Short/Blue Cone Photoreceptor

References The Human retina has 6.8 x 10^6 cones (Oesterberg) or at least more than 4 x 10^6 (Polyak). There are about 25,000 in the foveola and 110,000 to 115,000 in the fovea.

• Postsynaptic connections (Input)
  • Light
    • The cone is a photoreceptor, so its main source of input is the transduction of light.
    • Blue Cones: 430-440 nm maximally sensitive
  • Rods
    • Rods via electrical synapses
  • Cones
    • Red Cones via electrical synapses
    • Green Cones via electrical synapses
    • Blue Cones via electrical synapses
  • Horizontal Cells
    • Cat Type A Horizontal Cell
    • Cat Type B Horizontal Cell
    • NOT Primate Type H1 Horizontal Cell
    • Primate Type H2 Horizontal Cell

• Presynaptic connections (Output) Cones release Glutamate.
  • Rods
    • Rods via electrical synapses
  • Cones
    • Red Cones via electrical synapses
    • Green Cones via electrical synapses
    • Blue Cones via electrical synapses
  • Excites/Depolarizes Horizontal Cells
    • Cat Type A Horizontal Cell
    • Cat Type B Horizontal Cell
    • NOT Primate Type H1 Horizontal Cell
    • Primate Type H2 Horizontal Cell
  • Bipolar Cells
    • dendrites from up to 4 (mean,sd: 1.84 +- .7) Invaginating Blue Bipolar form the central element of the cone ribbon synapse. (Kouyama and Marshak, 1992)
    • Herr, Tiv, Sterling and Schein ARVO 1996
      • Only Invaginating Blue Bipolar form the central element of the cone ribbon synapse.
      • Pedicle #35 received 31 identified Invaginating Blue Bipolar contacts
      • Pedicle #52 received 47 identified Invaginating Blue Bipolar contacts

• Function - plot of blue spectrum
  • 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.
  • Cones hyperpolarize (negative resting potential becomes more negative) to a light increment.
  • Hyperpolarization = decrease in transmitter release.
  • Response has both a transient (10s of msec) and a sustained (100s of msec) component.
  • Though the physiological evidence isn't decisive, modeling (Smith...) suggests that the cone has a center-surround antagonism. Naturally, the horizontal cells are creditted with the surround.
  • Absolute sensitivity = -2 log photopic trolands
  • Damage possible at 8.5 log photopic trolands
  • Foveal summation area: 2-8 minutes of arc
  • Saturation is produced only for very brief stimuli
  • As photopigment absorbs light it becomes ineffective or bleached. General formulas for modeling photopigment depletion are available.
  • Schnapf et al 1990
    • An empirical equation describing response to a flash.
    • A theoretical equation describing response to a flash.
    • "Response to a brief flash is diphasic" with an initial reduction in dark current followed by a rebound increase resulting from "an increase in the number of open light-sensitive channels."
    • "peak amplitude of the single photon response was estimated as about 30 fA."
    • Half saturation reached at about 650 photoisomerizations. "Saturation curve was gentler than an exponential but steeper than a Michaelis relation." - Within this context the initial peak response is what is saturating.
    • Bleaching did not affect the kinetics or saturating amplitude of subsequent flash responses, though it did reduce flash sensitivity.
    • Membrane current in dark had a variance near .12 pA2 in 0-20 Hz band.
    • "The power spectrum of the dark noise resembled the spectrum of the dim flash response. Noise with the observed magnitude and spectral composition would be generated by photoisomerizations occurring at a rate of about 2400/sec.
    • Blue cone kinetics did not differe substantially from the kinetics of red and green cones.
    • The mean (n=3 all within 1mm of the fovea) maximum amplitude of the positive going portion of the flash response for blue cones was 8(sd=3)pA.
    • The mean maximum amplitude of the negative going portion of the flash response for blue cones was 4(sd=2)pA.
    • The flash producing the mean half saturation point for blue cones was 1732 (sd=750) photons um-2 at peak wavelength.
    • Time to peak of linear flash for blue cones was 61 (sd=1) msec. blue cones was 34 (sd=17) msec.
    • Estimated peak amplitude of the single photon response for blue cones was 10 (sd=0) fA.
  • Valeton and van Norren (1983) - Light adaptation of primate cones: an analysis based on extracellular data
    • Response vs. Intensity function is invariant across background light level (0-6 log td).
    • It takes the form of a Michaelis-Menten function with a half-saturation of 3.2 log td and an exponent of .74.

• Morphology
  • Outer segment - where pigment is.
    • Diameter in foveola: absent
    • Diameter in fovea outer edge: 3.5-4.0 micrometers
    • Diameter in parafovea: 4.5 micrometers
    • Diameter in perifovea: 5.0 micrometers
    • Diameter in distant periphery: 8.0-9.0 micrometers
  • Inner segment
    • Diameter in foveola: absent
    • Diameter in fovea outer edge: 1.3 micrometers
    • Diameter in parafovea: 1.5-2.0 micrometers
    • Diameter in perifovea: 2.0 micrometers
    • Diameter in distant periphery: 3.0 micrometers
  • axon - 50um long, 1.5um diameter
  • Pedicle - the presynaptic structure about 6 um diameter.
  • Kouyama & Marshak note ribbon synaptic complexes which are combinations of 2 or 3 ribbons each associated with the typical triad of dendrites. 6 of 29 ribbons were ribbon complexes with multiple central elements.
  • Herr, Tiv, Sterling and Schein ARVO 1996

    Central Element Morphology

    Pedicle ID #	Ribbons	# of Invaginating Processes
    		Singlet	Doublet	Triplet	Quad
    35	22	16	5	3	0
    47	23	9	10	4	2
    144 partial	10	6	4	0	0

    • S-cones have many more central elements than M & L cones due to more ribbons and many more invaginating contacts per ribbon.

• Array Characteristics
  • 3.5 minutes between B cones in baboon (Marc & Sperling, 1977)
  • absent within 10 minutes of the central fovea (Williams, MacLeod & Hayhoe, 1981)

• Compartmental Model Characteristics
  • light-modulated conductance in the outer segment
  • Leakage Membrane resistance (Rm) 20,000 Ohm-cm2
  • isolated cone dark voltage -20mV
  • membrane "leakage" potential -70mV
  • input resistance 700 MOhms
  • connected cone input resistance is 90MOhms
  • gap junctions simulated as a linear conductance

• Location
  • Phototransduction elements: Photoreceptor Layer
  • Nucleus: Outer Nucleus Layer (ONL)
  • Axon: Outer Plexiform Layer (OPL)

• Subclasses

• Synapse types
  • from Horizontal cell - GABAergic chemical synapses
  • from Cone - Glutamate chemical synapses

• Synapse Receptors
  • 600 docking cites.

• Neurotransmitters
  • Glutamate (Massey, 1990)

• Immunoreactivity/staining
  • Stained by Procion black dye.
  • antiserum JH455 labels in Marmoset, Cebus and Macaque - J. Nathans

• Nomenclature
  • Short wavelength cone = Blue cone

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