RETINA AND PHOTOTRANSDUCTION

Views:
 
Category: Entertainment
     
 

Presentation Description

No description available.

Comments

Presentation Transcript

RETINA AND PHOTOTRANSDUCTION:

RETINA AND PHOTOTRANSDUCTION Dr.Abhijit Gogoi University of Fiji

Eye Anatomy:

Eye Anatomy http://en.wikipedia.org/wiki/Eye Designed to focus light onto the retina

Photoreceptor Functions:

Photoreceptor Functions Monochromatic vision Single visual pigment Scotopic vision (low light conditions) “Night” vision High Sensitivity Often respond to single photon Slow response – stimuli added Peripheral vision “Warning” vision Wide distribution Covers large visual angle None in fovea Chromatic vision 3 visual pigments Trichromatic vision Photopic vision (high light conditions) Low sensitivity 1000x less than rods Often misconstrued as “Color” vision Detail vision Foveal location High spatial acuity (resolution) High density Less “escaped light” Fast response to stimuli Rods Cones

Microscopic Anatomy of the Retina:

Slide 4 Microscopic Anatomy of the Retina Photoreceptors: Cells that convert light energy into neural activity Direct (vertical) pathway: Horizontal cells, Amacrine cells : modify the responses of bipolar cells and ganglion cells via lateral connections Ganglion cells Output from the retina Photoreceptors bipolar cells ganglion cells

Microscopic Anatomy of the Retina:

Slide 5 Microscopic Anatomy of the Retina The Laminar Organization of the Retina Cells organized in layers Inside-out : Upper cells are relatively transparent Pigmented epithelium is critical to maintain photoreceptors and photopigments

Microscopic Anatomy of the Retina:

Slide 6 Microscopic Anatomy of the Retina Photoreceptor Structure Electromagnetic radiation to neural signals Four main regions Outer segment Stack of membraneous disks that contain photopigments Lights are absorbed by photopigments and lead to changes in membrane potential Inner segment Cell body Synaptic terminal Types of photoreceptors Rods have more disks and higher photopigments concentration - 1000 times more sensitive to light than cones Scotopic retina Cones detect colors Photopic retina

Microscopic Anatomy of the Retina:

Slide 7 Microscopic Anatomy of the Retina Regional Differences in Retinal Structure Varies from fovea to retinal periphery Peripheral retina Higher ratio of rods to cones Higher ratio of photoreceptors to ganglion cells - lower acuity More sensitive to light (rods are specialized for low light) Rods outnumber cones in the human retina (20 to 1)

Microscopic Anatomy of the Retina:

Slide 8 Microscopic Anatomy of the Retina Regional Differences in Retinal Structure Cross-section of fovea: Pit in retina due to lateral displacement of the cells above the photoreceptors Maximizes visual acuity by allowing light to strike photoreceptors directly (no scattering) Central fovea: All cones (no rods) 1:1 ratio with ganglion cells Area of high visual acuity

Phototransduction:

Phototransduction Eric Niederhoffer SIU-SOM Retina and photoreceptor Rhodopsin Excitation - recovery/adaptation Cyclic nucleotide gated channels

Retina and Photoreceptor:

Retina and Photoreceptor http://www.fz-juelich.de/inb/inb-1/Photoreception/ http://www.mardre.com/homepage/mic/tem/samples/bio/ros/rosdiag.html

Visual Pigments: Chromophore:

Visual Pigments: Chromophore Retinal (aldehyde derivative of Vitamin A) Aka retinaldehyde Absorption in near ultraviolet (330-365 nm) Induces photoisomerization h ν = energy required to promote retinal to an excited state Rotation around the double bond more energetically favored 11- cis retinal all- trans retinal +h ν *

PowerPoint Presentation:

Vitamin A = half a beta-carotene (get from carrots, leafy vegetables, liver, melons, pumpkins, papayas, etc) Sufficient energy provided by the photon to promote retinal to an excited state in which rotation around the bond is more energetically favored then relaxes to stay in this conformation

Visual Phototransduction:

Visual Phototransduction Conversion of electromagnetic radiation into electrical signals Absorption of electromagnetic radiation Triggering of a signaling cascade Change in electrical properties of the cell

Phototransduction :

Slide 14 Phototransduction Phototransduction in Rods Depolarization in the dark: “Dark current” -30 mV Due to steady influx of Na+ through cGMP gated sodium channel Constant production of cGMP by guanylyl cyclase Hyperpolarization in the light Light reduces cGMP to close Na+ channel - hyperpolarize transducin

Phototransduction :

Slide 15 Phototransduction Phototransduction in Rods Depolarization in the dark: “Dark current” to -30 mV Due to steady influx of Na+ through cGMP gated sodium channel Constant production of cGMP by guanylyl cyclase Hyperpolarization in the light Light reduces cGMP to close Na+ channel - hyperpolarize PDE

PowerPoint Presentation:

Slide 16 Rhodopsin Photopigment that absorb electromagnetic radiation Receptor protein that is embedded in the membrane of the stacked disks in the rod outer segments Receptor protein with a prebound chemical agonist [Opsin (GPCR) + retinal (vitamin A derivative)] Bleaching : change in conformation of retinal by light absorption leading to the activation of opsin (by dissociation) Phototransduction 11- cis- retinal all- trans- retinal

Rhodopsin:

Rhodopsin Metarhodopsin II (t 1/2 ~ 60 s) Rhodopsin* Rhodopsin bathorhodopsin lumirhodopsin metarhodopsin I Opsin + trans -retinal retinol cis -retinal RPE h 

Visual Phototransduction:

Visual Phototransduction Light → electrical signal http://en.wikipedia.org/wiki/File:Phototransduction.png

PowerPoint Presentation:

Replenishment of 11- cis retinal http://en.wikipedia.org/wiki/File:Visual_cycle_v2.png

Excitation - Recovery/Adaptation:

Excitation - Recovery/Adaptation CaM CNGC cGMP cGMP hv Rh c-tR Excitation Recovery/Adaptation T a T bg GDP GTP T a I I PDE PDE GMP GC Arr RK Rc Low [Ca 2+ ] cGMP cGMP GTP GTP T a GDP cGMP GTP Low [Ca 2+ ] GCAP-1 Rh tR PPP tR cR Na + Ca 2+ K + Ca 2+ Na + Rh tR RPE65

PowerPoint Presentation:

Slide 21 Light - retinal - opsin - transducin - PDE - cGMP - cGMP gated Na + channel Signal amplification : very low number of photons can be detected Phototransduction

Phototransduction :

Slide 22 Phototransduction Phototransduction in Cones Similar to rod phototransduction Rods are hyperpolarized constantly - saturated Daytime vision depends on cones, whose photopigments require more energy to become bleached Different opsins : major difference Red, green, blue cones Color detection Contributions of blue, green, and red cones to retinal signal Young-Helmholtz trichromacy theory of color vision Brain assigns colors based on a comparison of the readout of the three color types Color blindness - significant spectral abnormality (beyond normal variation), mostly due to genetic errors Abnormal red-green vision is the most common abnormality that is more frequently found in men Peak sensitivity of rods is to a wavelength of 500 nm (blue-green)

Phototransduction:

Slide 23 Phototransduction Dark and Light Adaptation Dark adaptation— Increasing the sensitivity to light (10 6 fold) Dilation of pupils - 2-8 mm diameter; 16 fold Regeneration of unbleached rhodopsin Adjustment of functional circuitry - signals from more rods are available to each ganglion cells All-cone daytime vision All-rod nighttime vision 20–25 minutes

Phototransduction:

Slide 24 Phototransduction Dark and Light Adaptation Calcium’s Role in Light Adaptation Blinding sensation reflects saturation of both rods and cones (hyperpolarization) Cones gradually adapt their membrane potential to - 35 mV cGMP-gated Na+ channel also allow Ca++ that inhibit guanylyl cyclase (balancing GC in the dark) Under the light, low Ca++ concentration in the hyperpolarized cones gradually activates GC, recovering cGMP - open the Na+ channel again Ca++ also affect photopigments and PDE to reset their responses to light What we see is the relative difference in light level, not the absolute level!

Retinal Processing:

Slide 25 Retinal Processing Only ganglion cells fire APs! All other cells respond with graded changes in membrane potential : difficult to detect! Transformations in the Outer Plexiform Layer Photoreceptors form synapses with bipolar cells and horizontal cells Output of photoreceptors is generated by dark rather than light Dark is the preferred stimulus When a shadow passes across a photoreceptor, it responds by depolarizing and releasing neurotransmitter glutamate

Colour Vision:

Colour Vision Trichromatic theory of colour vision There is only one type of rod and this responds strongly to bluish-green light Cones are divided into three categories, each of which has a different sensitivity to light There are red light receptors, green light receptors and blue light receptors. These cone sensitivities support the trichromatic theory as all colours of the visible spectrum can be seen by mixing the 3 primary colours (red, blue and green) White objects reflect all colours to eye, black absorbs all colours so no light to the eye.

Wavelengths of light absorbed by different cones:

Wavelengths of light absorbed by different cones

Colour Blindness:

Colour Blindness If you have normal vision you will see a figure seven in reddish brown dots. People with red-green colour blindness will not see the 7, why? These people lack red sensitive cones, but the green stimulated cones are stimulated by the red light, so all dots appear green

Color Vision:

Color Vision

Pigment Anatomy :

Pigment Anatomy 3 types of cones: short (S), middle (M), and long (L) wavelength sensitive. (S): 430 nm = blue (M): 530 nm = green (L): 560 nm = red

Pigment Anatomy :

Pigment Anatomy Origin of pigments Red/green from opsin gene on X-chromosome or sex chromosome. Show very similar amino acid seqs. (96%) Blue on chromosome 7 and rhodopsin on chromosome 3 are very different.

Photoreceptor Anatomy:

Photoreceptor Anatomy Example: if you stimulate all 3 types of cones about equally the result is white or no color.

Color Vision:

Color Vision “Blue” “Green” “Red” Red Wavelength Input Cone Signal to Brain Theories of Color Vision: Trichromatic Theory

Color Vision:

Color Vision “Blue” “Green” “Red” Yellow Equal Parts Red and Green = Wavelength Input Cone Signal to Brain Theories of Color Vision: Trichromatic Theory

Color Vision:

Color Vision Trichromatic theory of color vision: brain interprets the relative amounts of signaling from each of these cone types This means that some colors can be matched by a pair of wavelengths metamers : colors that have no definite single wavelength (e.g. yellow) This also means that any color can be matched by mixing (not more than) three different wavelengths Theories of Color Vision: Trichromatic Theory

Color Vision:

Color Vision Theories of Color Vision Trichromatic Theory can explain some aspects of colorblindness: most of us are trichromats someone missing one of the three cone types is a dichromat someone missing two is a monochromat someone missing all cone types is called a rod monochromat (very poor vision!)

Physiology of color blindness :

Physiology of color blindness Male dominant trait but females carry it. Females have 2 X chromosomes so trait is normally not expressed. Males have 1 X and 1 Y chromosome which means recessive traits will show in phenotype. As stated before, red and green pigments originate on the X chromosome. 1 in 20 males suffers from some form of color blindness. Most common is red-green color blindness which is caused by problem with M or L cones. However, they can still see red and green, but have trouble with light or desaturated colors.

Physiology of color blindness :

Physiology of color blindness A few types: Anomalous Trichromacy: have 3 photopigments, but only from 2 groups. Most common is deuteranomalous = 2 L photopigments. Dichromacy: missing 1 group of photopigments.

Color Vision:

Color Vision Trichromatic Theory can explain some aspects of colorblindness: dichromats have only two primaries: any color they can see can be matched with differing proportions of the two wavelengths to which they are sensitive most common is deuteranopia (~3% of men, <1% of women) - missing “green” cones Theories of Color Vision: Trichromatic Theory

Color Vision:

Color Vision White light is a mixture of wavelengths prisms decompose white light into assorted wavelengths OR recompose a spectrum into white light Wavelength and Color

PowerPoint Presentation:

Additive mixing is most intuitive: ADD wavelengths: red+green = yellow red+blue = magenta blue+green = cyan red+green+blue=white Color Mixing

authorStream Live Help