photoreception

1. PHOTORECEPTION
• Ability to detect
a small
proportion of the
electromagnetic
spectrum from
ultraviolet to
near infrared
Figure 7.27

2. PHOTORECEPTORS
 Organs range from single light-sensitive cells to
complex, image forming eyes
 Two major types
 Ciliary photoreceptors – have single, highly
folded cilium; folds form disks that contain
photo-pigments
 Rhabdomeric photoreceptors – apical
surface is covered with multiple out foldings
called microvillar projections
 Photo-pigments - molecules that absorb energy
from photons

3. VERTEBRATE PHOTORECEPTORS
 All are ciliary
photoreceptors
 Two types
 Rods
 Cones
Figure 7.29

4. CHARACTERISTICS OF RODS AND CONES
 Nocturnal animals have relatively more rods

5. PHOTOPIGMENTS
 Photopigments have two covalently bonded parts
 Chromophore – pigment that is a derivative of
vitamin A, e.g., retinal
 Opsin – G-protein-coupled receptors
 Steps in photoreception
 Chromophore absorbs energy from photon
 Chromophore changes shape
 Photoreceptor protein changes shape
 Signal transduction cascade
 Change in membrane potential
 Bleaching – process where activated retinal no
longer bonds to opsin, thereby activating opsin

6. PHOTOTRANSDUCTION
 Transduction
cascades differ
in rhabdomeric
and ciliary
photoreceptors

8. THE EYE
• Eyespots are single cells or regions of a cell that contain
photosensitive pigment, e.g., protist Euglena
• Eyes are complex organs

9. FLAT-SHEET EYES
• Provide some sense of light direction and
intensity
• Most often seen in larval forms or as accessory
eyes in adults

10. CUP-SHAPED EYES
• Retinal sheet is folded to form a narrow aperture
• Better discrimination of light direction and intensity
• Seen in the Nautilus

11. VESICULAR EYES
• Use a lens in the aperture to improve clarity and
intensity
• Lens refracts light and focuses it onto a single
point on the retina
• Present in most vertebrates

12. CONVEX EYE
•Photoreceptors radiate outward forming a convex
retina
•Present in annelids, molluscs, and arthropods

13. COMPOUND EYES
Most complex convex eyes found in arthropods
Composed of ommatidia
Form images in two ways
 Apposition compound eyes – ommatidium
operate independently; afferent neurons make
interconnection to generate an image
 Superposition compound eyes – ommatidium
work together to form an image on the retina

14. THE VERTEBRATE EYE
 Forms bright, focused
images
 Parts
 Sclera – white of
the eye
 Cornea –
transparent layer
 Choroid –
pigmented layer
 Tapetum – layer in
the choroid of
nocturnal animals
that reflects light

15. THE VERTEBRATE EYE, CONT.
 Parts
 Iris – two layers of
pigmented smooth
muscle
 Pupil – opening in iris
 Lens – focuses image
 Ciliary body – muscles
for changing lens shape
 Aqueous humor – fluid in
the anterior chamber
 Vitreous humor –
gelatinous mass in the
posterior chamber

16. IMAGE FORMATION
• Refraction – bending light
rays
• Both the cornea and the
lens act as converting
lens to focus light on the
retina
• In terrestrial vertebrates,
most of the refraction
occurs between the air
and the cornea

17. IMAGE ACCOMMODATION
• Accommodation - incoming light rays must converge on the retina to
produce a clear image
• Focal point – point at which light waves converge
• Focal distance – distance from a lens to its focal point
• Distant object: light rays are parallel when entering the lens
• Close object: light rays are not parallel when entering the lens and
must be refracted more
• Light rays are focused on the retina by changing the shape of the
lens

18. THE RETINA
• Arranged into several
layers
• Rods and cones are are
at the back and their tips
face backwards
• Axons of ganglion cells
join together to form the
optic nerve
• Optic nerve exits the
retina at the optic disk
(“blind spot”)

19. THE FOVEA
• Small depression in
the center of the
retina where
overlying bipolar and
ganglion cells are
pushed to the side
• Contains only cones
• Provides the
sharpest images
Figure 7.37a

20. SIGNAL PROCESSING IN THE RETINA
 Rods and cones form different images
 Rods
 Principle of convergence – as many as 100 rods
synapse with a single bipolar cell  many bipolar
cells synapse with a ganglion cell
 Large visual field
 Fuzzy image
 Cones
 One cone synapses with one bipolar cell which
connects to one ganglion cell
 Small visual field
 High resolution image

21. SIGNAL PROCESSING IN THE RETINA, CONT.
 Complex “on”
and “off” regions
of the receptive
fields of
ganglion cells
improve their
ability to detect
contrasts
between light
and dark
Figure 7.39

22. THE BRAIN PROCESSES THE VISUAL SIGNAL
• Optic nerves 
optic chiasm 
optic tract  lateral
geniculate nucleus
 visual cortex
Figure 7.41

23. COLOR VISION
 Detecting different wavelengths
of light
 Requires multiple types of
photoreceptors with different
maximal sensitivities
 Humans: three (trichromatic)
 Most mammals: two
(dichromatic)
 Some bird, reptiles and fish:
three, four, or five
(pentachromatic)