Lecture 20: Cellular Basis of Vision: Phototransduction

Question 1

Accessory structures focus image on the retina

Answer 1

Many accessory structures

(e.g. cornea, lens) assist in forming a SHARP and FOCUSED image on the retina

Question 2

Retinal layers = 3

Answer 2

1. Photoreceptors – rods and cones (specialized receptor cells)
2. Neural network – bipolar neurons, horizontal and amacrine cells
3. Ganglion cells – output neurons

Question 3

Structure of photoreceptors: RODS VS CONES

Answer 3

SLIDE 6
Rods:

• Give us the ability to see in low light

Cones:
* Less sensitive
* Give the perception of
colour

Question 4

Photoreceptor membrane potentials =4

Answer 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

Question 5

Hyperpolarization of photoreceptors is
the result of closing of Na+ channels

Answer 5

Hyperpolarization of photoreceptors is
the result of closing of Na+ channels

Question 6

G-protein mediated cascade results
in Na+ channel closure =5

Answer 6

1. Light energy absorbed by rhodopsin
2. Activates the G-protein
   transducin
3. Activates phosphodiesterase
4. Inactivates cGMP
5. Closure of Na+ channels leads to hyperpolarization

Question 7

Enzyme cascade gives amplification resulting in great sensitivity of vision

Answer 7

Absorption of a single photon can cause a disproportionate affect on membrane potential

Question 8

7steps…. Enzyme cascade gives amplification resulting in great sensitivity of vision

Answer 8

1. one rhodopsin molecule absorbs one photon
2. 500 transducin molecules are activated
3. 500 phosphodiesterase molecules are activated
4. 10^5 cyclic GMP molecules are hydrolyzed
5. 250 Na+ channels close
6. 10^6-10^7 ions per second are prevented from entering the cell for a period of aprox 1 second
7. Rod cell membrane is hyperpolarised by 1mV.

Question 9

How does hyperpolarisation cause
excitation in the retinal ganglion?

=4

Answer 9

SLIDE 11

1. Light causes bipolar cells to depolarize because of REDUCTION in inhibitory neurotransmitter release
   from photoreceptors
2. Depolarization, but no AP
3. Excitatory neurotransmitter is released from bipolar cell
4. AP generated

Question 10

Absorption spectra of photoreceptors = 4

Answer 10

S cones = 420nmn

rodes = 500nm

M cones = 532 nm

L cones = 558 nm

Question 11

Absorption spectra of photoreceptors = CONES

Answer 11

CHANGING the relative ACTIVATION of CONES will CHANGE our WHOLE COLOUR PERCEPTION.

Question 12

photobleaching

Answer 12

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.

Question 13

Colour ‘blindness’

Answer 13

• 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)

Question 14

Ishihara colour chart

Answer 14

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.

Question 15

Visual acuity =5

Answer 15

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

Question 16

Remember lateral inhibition? =3

Answer 16

1. 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!

Question 17

Lateral inhibition is mediated by
horizontal cells

Answer 17

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

Question 18

Lateral inhibition allows sharp contrast

ACTIVATION VS REDUCED DEACTIAVTION

KEY POINT

Answer 18

Key point: LATERAL INHIBITION INCREASES OUR ABILITY TO IDENTIFY, CONTRAST AND DETECT EDGES OF OBJECTS.

1. ACTIVATION of CENTRAL receptors and DEACTIVATION of PERIPHERAL
   receptors = SHARP CONTRAST
2. REDUCED DEACTIVATION OF PERIPHERAL RECEPTORS MAKES EDGE HARDER TO SEE

Question 19

Summary =6
Trandsuction? RodsvsCones,
photoreceptors, cones?,

response of sensory
neurons in retina?, receptive feilds of sensory neurons?

Answer 19

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