1. Accessory Structures of the Eye
Figure 17–3

2. Neural Tunic (Retina)
Figure 17–4c

3. Retina
• Rods, cones are types of photoreceptors
Figure 17–6

4. 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

5. Photoreceptor
Figure 17–13a

6. • Rods and cones synapse with neurons
called bipolar cells
• Bipolar cells then synapse with neurons
called ganglion cells
Figure 17–6a

7. 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

8. Optic Disc
• Circular region just medial to fovea
• Origin of optic nerve
Figure 17–6b, c

9. Blind Spot
Figure 17–7

10. Visual Pigments
• Where light absorption
occurs
• Derivatives of rhodopsin
(opsin plus retinal)
• Retinal:
synthesized from vitamin A
Figure 17–13b

11. Phototransduction
1) 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

12. In dark,
photoreceptors are
activated (stimulated)
Dark Current
In light,
photoreceptors are
inactivated (inhibited)
Figure 17–14

13. Bleaching
• Rhodopsin molecule breaks down into
retinal and opsin
Figure 17–15

14. 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

15. Pupillary Muscle Reflexes
Figure 17–5

16. Color Sensitivity
• Integration of
information from
red, green, and
blue cones
• All wavelengths
reflected off an
object looks white
Figure 17–16

17. Color Blindness
• Inability to detect certain colors
Figure 17–17

18. Visual Pathway
• Begins at photoreceptors
• Ends at visual cortex of cerebral
hemispheres
• Message crosses 2 synapses before goes
toward brain:
–photoreceptor --> bipolar cell
–bipolar cell --> ganglion cell (sensory neuron)
“processing in retina”

19. Convergence
• Each ganglion cell
receives input from
many photoreceptors
• Therefore,
Receptive field of
ganglion cell is
monitored by many
photoreceptors

20. M Cells
• ganglion cells that monitor rods
• Provide information about:
–general form of object
–motion
–shadows in dim lighting
P Cells
• ganglion cells that monitor cones
• Provide information about:
–edges
–fine detail
–color

21. Visual Pathway
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
Figure 17–19

22. Optic Tracts
5. Axons from both
eyes projecting from
optic chiasm to lateral
geniculi
Optic Radiations
6. Bundles of projection
fibers from lateral
geniculates to visual
cortex

23. 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

24. Depth Perception
• By comparing relative
positions of objects
between left–eye and
right–eye images
But, at great distances:
- Previous familiarity
- Occlusion
- Perspective
- Motion parallax
- Shadows and light
Figure 17–19