Ch09

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