How do photoreceptors work?

Transcription

1

2

3 How do photoreceptors work? Convert energy from light into nerve signals No easy feat!!

4 200 femtoseconds!!

5 Phototransduction Isomerization Opsin - transducin Transducin - PDE PDE - cgmp to GMP Low cgmp closes ion channels hyperpolarized Isomerization Opsin - transducin Transducin - PLC PLC - produces IP 3 IP 3 - release of Ca +2 Ca +2 opens ion channels depolarized

6 Photopigments can be characterized according to the efficiency with which they absorb light S Rod M L Absorbance Wavelength (nm)

7 Principle of Univariance: Photoreceptors cannot register the wavelength of the photons they catch - i.e., the output depends on quantum catch, but not upon what quanta are caught.

8 Monochromat with 1 cone type Incident Photons Pigment Absorptance Number Absorbed λ 1 λ 2 Fraction of incident light absorbed λ 1 λ Wavelength (nm)

9 Monochromat with 1 cone type Fraction of incident light absorbed Incident Photons Pigment Absorptance Number Absorbed λ 1 λ 2 λ 1 λ 2 A Perfect Match! Wavelength (nm)

10 Monochromat with 1 cone type Fraction of incident light absorbed Incident Photons Pigment Absorptance Number Absorbed λ 1 λ 2 λ 1 λ Wavelength (nm) λ 2 Unable to discriminate!

11

12 Dichromat with 2 cone types λ 1 λ 2 Fraction of incident light absorbed λ 1 λ 2 No Match Possible Wavelength (nm)

13 12 10 Test R & G Primaries R/R + G

14 2 types of L pigment in normal males Frequency Peak of L pigment

15 Comparative color vision

16 Direct techniques for measuring photopigment spectra: Micorspectrophotometry (MSP) Electroretinography (ERG) Suction electrode recordings Genetic analysis (indirect??)

17 L UV S M L M Rh S UV

18

19 L L M M Rh S UV

20 L, M & S or L, L & UV??

21

22 Relative Sensitivity Wavelength (microns)

23

24

25

26

27

28

29

30 1.2 1 Absorbance Wavelength (nm)

31 Mechanisms of Spectral Tuning Changes in the opsin protein Ocular filters (lens, oil droplets) Altering the chromophore Different optical density

32

33 Helix 5 Helix 1 Helix 2 Helix 3 Helix 6 Helix 7 Helix dimorphic sites: spectral tuning sites: long- vs middle-wave determining sites chromophore attachment site Neitz & Neitz (1998)

34 12 10 Test R & G Primaries R/R + G

35 2 types of L pigment in normal males Frequency Peak of L pigment

36 Gene transcription mrna translation Protein (opsin) transduction Behavior

37 Exon 2 Exon 3 Exon 4 No Total = 159 Amino Acid I V Y M V V V A T S V T I S L I A I S I A M Frequency Variants of the L gene in individuals with normal color vision

38 Adding an oil droplet in the inner segment effectively narrows the overall relative spectral sensitivity of the cone relative to the pigment...

39 Common chromophores used in vision Retinal - most widely used 3-dehydroretinal - freshwater fishes, amphibians and reptiles 3-hydroxyretinal - insects (moths, flies, butterflies)

40 1 Relative Absorbance Increasing optical density broadens the absorbance spectrum Wavelength

41 Evolution of Trichromacy in primates

42 L L M M Rh S UV

43

44 Duplication 14 KB 236 base pairs 25 KB

45 Divergence Absor pti on (per cent of ma xi mu m Wavelength (nm)

46 Selective expression 236 bp 14 KB Neural circuitry already in place?

47 One Cell-Type Model Stochastic Pigment-Gene Choice random choice mechanism??? Second Order Neurons L M L vs. M gene choice determines cell type

48 Evidence that the neural circuitry was in place - New World primates X 560 Y X 545 Y X 530 Y Wavelength (nm) Wavelength (nm) Wavelength (nm) X 560 X Wavelength (nm) X 545 X Wavelength (nm)

49

50 L M S +? Achromatic channel L M S - Red-green channel L M + - Blue-yellow channel S

51

52

53

54

55

56

57 * HS YY AN AP nasal AP temporal MD JP JC * * RS JW temporal BS JW nasal 5 arcmin * A. Roorda & D. R. Williams, Nature 1999

58 AP nasal BS

59 n = 62 from ERG Frequency L/M Proportion (%L)

60 How does this variation arise??

61 One Cell-Type Model Stochastic Pigment-Gene Choice random choice mechanism??? Second Order Neurons L M L vs. M gene choice determines cell type

62 Selective expression 236 bp 14 KB

63 Color Vision Defects in Humans X-linked inheritance pattern About 8% of Caucasian males have a R/G CVD About 15% of Caucasian females are carriers

64 Forms of Red-Green Color Blindness Deutan: missing functional M pigment Absorption Deuteranope S L Wavelength (nm) Absorption Deuteranomalous 120 S L L Wavelength (nm) Protan: missing functional L pigment Protanope Protanomalous S M S M M Absorption Absorption Wavelength (nm) Wavelength (nm)

65

66 L/M Gene Array 1) Head-to-tail tandem array on the X chromosome, 2) L and M genes are highly homologous, 3) Susceptible to unequal homologous recombination.

67 Unequal crossover generates genetic variation in the pigment genes

68 Inheritance Pattern X n X p X n Y X p Y X n X p X n X n X n Y X p Y X n X n X n X p X n Y X p X p X p Y X n Y X n X p X n X p - carrier - affected male - normal male - affected female - normal female

69 Sharpe & Jägle, 2001

70

71

72

73

74 Characteristics of Color Vision Defects Congenital Present at birth Type and severity is the same thru life Type can be classified precisely Both eyes equally affected Visual acuity is often unaffected Predominantly protan or deutan Higher incidence in males Acquired Onset after birth Type and severity fluctuates Type may not be easily classified. Combined defects occur Monocular differences often occur Visual acuity is often reduced Predominantly tritan Equal incidence in males and females

75 Points of discussion... L:M ratio and color perception Why aren t we pentachromatic?? Did R/G vision really evolve this way? Q & A