Rod Photoreceptor

References

The human retina has 110-125 x 10^6 rods (Oesterberg)

• Postsynaptic connections (Input)
  • Light
    • The rod is a photoreceptor, so its main source of input is the transduction of light.
  • Rods
    • only in lower vertebrates Rods via electrical synapses
  • Cones
    • Red Cones via electrical synapses
    • Green Cones via electrical synapses
    • Blue Cones via electrical synapses
  • Horizontal Cells
    • Type B Horizontalvia conventional synapse. Note that this synapse may be bidirectional.

• Presynaptic connections (Output) -Diagram
  • Rods
    • Rods via electrical synapses
  • Cones
    • Red Cones via electrical synapses
    • Green Cones via electrical synapses
    • Blue Cones via electrical synapses
  • Horizontal Cells
    • 2-3 Type B Horizontal via conventional synapse.
  • Rod Bipolar
    • 1 or 2 rod bipolar via a ribbon synapse.

• Function
  • Phototransduction cascade - the hypothesized series of steps required to transform photon absorption into the closing of channels. notes mostly from Dowling's 'Retina' and Hille's 'Ionic Channels of Excitable Membranes' (nice web image)
    • Pre-absorption rhodopsin: retinal (R or R) or 11-cis-retinaldehyde
      • see Dowling's 'Retina' page 199 for rhodopsin -> all-trans which includes metarhodopsin I & II, pararhodopsin and opsin.
    • Post-absorption rhodopsin: (R* or Rh*) all-trans-retinaldehyde
    • R* stimulates Transducin, a G-protein (G_T)
    • This stimulation, or the act of stimulation, causes transducin to exchange a guanosine diphosphate (GDP) molecule for a gaunosine triphosphate (GTP) molecule.
    • Transducine with GTP activates Phosphodiesterase (PDE)
    • PDE changes cyclic GMP (cGMP) into an inactive for, GMP
    • cGMP acts as a second messenger and opens channels, decreasing cGMP results in channels closing.
    • Channels close making the cell hyperpolarize.
  • Absolute threshold = about -4.4 log photopic trolands
  • Saturation begins at about 2.9 log photopic trolands
  • Hood's "Sensitivity to Light" page 5-30: "By about 2,000 scotopic td, the rod system is incapable of discriminating between two lights that differ in intensity by as much as a factor of 100." Contrasts on a 120 cd/m2 background are undetectable.
  • Maximally sensitive to 500 nm wavelength.
  • Spatial summation up to 1-2 degrees (graham & Bartlett, 1939)
  • Temporal summation up to 100 msec (Graham & Margaria, 1935)
  • Temporal integration ~300 ms
  • As photopigment absorbs light it becomes ineffective or bleached. General formulas for modeling photopigment depletion are available.
  • Perpetual glutamate release in the dark, but glutamate release decreases in the light..
  • Responsible for the A-wave in the ERG.

• Morphology
  • Diameter
    • Outer edge of fovea: 1.0 micrometers
    • Parafovea: 1.0-1.5 micrometers
    • Perifovea: 1.5 micrometers
    • Distant Periphery: 1.5-2.5 micrometers
  • Outer Segment
  • Inner Segment
  • Spherule - Presynaptic structure

• Array characteristics

• Compartmental Model Characteristics

• Location

• Subclasses

• Synapse types
  A single ribbon synapse.

• Synapse Receptors
  • 130 docking cites

• Neurotransmitters
  • Glutamate is released.

• Immunoreactivity

• Nomenclature

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