2. LIGHT
Vision is probably the most important sense in humans and animals. This system works by
transducing the property of light into a complex visual percept
Light is an electromagnetic radiation visible to the eye. It’s defined by 3 parameters:
wavelength (distance btw two peaks or troughs)
frequency (number of waves per second)
amplitude (difference btw wave trough and peak)
The energy content of a radiation is
proportional to his frequency.
Only a small part of the
electromagnetic spectrum is visible
to our eyes
3. LIGHT
Optics is the study of light rays and their interactions
Reflection: bouncing of light rays off a surface
Absorption: transfer of light energy to a particle or surface
Refraction: changing of a direction due to change in speed of light rays, due to the passing from one
medium to another
4. ANATOMY OF THE EYE
Pupil: Opening where light enters
the eye
Sclera: White of the eye
Iris: Gives color to eyes. Contains 2
muscles that give size to the pupil
Cornea: Glassy transparent external
surface of the eye
Extraocular muscles: move the
eyeball in the orbit
Optic nerve: Bundle of axons from
the retina
5. THE RETINA
Optic disk: where blood vessels
originate and axons leave the retina
Macula: part of retina for central
vision
Fovea: marks the center of the retina
6.
7. CROSS SECTION OF THE EYE
Ciliary muscles: Ligaments that suspend lens
Lens: Change shape to adjust focus. It divides eyes into two compartments:
1) anterior chamber containing aqueous humor
2) posterior chamber containing vitreous humor
zonule fibers retina
iris
lens
fovea
light
cornea
aqueous humor
optic nerve
ciliary muscles
vitreous humor
sclera
8. IMAGE FORMATION
Eye collects light, focuses on retina, forms images.
The cornea is the site of most of the refractive power of the eye
Focal distance: from refractive
surface to the point where the
rays converges. Depends on
the curvature of the cornea
11. IMAGE FORMATION
The Pupillary Light Reflex
Depends on connections between retina and brain stem neurons that control
muscle around pupil and aim to continuously adjust to different ambient light
levels. It is consensual for both eyes
The Visual Field
Amount of space viewed by the retina when the
eye is fixated straight ahead
Visual Acuity
Ability to distinguish two nearby points
Visual Angle: Distances across the retina
described in degrees
12. MICROSCOPIC ANATOMY OF THE RETINA
Photoreceptors: cells that convert light energy into neural activity
In the Retina cells are organized in layers . Inside-out
13. MICROSCOPIC ANATOMY OF THE RETINA
Photoreceptor Structure
Transduction of electromagnetic radiation to
neural signals
Four main regions
1) Outer segment
2) Inner segment
3) Cell body
4) Synaptic terminal
Types of photoreceptors
Rods (scotopic vision-dark) and cones
(photopic vision-light)
14. MICROSCOPIC ANATOMY OF THE RETINA
Regional Differences in Retinal Structure
Varies from fovea to retinal periphery
In peripheral retina there is higher ratio of
rods to cones, and higher ratio of
photoreceptors to ganglion cells resulting in
more sensitive to light
In the fovea (pit in retina) visual acuity is
maximal. In Central fovea there are only
cones (no rods) and 1:1 ratio with ganglion
cells
15. PHOTOTRANSDUCTION
Phototransduction in Rods
Depolarization in the dark: “Dark current” and hyperpolarization in the light
One opsin in rods: Rhodopsin
Receptor protein that is activated by light
G-protein receptor Photopigment
16. PHOTOTRANSDUCTION
Depolarization in the dark:
“Dark current” and
hyperpolarization in the light:
Constant inward sodium
current
Light activate an enzime that
destroy the cGMP, causing
the closing of Na+ channel
18. PHOTOTRANSDUCTION
Phototransduction in Cons
Similar to rod phototransduction
Different opsins sensitive to different wavelengths: Red, green, blue
Color detection is determined by the relative
contributions of blue, green, and red cones to
retinal signal (Young-Helmholtz trichromacy
theory of color vision)
Dark and Light Adaptation is the transition
from photopic to scotopic vision (20-25
minutes). It’s determined by:
Dilation of pupils
Regeneration of unbleached
rhodopsin
Adjustment of functional circuitry
19. RETINAL PROCESSING
Photoreceptors release glutamate when depolarized
Bipolar Cells. Can be categorized in 2 classes: OFF bipolar cells (they respond to
glutamate by depolarizing) and ON bipolar cells (they respond to glutamate by
hyperpolarizing) . Light off or on causes depolarization
21. RETINAL PROCESSING
Two types of ganglion cells in monkey and human retina
M-type (Magno) and P-type (Parvo) – 5 and 90 % of the ganglion cell population. The rest 5 % is non-P
and non-M cells
M-type: larger receptive
field, faster conduction of AP,
more sensitive to low contrast stimuli
Color-Opponent Ganglion Cells
