L10: Photoreception

Overview of Eye Types

  • The study of eyes encompasses various types of visual systems across different species.

Phylogenetic Placement of Eye Types

  • Flatworms (Platyhelminthes)

    • Defined as "flat plate worm."

    • Possess a simple visual structure known as a cup eye.

    • Example: Planarians (illustrated above).

  • Vertebrates

    • The most familiar group when discussing complex eyes.

    • Camera Eye: Characterized by a lens that focuses light.

  • Arthropods

    • Have compound eyes, diverging from camera eyes.

  • Phylum Mollusca

    • Diverse invertebrate group with varied eye types, including:

    • Cephalopods: Possess camera-like eyes similar to vertebrates.

    • Nautilus: Features a pinhole eye.

    • Bivalves: Have mirror eyes.

    • Limpets and Abalones: Show compound eye adaptations.

Mechanics of Photoreception

  • Photoreceptors are specialized sensory cells that respond to light (photons).

Types of Photoreceptors

  • Ciliary Photoreceptors:

    • Pack photoreceptors into discs mounted on a cilium.

    • Rod Cells: Rectangular shape, sensitive to low light.

    • Cone Cells: Triangular shape, facilitate color vision.

  • Photopigments and Chromophores:

    • Contain molecules that change configuration when struck by light.

    • Process of isomerization:

    • Compound retinol (derived from Vitamin A) changes from a cis form to a trans form upon photon absorption.

    • Requires the enzyme isomerase for recycling back to cis form using ATP (known as regeneration).

Photoreceptor Adaptation and Function

  • Depolarization Mechanism:

    • Electrical synapses transmit the signal to afferent neurons rapidly.

    • Invertebrates often use rhabdomeric photoreceptors, which differ from vertebrates in structure and function.

  • Visual Processing Comparisons:

    • Invertebrates utilize rhabdomeric systems which involve G-protein-coupled receptors (GPCRs).

    • Vertebrates primarily utilize ciliary-type photoreceptors that interact differently with G proteins (inhibitory).

Comparison of Eye Structures

Compound Eye vs. Camera Eye:

  • Camera Eye:

    • Single, concave lens that focuses light.

    • Examples include some mollusks like squids and octopuses.

  • Compound Eye:

    • Composed of numerous ommatidia; each with its own lens.

    • Possesses high field of view but low image resolution (e.g., dragonflies).

    • Apposition Compound Eye: Suitable for bright lighting conditions.

    • Superposition Compound Eye: Enhances sensitivity to dim light.

Unique Adaptations across Taxa

Octopus vs. Vertebrate Eye

  • Octopus Eye:

    • Rods and cones face light directly, allowing maximum photon capture.

  • Vertebrate Eye:

    • Features an inverted retina, requiring light to pass through nerve layers and blood vessels before reaching photoreceptors.

    • Leads to a blind spot where optic nerve exits the retina.

Night Vision and Depth Perception

  • Various adaptations enhance vision in low-light conditions (e.g., tapetum lucidum in many mammals).

  • Depth perception is enhanced in vertebrates through binocular vision and the optic chiasm, allowing visual field overlap.

Color Vision

Types of Color Receptors

  • Rods: Sensitive to low light (max absorption at 500 nm).

  • Cone Types:

    • Short (blue), medium (green), and long (red) wavelength sensitivity.

    • Trichromatic Vision: Present in Old World primates due to gene duplication in opsin genes on the X chromosome.

    • Dichromatic Vision: Most mammals, limited to blue and green sensitivity.

Evolution and Variation in Color Vision

  • Primates have evolved color vision to aid in food selection (e.g., detecting ripe fruits/eliminating visual noise).

  • Some species, like the mantis shrimp, possess advanced color detection abilities (up to 12 cone types).

Conclusion

  • Evolution has led to significant diversity in eye structure and function adaptable to different environments, capturing light in various ways across taxa.