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Lecture 5
Sensory Physiology: Photoreception
•Cross Section In the Cochlea:
oOuter hairs amplify quiet sound
Change shape in response to sound
Do not release neurotransmitter
The change in the shape of the hair cell increases the
movement of the basilar membrane increased stimulus
to inner hair cells
Inhibited by efferent neurons in response to loud sounds
•Cristae Detect Angular Acceleration
•Photoreception
oAbility to detect small proportion of the electromagnetic wave:
from near infared to near UV
A small proportion of the electromagnetic spectrum from
ultraviolet to near infared
Ability to detect this range of wavelengths supports idea
that animals evolved in water
•Wavelengths around the visible light travels well in
water; other wavelengths do not
•Electromagnetic Spectrum
oMost creatures don’t detect
oVisible light – for humans
oVisible light is must less attenuated in water
•Photoreceptors
oRange from single light-sensitive cells to complex, image-
forming eyes
oTwo major types of photoreceptor cells:
Ciliary photoreceptors
•Have a single, highly folded cilium
•Folds form disks that contain photopigments
Rhabdomeric photoreceptors
•Apical surface covered with multiple outfoldings
called microvillar projections
•Microvillar projections contain photopigments
oPhotopigments
Molecules that absorb energy from photons
•Phylogeny of Photoreceptors
oVertebrates have ciliary receptors, but other chordata have both
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oDon’t have to know details of evolutionary tree
•Vertebrate Photoreceptors
oHave ciliary photoreceptors
Rods
Cones
oBoth have inner and outer segments
oOuter segment (an extension of the colium) – photopigments
oInner segment - forms synapses with other cells
•Rods and Cones
oDifference – the rods are sensitive to dim light, cones are
sensitive to bright light
oMore than one type of cones in mammals (color) whereas one
type of rod (black and white only)
•Diversity in Rod and Cone Shape
oDiverse among vertebrates
oShape does not determine properties of photoreceptor
Properties depend on its photopigment
•Photopigments
oHave two covalently bonded parts
Chromophore
•Derivative of vitamin A
•Ex.. retinal
•Contains carbon-carbon double bonds
•Absorption of light converts bond from cis to trans
oOpsin
G-protein could receptor protein
Opsin structure determines photopigment characteristics
Ex. Wavelength of light absorbed
•Retinal
oLight changes the C bond at position 12, from cis to all trans
•Phototransduction
oLight arrives – photopigment is released – now G protein can be
activated
Vertebrate – ligand gated channel that is gated by ligand;
when there is light, the channel closes – less Na flowing
in (hyperpolarization)
Rhabdomeric – depolarization
•Phototransduction
oSteps in photoreception:
oChromophore absorbs energy from photon
oChromophore changes shape
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Double bond isomerizes from cis to trans
oActivated chromophore dissociates from opsin
“Bleaching”
oOpsin activates G-protein
oFormation of second messenger
oIon channels open or close
oChange in membrane potential
•The Eye
oEyespots
Cells or regions of a cell that contain photosensitive
pigment – ex. Protist Euglena
oEyes are complex organs
Detect direction of light
Light-dark contrast
Some can form an image
•Types of Eyes
oFlat sheet eyes
Some sense of light direction and intensity
Often in larval forms or as accessory eyes in adults
Layer of photoreceptor cells – no shape for it – pigments
layer (epithelial cell)
Layer to absorb light
oCup-shaped eyes
Retinal sheet is folded to form a narrow aperture
Discrimination of light direction and intensity
Lightdark contrast
Image formation
•Poor resolution
•Depends on how narrow the opening is
•When opening starts to close – similar to camera –
image formation
oVesicular eyes (present in most vertebrates)
Lens in the aperture improves clarity and intensity
Lens refracts light and focuses it onto a single point on
the retina
Image formation
oGood resolution
oConvex eyes (annelids, mollusks, anthropods)
Bees etc.
Photoreceptors radiate outward
oConvex retina
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