Photoreception (1)

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

7. The Eye Eyespots are single cells or regions of a cell that contain photosensitive pigment, e.g., protist Euglena Eyes are complex organs

8. Flat-sheet Eyes Provide some sense of light direction and intensity Most often seen in larval forms or as accessory eyes in adults

9. Cup-shaped Eyes Retinal sheet is folded to form a narrow aperture Better discrimination of light direction and intensity Seen in the Nautilus

10. 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

11. Convex Eye Photoreceptors radiate outward forming a convex retina Present in annelids, molluscs , and arthropods

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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 ”)

18. 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

19. 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

20. 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

21. The Brain Processes the Visual Signal Optic nerves  o ptic chiasm  optic tract  l ateral geniculate nucleus  v isual cortex Figure 7.41

22. 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 )