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THE EYE
LIGHTVision 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
LIGHTOptics is the study of light rays and their interactions
Reflection: bouncing of light rays off a surfaceAbsorption: transfer of light energy to a particle or surfaceRefraction: changing of a direction due to change in speed of light rays, due to the passing from one medium to another
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
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
CROSS SECTION OF THE EYECiliary muscles: Ligaments that suspend lensLens: Change shape to adjust focus. It divides eyes into two compartments:1) anterior chamber containing aqueous humor 2) posterior chamber containing vitreous humor
lens
iris
light
cornea
aqueous humor
ciliary muscles
scleravitreous humor
optic nerve
fovea
retinazonule fibers
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
IMAGE FORMATION
Accommodation by the LensChanging shape of lens allows for extra focusing power
IMAGE FORMATION
IMAGE FORMATION
The Pupillary Light ReflexDepends 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 FieldAmount of space viewed by the retina when the eye is fixated straight ahead
Visual AcuityAbility to distinguish two nearby pointsVisual Angle: Distances across the retina described in degrees
MICROSCOPIC ANATOMY OF THE RETINAPhotoreceptors: cells that convert light energy into neural activity
In the Retina cells are organized in layers . Inside-out
MICROSCOPIC ANATOMY OF THE RETINA
Photoreceptor StructureTransduction of electromagnetic radiation to neural signals
Four main regions1) Outer segment2) Inner segment3) Cell body4) Synaptic terminal
Types of photoreceptorsRods (scotopic vision-dark) and cones (photopic vision-light)
MICROSCOPIC ANATOMY OF THE RETINA
Regional Differences in Retinal StructureVaries 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
PHOTOTRANSDUCTION
Phototransduction in RodsDepolarization in the dark: “Dark current” and hyperpolarization in the lightOne opsin in rods: Rhodopsin
Receptor protein that is activated by light
G-protein receptor Photopigment
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
PHOTOTRANSDUCTION
PHOTOTRANSDUCTION
Phototransduction in ConsSimilar to rod phototransductionDifferent 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 trichromacytheory of color vision)
Dark and Light Adaptation is the transition from photopic to scotopic vision (20-25 minutes). It’s determined by:
Dilation of pupilsRegeneration of unbleached rhodopsinAdjustment of functional circuitry
RETINAL PROCESSING
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
Photoreceptors release glutamate when depolarized
RETINAL PROCESSING
Ganglion Cell Receptive Fields On-Center and Off-Center cellsResponsive to differences in illumination
M-type: larger receptivefield, faster conduction of AP,more sensitive to low contrast stimuli
RETINAL PROCESSING
Color-Opponent Ganglion Cells
Two types of ganglion cells in monkey and human retinaM-type (Magno) and P-type (Parvo) – 5 and 90 % of the ganglion cell population. The rest 5 % is non-P and non-M cells
RETINAL PROCESSING