Middle/Green Cone Photoreceptor
Input | Output | Function | Morphology | Array | Comp. Model | Location | subclass | Syn Receptor | Transmitters | Immunocytochemistry | Nomenclature
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.
- Green Cones: 535-540 nm maximally sensitive
- Rods
- Cones
- Horizontal Cells
- Presynaptic connections (Output)
Cones release Glutamate.
- Rods
- Cones
- Excites/Depolarizes Horizontal Cells
- Bipolar Cells
- 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.
- 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.
- The mean (n=11, 5 within 1mm of the fovea) maximum amplitude of the positive going portion of the flash response for green cones was 16 (sd = 4) pA.
- The mean maximum amplitude of the negative going portion of the flash response for green cones was 10 (sd = 3) pA.
- The flash producing the mean half saturation point for green cones was 1639 (sd = 695) photons um-2 at peak wavelength.
- Time to peak of linear flash for green cones was 51 (sd = 13) msec.
- Integration time for green cones was 19 (sd = 10) msec.
- Estimated peak amplitude of the single photon response for green cones was 26 (sd = 18) 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: 1.0-1.5 micrometers
- 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: 1.0 micrometers
- 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 - 50 micrometers long, 1.5 micrometer diameter
- Pedicle - the presynaptic structure
- Array characteristics
6 micrometer spacing in area centralis of cat
- Compartmental Model Characteristics
- Array 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
- Neurotransmitters
- Immunoreactivity
- Nomenclature
- Middle wavelength cone = Green cone
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© 1995 Lance Hahn(lance@retina.anatomy.upenn.edu)
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