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Photoreception (1)
1. Photoreception is the ability to detect light and involves photoreceptors like rods and cones that contain photopigments. 2. In the eye, light is refracted by the cornea and lens to form an image on the retina, with the fovea providing the sharpest vision. 3. Signals from photoreceptors are processed in the retina and optic nerve before reaching the visual cortex of the brain for interpretation.
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Photoreception (1)
- 1. PHOTORECEPTION • Ability todetect a small proportion of the electromagnetic spectrum from ultraviolet to near infrared Figure 7.27
- 2. PHOTORECEPTORS Organs rangefrom 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 Allare ciliary photoreceptors Two types Rods Cones Figure 7.29
- 4. CHARACTERISTICS OF RODSAND CONES Nocturnal animals have relatively more rods
- 5. PHOTOPIGMENTS Photopigments havetwo 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 inrhabdomeric and ciliary photoreceptors
- 8. THE EYE • Eyespotsare single cells or regions of a cell that contain photosensitive pigment, e.g., protist Euglena • Eyes are complex organs
- 9. FLAT-SHEET EYES • Providesome sense of light direction and intensity • Most often seen in larval forms or as accessory eyes in adults
- 10. CUP-SHAPED EYES • Retinalsheet is folded to form a narrow aperture • Better discrimination of light direction and intensity • Seen in the Nautilus
- 11. VESICULAR EYES • Usea 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 radiateoutward forming a convex retina •Present in annelids, molluscs, and arthropods
- 13. COMPOUND EYES Most complexconvex 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 • Arrangedinto 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 • Smalldepression 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 INTHE 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 INTHE 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 PROCESSESTHE VISUAL SIGNAL • Optic nerves optic chiasm optic tract lateral geniculate nucleus visual cortex Figure 7.41
- 23. COLOR VISION Detectingdifferent 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)