of the pupil, making it small in bright light, and large in dim light, is called the
a.) retina
b.) iris
c.) sclera
d.) eyelid
e.) cornea

4. Vitreous Chamber (contains vitreous Macula humor) Anterior Chamber
aqueous
humor)
2.) The central part of the retina which is responsible for straight-ahead vision is called
a.) blind spot
b.) cornea
c.) lens
d.) macula
e.) pupil
Fovea
centralis to
thalamus
Blind spot
Zonules

5. 1. 2. 3. 4. 5. 6.
3.) The following are cells found in the retina, except:
a.) amacrine cells
b.) rod cells
c.) cone cells
d.) bipolar cells
e.) hepatocytes

6. 4.) Which neurotransmitter do rod cells and cone cells release?
a.) glutamate
b.) serotonin
c.) glucagon
d.) oxytocin
e.) histamine

7. 5.) Which of the following animals would probably have the least number of cone cells in its retina?
a.) human
b.) cheetah
c.) bat
d.) bald eagle
e.) giraffe

8. 6. ) Rhodopsin is a protein embedded in the membrane of rod cells
6.) Rhodopsin is a protein embedded in the membrane of rod cells. Which of the following statements is true about rhodopsin?
a.) It is a complex between the vitamin A derivative, retinal, and an opsin protein.
b.) It is present in cone cells as well.
c.) It is does not contain alpha helices.
d.) All of the above.
e.) None of the above.

9. In vision, visual pigments are required:
chromophore + opsin
visual pigment 1
a carotenoid lipid
a protein
Two important chromophores in vision:
Retinal 1 and Retinal 2, both derived from
Vitamin A1 and Vitamin A2, respectively
(see below): 1
C3-C4 is a double bond in Retinal 2
7.) Which of the following reactions requires light energy?
a.) conversion of vitamin A to retinal.
b.) isomerization of 11-cis retinal to all-trans retinal
c.) conversion of retinal to retinoic acid
d.) combination of opsin and retinal to form rhodopsin
e.) conversion of retinal A1 to retinal A2
alcohol
functionality
aldehyde
functionality
[O]
Vitamin A1 (Retinol 1)
Vitamin A2 (Retinol 2)
Retinal 1
Retinal 2

10. The RETINA is the nerve center of the visual process:
Two important chromophores in vision:
Retinal 1 and Retinal 2:
Two important opsins in vision:
Scotopsin and Photopsin:
Chromophore Opsin Visual Pigment
Retinal Scotopsin (rods) = Rhodopsin
Retinal Photopsin I (cones) = Porphyropsin
Retinal Photopsin II (cones) = Iodopsin
Retinal Photopsin III (cones) = Cyanopsin
8.) Cone cells contain three different types of visual pigments, photopsins I, II and III.
This feature is responsible for which of the following properties of cone cells?
a.) They are generally smaller in size than rod cells.
b.) They are much more in number than rod cells in humans.
c.) They enable the animal to perceive color.
d.) They are shaped like cones.
e.) They are able to interact closely with the cell membrane.
chromophores and opsins
combine to form the visual pigments
in the retina.
NOTE:
Opsins are GPCRs

11. The Molecular Basis of Visual Excitation: An Example of Signal Transduction
photon
of light
rhodopsin or
photopsin
cell
membrane
rhodopsin or
photopsin
cell
membrane
G-protein
G-protein
out
out in in
changes
conformation
becomes
active
open Ca++/Na+ ion channel
rhodopsin or
photopsin
9.) The family of proteins called opsins are always found associated with G-proteins in the cell.
Which particular G-protein is rhodopsin associated with?
a.) Transducin
b.) Ras
c.) Rho
d.) α-crystallin
e.) phosphodiesterase
G-protein
cGMP
out in
GMP
activates
phosphodiesterase
phosphodiesterase
(PDE)
closed Ca++/Na+ ion channel

12. becomes hyperpolarized
(Con’t)
Na+
out
out
++++++ in in
reduced Na+ permeability
photoreceptor cell
becomes hyperpolarized
modulation by horizontal
cells and amacrine cells
depolarized cell
in dark:
glutamate
differential effect
on bipolar cells
signal sent to
ganglion cells
brain
10.) When a photoreceptor cell (rod or cone cell) becomes hyperpolarized due to the closure of the Na+ ion channels, the direct result is:
a.) the cell dies due to the high voltage generated
b.) the cell drastically reduces releasing the neurotransmitter, glutamate
c.) the cell undergoes mitosis
d.) the cell leaves the retina and migrates to the vitreous chamber where there are less charges
e.) the cell increases its output of the neurotransmitter, glutamate.
w/ light:
glutamate
++++++
complicated, multi-step process
hyperpolarized cell

13. The Principle of Amplification During Signal Transduction (“Phototransduction”):
1 rhodopsin molecule absorbs 1 photon
500 transducin molecules activated
5,000 PDE molecules activated
10^5 cGMP molecules hydrolyzed to GMP
~250 Na+ channels closed
11.) The principle of signal amplification during signal transduction results in the
a.) inactivation of one PDE molecule per photon absorbed
b.) production of 100 photons from one photon absorbed
c.) activation of one transducin molecule per photon absorbed
d.) closure of one Na+ ion channel per photon absorbed
e.) blockage of 10 million Na+ ions due to a single photon
10^7 Na+ ions blocked for 1 sec.
rod cell membrane hyperpolarized by 1 millivolt
Processing

14. The Camera vs. The Human Eye
aperture
The Camera vs. The Human Eye
diaphragm
                                                                            
12.) The part of the eye that corresponds to the film in a regular camera is the:
a.) iris
b.) lens
c.) pupil
d.) retina
e.) cornea
outer glass
covering
Iris

15. An Important Concept: ACCOMMODATION
14.) Accommodation refers to the ability of the lens to
a.) change shape in response to close and distant vision
b.) change its color in response to varying degrees of sunlight
c.) absorb photons of light
d.) block ultraviolet rays from the sun
e.) magnify the size of the images of objects

16. constants between 0.0 and 1.0;
COLOR PERCEPTION
excited blue
cone cells
excited green
cone cells
excited red
cone cells
C = a*B + b*G + c*R
resulting
color perceived
15.) We humans are able to see in color because we
a.) have three different types of rod cells which absorb in three different regions of the electromagnetic spectrum
b.) have three different types of cone cells which absorb in three different regions of the electromagnetic spectrum
c.) have a very advanced form of brain and central nervous system
d.) possess irises of different colors depending on our different races
e.) have pupils that allow light rays from the colored regions of the electromagnetic spectrum
constants between 0.0 and 1.0;
Total = 1.00

17. Diseases/Defects of the Human Eye:
Nearsightedness (myopia)
Farsightedness (
hyperopia)
Cataract
Glaucoma
Color blindness
Macular degeneration
Presbyopia
Dry eye syndrome
diabetic retinopathy
16.) In retinitis pigmentosa, which of the following cell type/s are affected?
a.) cones only
b.) rods and cones
c.) rods only
d.) neither rods nor cones
e.) amacrine cells
17.) Dryness of the eye in DES (Dry Eye Syndrome) is due to:
a.) failure of the lacrimal gland to produce water for tears
b.) failure of the small glands in the eyelids to produce oil to cover the watery portion of the tears to prevent evaporation
c.) adverse reaction of the eye to hot weather conditions
d.) adverse reaction of the eye to dry weather conditions
e.) failure to drink eight glasses of water per day
18.) Person A has cataract; person B has astigmatism; person C is nearsighted; person D has a mutation in his photopsin II gene; person E has DES.
Which of them is probably color-blind?
a.) Person A
b.) Person B and C
c.) Person D
d.) Person E
e.) The one who is not racist.
Astigmatism
retinal detachment
Retinitis pigmentosa