Sensory Systems 3

Photoreceptors

  • photoreceptors covert light energy into changes in membrane potential

  • Rhabodomeric photoreceptors

  • Ciliary photoreceptors

  • Ciliary vs Rhabdomeric photoreceptors

Both classes of photoreceptor are found throughout the animal kingdom

  • The photoreceptive opsin are seven-membrane spanning GPCRs

  • Opsins covalently bind vitamin-A-derived chromophores (pigments)

  • Photons cause the isomerization of chromophore

  • Isomerization of chromophore results in changes in opsin

  • Rhodopsin’s conformational change induces G-protein signaling

  • Rhabdomeric photoreceptors employ “R-opsins” which signal through Gq

  • Cilary photoreceptors express “C-opsins” which signal through

Light induced events

  • opsin covalently bonded to vit.A derived chromophores

  • chromophores are usually in a cis-conformation

  • photon sensing causes isomerization to trans-conformation

  • isomerization leads to changes in the opsion

    • dissociation of chromophore from opsin (called bleaching)

    • conformotionchange in opsin

  • G-protein signaling causes changes in membrane potential

Mammalian eye

  • allows formation of a bright, focused image

Nocturnal animal

  • choroids of nocturnal animals contain a reflective layer

Cornea and lens are converging (convex) lenses

  • most refraction occurs at the cornea

  • the lens is responsible for the fine-tuning the focus of the retina

  • most of focusing done at the cornea nor the lens

Accommodation refers to the ability of the eye to focus light from different distances

Mammals have two types of photoreceptor cells

  • Rods

    • ciliary photoreceptors

    • rod-shaped outer segment

    • sensitive to very dim light

  • Cones

    • ciliary photoreceptors

    • cone-shaped outer segment

    • sensitive to brighter light

Many rods synapse on single bipolar cell

Cones in the fovea centrals connect to a single ganglion cell via a single bipolar cell

  • The absorbance spectra of human rods and cones are the basis for color discrimination

  • Activated horizontal cells inhibit neighboring bipolar cells

  • The neural circuitry of retinal processing

    • 1) light hyper polarizes photoreceptor, reducing transmitter release

    • 2) a decrease in inhibitory signaling depolarizes the bipolar cell

    • 3) depolarization increases excitation of ganglion cell

  • There are many variation on the pattern of retinal connectivity