Lecture12 6 13

Figure 17–3

Accessory Structures of the Eye

Neural Tunic (Retina)

Figure 17–4c

Retina • Rods, cones are types of photoreceptors

Figure 17–6

Rods • Highly sensitive to light, do not discriminate colors• Rods dominate peripheral areas of retina • Provide low-resolution black–and–white vision in

dimly lit environments

Cones

• Sensitive to colored light• Densely clustered in fovea• Provide high–resolution color vision in brightly lit

environments

Photoreceptor

Figure 17–13a

• Rods and cones synapse with neurons called bipolar cells

• Bipolar cells then synapse with neurons called ganglion cells

Figure 17–6a

Horizontal Cells• Where receptors synapse with bipolar cells

Amacrine Cells• Where bipolar cells synapse with ganglion

cells

• Both facilitate or inhibit communication between photoreceptors and ganglion cells

• Alter sensitivity of retina

Optic Disc

• Circular region just medial to fovea

• Origin of optic nerve

Figure 17–6b, c

Blind Spot

Figure 17–7

Visual Pigments

Figure 17–13b

• Where light absorption occurs

• Derivatives of rhodopsin (opsin plus retinal)

• Retinal:synthesized from vitamin A

Phototransduction1) Photon strikes retinal portion of rhodopsin

2) Opsin is activated:– Goes from 11-cis form to 11-trans form

3) Opsin activates transducin (G protein), then activates phosphodiesterase (PDE)

4) Cyclic-GMP (cGMP) levels decline; gated sodium channels close

5) Dark current is reduced; neurotransmitter release declines

Figure 17–14

In dark, photoreceptors are activated (stimulated)

Dark Current

In light, photoreceptors are inactivated (inhibited)

Figure 17–15

Bleaching• Rhodopsin molecule breaks down into

retinal and opsin

Dark-Adapted State

• Pupil dilates • Most pigments can be activated• A single photon can be detected

Light-Adapted State

• Pupil constricts

• Bleaching of visual pigments occurs

• “Blinding” when go from dark to light room

Pupillary Muscle Reflexes

Figure 17–5

Figure 17–16

Color Sensitivity

• Integration of information from red, green, and blue cones

• All wavelengths reflected off an object looks white

Color Blindness • Inability to detect certain colors

Figure 17–17

Visual Pathway

• Begins at photoreceptors

• Ends at visual cortex of cerebral hemispheres

• Message crosses 2 synapses before goes toward brain:

“processing in retina”

–photoreceptor --> bipolar cell

–bipolar cell --> ganglion cell (sensory neuron)

Convergence

• Each ganglion cell receives input from many photoreceptors

• Therefore,

Receptive field of ganglion cell is monitored by many photoreceptors

M Cells• ganglion cells that monitor rods• Provide information about:

P Cells• ganglion cells that monitor cones• Provide information about:

–general form of object–motion–shadows in dim lighting

–edges–fine detail–color

Visual Pathway

Figure 17–19

1. Axons from ganglion cells converge on optic disc

2. Penetrate wall of eye

3. Proceed toward diencephalon as optic nerve (II)

4. 2 optic nerves (1 for each eye) reach diencephalons at optic chiasm

Optic Radiations6. Bundles of projection

fibers from lateral geniculates to visual cortex

Optic Tracts5. Axons from both eyes

projecting from optic chiasm to lateral geniculi

Visual Cortex

7. Info from right or left fields of vision arrive at visual cortex of opposite occipital lobe:

– left half arrives at right occipital lobe

– right half arrives at left occipital lobe

Depth Perception

• By comparing relative positions of objects between left–eye and right–eye images

Figure 17–19

But, at great distances:

- Previous familiarity

- Occlusion

- Perspective

- Motion parallax

- Shadows and light