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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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