l.m. of embryonic eye conjunctiva cornea lens
Iris forming
Vitreous humour forming
Choroid
(pigmented) layer forming
Retina forming

Embryonic eye development retina
Spherical lens cornea

External anatomy of the eye
Pigmented iris – circular and longitudin al muscles
Pupil – diameter controlled by iris muscles
Curved transparent cornea – responsible for refraction of light sclerotic

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

g h i
j a b f c e d

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

• Dark & Light Adaptation
•
Adaptation - process by which the eye becomes
• more or less sensitive to light

Do objects possess colour?
Is a lemon “yellow”?
NO!
Light has no colour
Is a chili pepper “red”?

Human eye has 3 types of cone receptors sensitive to different wavelengths of light.
Helmholtz 1852
Short Medium Long
People see colours because the eye does its own “colour mixing” by varying ratio of cone neural activity

• 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 hyper polarization 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
More abundant
Periphery of retina
Black & White
Poor definition
Night Vision
Less abundant
Center of retina
Color
High resolution
Daytime

• 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?

• Cones
– Phototopic
– Chromatic
– Fast
– Foveal
• Rods
– Scotopic
– Achromatic
– Slow
– Peripheral vision

A rod cell

Figure 6.4 Visual path within the eyeball
The 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.

• Fovea a 0.3 mm spot with cone-only distribution: highest acuity and color rendition
• Blind spot where optic nerve leaves the retina

Rod cells
B-P Cells
Gcells
LIGHT

• Integrator neurons
– Horizontal cells
– Bipolar cells
– Amacrine cells
– Ganglion cells
• Cones
– Cone > Bipolar cell > Ganglion cell
• Rods
–
Rod > Bipolar cell > Amacrine cell >
Ganglion cell

• 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

• 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.

• Optic nerve
• Optic chiasm
• Lateral geniculate body
• Optic radiation
• Visual cortex http://www.brother.com/usa/printer/advanced/lcv/light1.html