EYES!

conjunctiva

cornea

lens

Iris forming

Retina forming

Choroid (pigmented) layer forming

Vitreous humour forming

l.m. of embryonic eye

Embryonic eye development

Spherical lens

cornearetina

External anatomy of the eye

Curved transparent cornea – responsible for refraction of light

Pigmented iris – circular and longitudinal muscles

Pupil – diameter controlled by iris muscles

sclerotic

Figure 6.2  Cross section of the vertebrate eyeNote how an object in the visual field produces an inverted image on the retina.

Label the following:

a

b

c

d

e

f

g

h

i

j

Figure 6.18  An illustration of lateral inhibitionDo you see dark diamonds at the “crossroads”?

• Dark & Light Adaptation

• Adaptation - process by which the eye becomes

• more or less sensitive to light

Cones and Colour

Colour VisionDo objects possess colour?

Is a lemon “yellow”?

Is a chili pepper “red”?

NO! Light has no colour

Trichromatic Theory of Colour Vision

Helmholtz 1852

Human eye has 3 types of coneHuman eye has 3 types of conereceptors sensitive to differentreceptors sensitive to different

wavelengths of lightwavelengths of light.

Short Medium Long

People see colours because thePeople see colours because theeye does its own “colour mixing”eye does its own “colour mixing”

by varying ratio of coneby varying ratio of coneneural activityneural activity

Bleaching

• Bleaching occurs when you have looked at a red picture too long the red iodopsin has being bleached so when you look at white paper the red iodopsin is temporally out of order.

Transduction

Both Rods and Cones contain photopigments (chemicals that release energy when struck by light)

11-cis-retinal is transformed into all-trans-retinal in light conditions

this results in hyperpolarization of the photoreceptor

the normal message from the photoreceptor is inhibitory…

Light inhibits the inhibitory photoreceptors and results in depolarization of bipolar and ganglion cells

• Retina– Several layers

of cells in inner surface of choroid

– Contains photoreceptors - Rods & Cones

DaytimeNight Vision

High resolutionPoor definition

ColorBlack & White

Center of

retinaPeriphery of

retina

Less abundantMore abundant

ConesRods

Rods & Cones: Distibution• Rod density high away from

the center– The more sensitive rods

(~100_rods-1_neuron map) help track peripheral image motion

– ~120 million rods in retina

• Cone density high near the center– The 0.3 mm dia fovea has only

high density of cones (1_cone-1_neuron map) helps form sharp brilliantly colored images

– ~6-7 million cones in retina

A rod cell (upper) and a cone cell

From which direction would light come?

The Photo-receptors: Rods & Cones

• Cones– Phototopic– Chromatic– Fast– Foveal

• Rods– Scotopic– Achromatic– Slow– Peripheral vision

A rod cell

Direction of light

Figure 6.4  Visual path within the eyeballThe receptors send their messages to bipolar and horizontal cells, which in turn send messages to the amacrine and ganglion cells. The axons of

the ganglion cells loop together to exit the eye at the blind spot. They form the optic nerve, which continues to the brain.

Rods & Cones: Fovea & Blind Spot

• Fovea a 0.3 mm spot with cone-only distribution: highest acuity and color rendition

• Blind spot where optic nerve leaves the retina

retina

LIGHT

G-cells

B-P Cells

Rod cells

Retinal signal processing

• Integrator neurons– Horizontal cells– Bipolar cells– Amacrine cells– Ganglion cells

• Cones– Cone > Bipolar cell > Ganglion cell

• Rods– Rod > Bipolar cell > Amacrine cell >

Ganglion cell

Rods & Cones

• Photosensitive protein is rhodopsin, membrane protein, that modulates membrane ion conductivity via a biochemical cascade once it absorbs a photon, with the cell getting hyperpolarized as a function of light

• Different amino-acid sequences in the ‘opsin’ segments of rhodopsin give the different color sensitivities of rods & cones

Bipolar Cells

• Many Rod cells are connected to one bipolar cell which means that when only one of the Rod cells are activated an impulse is sent to the brain.

• One Cone cells is connected to one bipolar cell which means that the light needs activate each Cone cell to send an impulse. This is why the Cone cells have a higher acuity and why they cant function in the dark.

Link to brain: Primary pathway

• Optic nerve• Optic chiasm• Lateral geniculate

body• Optic radiation• Visual cortex

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