PHOTORECEPTION• Ability to

detect a small proportion of the electromagnetic spectrum from ultraviolet to near infrared

Figure 7.27

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

VERTEBRATE PHOTORECEPTORS All are ciliary

photoreceptors Two types

Rods Cones

Figure 7.29

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

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 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

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

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

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

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

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 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

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

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

THE BRAIN PROCESSES THE VISUAL SIGNAL

• Optic nerves optic chiasm optic tract lateral geniculate nucleus visual cortex

Figure 7.41

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)