Photoreception involves the ability to detect light across the electromagnetic spectrum using light-sensitive organs like eyes. There are two main types of photoreceptors - ciliary and rhabdomeric. Photopigments contain a chromophore molecule like retinal that absorbs photons and changes shape, triggering signal transduction cascades in the photoreceptor cell. Eyes have evolved from simple eyespots in single cells to complex image-forming eyes like the vertebrate eye, which uses refraction by the cornea and lens to focus light onto the retina. In the retina, rods are more numerous and provide low-light vision while cones provide high-resolution color vision in bright light. The visual signal is then processed

1. Photoreception
Abilitytodetectasmall proportionof theelectromagneticspectrum
from ultraviolet to near infrared. Organs range from single light-
sensitive cells to complex, image forming eyes
Two major types
 Ciliary photoreceptors – have single, highlyfolded 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
Vertebrate Photoreceptors
All are ciliary photoreceptors. There are two types of
photoreceptors: Rods and Cones
Characteristics of Rods and Cones
Nocturnal animals have relatively more rods
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
Phototransduction
Transduction cascades differ in rhabdomeric and ciliary photoreceptors

2. PLC = Phospho lipase C
PIP2 = Phospho tidyel inositol biphosphate
DAG = di-acyl glycerol
IP3 = inositol tri-phosphate
TRP = tryptophan
PDE = phosphor di-esterase
cGMP = cyclic Gwanosine mono-phosphate
The Eye
Eyespots are single cells or regions of a cell that contain photosensitive pigment, e.g., protist Euglena
Eyes are complex organs
Flat-sheet Eyes
Provide some sense of light direction and intensity
Most often seen in larval forms or as accessory eyes in adults
Cup-shaped Eyes
Retinal sheet is folded to form a narrow aperture
Better discrimination of light direction and intensity
Seen in the Nautilus
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
Convex Eye
Photoreceptors radiate outward forming a convex retina
Present in annelids, molluscs, and arthropods
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 –ommatidiumworktogether
to form an image on the retina
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

3.  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
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,mostof the refractionoccursbetweenthe airandthe cornea
Image Accommodation
Accommodation- incominglightraysmustconvergeonthe
retina to produce a clear image
Focal point – point at which light waves converge
Focal distance – distance from a lens to its focal point
Distantobject:lightraysare parallel whenenteringthe lens
Close object: light rays are not parallel when entering the
lens and must be refracted more
Light rays are focusedonthe retinaby changingthe shape
of the lens
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”)
The Fovea
Small depressioninthe centerof the retinawhere overlyingbipolarand
ganglion cells are pushed to the side
Contains only cones
Provides the sharpest images
Signal Processing in the Retina
Rods and cones form different images
Rods
o Principle of convergence– as many as 100 rods synapse
with a single bipolar cell  many bipolar cells synapse
with a ganglion cell
o Large visual field
o Fuzzy image
Cones
o One cone synapseswithone bipolar cell whichconnects
to one ganglion cell
o Small visual field
o High resolution image

4. Complex “on” and “off” regions of the receptive fields of ganglion cells improve their ability to detect contrasts
between light and dark
The Brain Processes the Visual Signal
Opticnerves  opticchiasm optictract  lateral geniculatenucleus
 visual cortex
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)