Lecture 20: Cellular Basis of Vision: Phototransduction Flashcards
Accessory structures focus image on the retina
Many accessory structures
(e.g. cornea, lens) assist in forming a SHARP and FOCUSED image on the retina
Retinal layers = 3
- Photoreceptors – rods and cones (specialized receptor cells)
- Neural network – bipolar neurons, horizontal and amacrine cells
- Ganglion cells – output neurons
Structure of photoreceptors: RODS VS CONES
SLIDE 6
Rods:
- Give us the ability to see in low light
Cones:
* Less sensitive
* Give the perception of
colour
Photoreceptor membrane potentials =4
1 * Photoreceptors are
depolarized in the dark
2 * Receptor POTENTIAL is
HYPERPOLARIZING
3 * Response is delayed
4 * Light causes a REDUCTION in neurotransmitter release
from the photoreceptor
Hyperpolarization of photoreceptors is
the result of closing of Na+ channels
Hyperpolarization of photoreceptors is
the result of closing of Na+ channels
G-protein mediated cascade results
in Na+ channel closure =5
- Light energy absorbed by rhodopsin
- Activates the G-protein
transducin - Activates phosphodiesterase
- Inactivates cGMP
- Closure of Na+ channels leads to hyperpolarization
Enzyme cascade gives amplification resulting in great sensitivity of vision
Absorption of a single photon can cause a disproportionate affect on membrane potential
7steps…. Enzyme cascade gives amplification resulting in great sensitivity of vision
- one rhodopsin molecule absorbs one photon
- 500 transducin molecules are activated
- 500 phosphodiesterase molecules are activated
- 10^5 cyclic GMP molecules are hydrolyzed
- 250 Na+ channels close
- 10^6-10^7 ions per second are prevented from entering the cell for a period of aprox 1 second
- Rod cell membrane is hyperpolarised by 1mV.
How does hyperpolarisation cause
excitation in the retinal ganglion?
=4
SLIDE 11
- Light causes bipolar cells to depolarize because of REDUCTION in inhibitory neurotransmitter release
from photoreceptors - Depolarization, but no AP
- Excitatory neurotransmitter is released from bipolar cell
- AP generated
Absorption spectra of photoreceptors = 4
S cones = 420nmn
rodes = 500nm
M cones = 532 nm
L cones = 558 nm
Absorption spectra of photoreceptors = CONES
CHANGING the relative ACTIVATION of CONES will CHANGE our WHOLE COLOUR PERCEPTION.
photobleaching
Photobleaching is the CHEMICAL ALTERATION of the INDICATOR DYE, be it a FLUROPHORE OR COLOMETRIC DYE
—SO that it is UNABLE TO FLUORESCE due to the DESTRUCTIONof COVALENT or NON-COVALENT due to NON-SPECIFIC BINDING CAUSED BY EXCITATION LIGHT.
Colour ‘blindness’
- Occurs due to MUTATION in GENES ENCODING the PROTEIN COMPONENT of
PHOTOPIGMENTS
–ABNORMALITIESOF COLOUR DISCRIMINATION
- TOTAL COLOUR VISION LOSS = ROD MONOCHROMACY is very rare (1/30,000)
Ishihara colour chart
The Ishihara plates are widely used as a test for colour vision.
Originally designed for the purpose of detecting congenital red-green colour blindness, the test also has some value in demonstrating acquired colour vision defects.
Visual acuity =5
1 * Visual acuity: ability to discriminate small differences in the location of objects within our visual field
2 * If image is directed to the FOVEA: VERY HIGH VISUAL ACUITY
3 * If image is directed to PERIPHERAL PARTS OF RETINA: LESS ACCUITY due to LOWER DENSITY OF RECEPTORS
4 * Works in a similar fashion to the receptor fields in the skin
5 * Can map out receptive fields
Remember lateral inhibition? =3
- Occurs in somatosensory system to
RESTRICT THE SPREAD OF ORIGINAL STIMULI
2 * Due to the PRESENCE OF INHIBITORY INTERNEURONS
3 * Also seen in the visual system!
Lateral inhibition is mediated by
horizontal cells
1 * Photoreceptors= VERY SMALL AREAS IN THE VISUAL FIELD THAT THEY ARE RESPONSIVE TO
2 * HORIZONTAL CELLSPREVENT THE SPREAD OF THE EXCITATORY SIGNAL TO ADJACENT PHOTORECEPTORS.
1: GANGLION CELL IS directly EXPOSED TO LIGHT
2: Adjacent ganglion cell
Lateral inhibition allows sharp contrast
ACTIVATION VS REDUCED DEACTIAVTION
KEY POINT
Key point: LATERAL INHIBITION INCREASES OUR ABILITY TO IDENTIFY, CONTRAST AND DETECT EDGES OF OBJECTS.
- ACTIVATION of CENTRAL receptors and DEACTIVATION of PERIPHERAL
receptors = SHARP CONTRAST - REDUCED DEACTIVATION OF PERIPHERAL RECEPTORS MAKES EDGE HARDER TO SEE
Summary =6
Trandsuction? RodsvsCones,
photoreceptors, cones?,
response of sensory
neurons in retina?, receptive feilds of sensory neurons?
1 * The eye has complex accessory structures to allow the transduction
of light
2 * The eye uses specialized receptor cells (rods and cones) to transduce light
3 * Photoreceptors generate hyperpolarizing receptor potentials and
GPCRs in their transduction mechanism
4 * The combined activation of cones gives the perception of colour
5 * Responses of sensory neurons in retina result from complex excitatory and inhibitory interactions
6 * Receptive fields of sensory neurons are suited to detection of spatial contrast
