photoreception (1)
TRANSCRIPT
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PHOTORECEPTION• Ability to
detect a small proportion of the electromagnetic spectrum from ultraviolet to near infrared
Figure 7.27
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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
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VERTEBRATE PHOTORECEPTORS All are ciliary
photoreceptors Two types
Rods Cones
Figure 7.29
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CHARACTERISTICS OF RODS AND CONES
Nocturnal animals have relatively more rods
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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
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PHOTOTRANSDUCTION
Transduction cascades differ in rhabdomeric and ciliary photoreceptors
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THE EYE• Eyespots are single cells or regions of a cell that
contain photosensitive pigment, e.g., protist Euglena• Eyes are complex organs
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FLAT-SHEET EYES• Provide some sense of light direction and
intensity• Most often seen in larval forms or as
accessory eyes in adults
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CUP-SHAPED EYES
• Retinal sheet is folded to form a narrow aperture
• Better discrimination of light direction and intensity
• Seen in the Nautilus
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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
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CONVEX EYE
•Photoreceptors radiate outward forming a convex retina
•Present in annelids, molluscs, and arthropods
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COMPOUND EYES
Most complex convex eyes found in arthropodsComposed 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
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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
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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
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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
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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
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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”)
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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
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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
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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
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THE BRAIN PROCESSES THE VISUAL SIGNAL
• Optic nerves optic chiasm optic tract lateral geniculate nucleus visual cortex
Figure 7.41
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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)