1. Light & Vision: Accessory Structures → Receptor Cells
Eyespots & Eyecups (Ocelli)
flatworm
microvilli
(rhodopsin)
photoreceptors
Light/Dark + Direction
pigment
insect
screens light
from behind
Image Forming Eyes
Short Wavelength = Spatial Resolution
Pinhole (Diffraction)
Mirror
Refractive
Compound
retina
lens
• various focusing
systems
• low resolution
• high sensitivity
• high resolution
• wide/deep field

2. Light & Vision: Receptor Cells → Light Absorption
Photoreceptors: membrane bound pigments
Cones
Rods
lamellae
rods
• multiple Opsins
diff absorption max
humans = 3
shrimp = 10
• single Rhodopsin
• insensitive to red
• signal processing:
- ‘Intensity’ only
(monchromatic)
• signal processing:
red + green = ‘Intensity’
differences = ‘Color’
• low sensitivity, high resolution (fovea)
• high sensitivity, low resolution
synapses
• not many blue!
• bleach rapidly
= night vision
+ chromatic aberration
= poor acuity in blue
light
compound eyes
@ 90° orientation?
=> Polarized Light!

3. Light & Vision: Light Absorption → Receptor Potentials
‘Dark Current’
Channel
Na+
out
Edark = ~ 0 mV in
cGMP G G
lammela K+
light
PDE
GMP
rhodopsin Em
bleach & recycle Retinal
dark
+ Light EK
(odorant receptors!)
Hyperpolarizing Receptor Potential!
(Invertebrates = Depolarize!)
‘Dark
Current’
integration by brain = vision
integration by retina
‘hyper- polarized’
‘depolarized’
⇓glutamate
=> excites & inhibits neurons!
glutamate