phototransduction notes

Phototransduction Cascade

Ocular Pigments

  • Ocular pigments located within photoreceptors in the retina.
  • Two types of photoreceptors in the vertebrate retina: rods and cones.
    • Rods: Responsible for black and white vision, operate at low light intensity.
    • Cones: Responsible for color vision, function at high light intensity.

Structure and Function of Rods and Cones

  • Rods have outer segments that contain stacks of flattened membrane discs.
    • Disc and plasma membrane contain Rhodopsin (visual purple).
    • Rhodopsin: Composed of a protein (opsin) and a prosthetic group – 11-cis retinal (derived from Vitamin A).
  • Cones mediate color vision using distinct opsins that are closely related but not identical to those in rods.

Visual Pigments

  • Retinal: A light-absorbing molecule that combines with proteins (opsins) to form four types of visual pigments.
    • The type of opsin determines which wavelengths of the visible spectrum retinal can absorb.
    • The absorption spectra of cones overlap; this overlap allows the perception of intermediate hues.
    • Example: Yellow light stimulates red and green cones, with higher activation of red cones leading to the perception of orange; all cones activated produce the perception of white.
  • Retinal isomers: Different 3D configurations of retinal are present.
    • 11-cis-retinal: Bent form of retinal.
    • All-trans-retinal: Straight form.
    • The conversion from bent to straight (11-cis to all-trans) initiates reactions leading to electrical impulses along the optic nerve.

Phototransduction

  • Phototransduction: The process through which light energy is converted into a graded receptor potential; the process begins when a visual pigment captures a photon of light.
Capturing Light
  • Deep purple pigment in rods is rhodopsin, arranged in the rod’s outer segment.
  • Process includes three main steps:
    1. Pigment Synthesis: Opsin and 11-cis-retinal combine to form rhodopsin in the dark.
    2. Pigment Bleaching: When rhodopsin absorbs light, retinal changes to the all-trans isomer, resulting in retinal and opsin separating (breakdown of rhodopsin).
    3. Pigment Regeneration: All-trans-retinal is converted back to 11-cis-retinal, and rhodopsin is regenerated in the outer segments.

Light Transduction Reactions

  • Light-activated rhodopsin activates a G protein called transducin.
  • Transducin activates phosphodiesterase (PDE), which breaks down cyclic GMP (cGMP).
    • In darkness, cGMP keeps cation channels of the outer segment open, allowing Na+ and Ca2+ to flow in and depolarize the cell.
    • In light, the breakdown of cGMP causes these channels to close, leading to cell hyperpolarization, which is the signal for vision.
G Protein Signaling Mechanism
  • Described as a molecular relay race:
    • Light (1st messenger) activates a visual pigment.
    • This switches on a G protein (transducin) which activates an enzyme (PDE).
    • PDE acts as a 2nd messenger, leading to the conversion of cGMP to GMP as cGMP levels fall.
    • Closing of cGMP-gated cation channels results in hyperpolarization of the cell.

Information Processing in the Retina

  • Photoreceptors and bipolar cells generate graded potentials (EPSPs and IPSPs), not action potentials (APs).
  • Hyperpolarization of photoreceptor cells due to light leads to a cessation in the release of inhibitory neurotransmitter glutamate to bipolar cells.
  • Bipolar cells (now depolarized) release neurotransmitters onto ganglion cells.
  • Ganglion cells then generate action potentials transmitted through the optic nerve to the brain.

Signal Transmission in the Retina

  • Photoreceptor Depolarization:
    • Dark: cGMP-gated channels open, allowing Na+ and Ca2+ influx, leading to photoreceptor depolarization.
    • In the dark, neurotransmitter (glutamate) is released continuously to bipolar cells, generating IPSPs.
  • Light effect:
    • In the light: cGMP-gated channels close, stopping cation influx.
    • Photoreceptor hyperpolarizes, resulting in no neurotransmitter release (stopping IPSPs in bipolar cells).
    • This allows bipolar cells to depolarize and release neurotransmitter to ganglion cells, which generates EPSPs.
    • Final outcome: Action potentials propagate along the optic nerve when light is present, enabling vision.

Electrical Signal Events (Diagram Summaries)

  • In the Dark:

    1. cGMP-gated channels open, allowing cation influx.
    2. Photoreceptor depolarizes and voltage-gated Ca2+ channels open at synaptic terminals.
    3. Neurotransmitter released continuously, causing IPSPs in bipolar cell.
    4. Bipolar cells are inhibited, hence no EPSPs occur in ganglion cells.
    5. No action potentials propagated along optic nerve.

  • In the Light:

    1. cGMP-gated channels close, stopping cation influx.
    2. Photoreceptor hyperpolarizes.
    3. Voltage-gated Ca2+ channels close in synaptic terminals, leading to no neurotransmitter release.
    4. Lack of IPSPs in bipolar cell results in its depolarization.
    5. This depolarization opens voltage-gated Ca2+ channels, allowing neurotransmitter release.
    6. EPSPs occur in ganglion cell, resulting in action potentials propagating down the optic nerve, leading to visual perception.