Transducing of Light Energy into Electrical Energy

Receptors have outer segments, which contain:

• Visual WHAT molecules, which have two components:

• WHAT, a large WHAT

• WHAT, a light-sensitive molecule

Receptors have outer segments, which contain:

• Visual PIGMENT molecules, which have two components:

• OPSIN, a large PROTEIN

• RETINAL, a light-sensitive molecule

Visual transduction occurs when the retinal absorbs a WHAT.

• The retinal changes its WHAT, which is known as WHAT.

Visual transduction occurs when the retinal absorbs a PHOTON.

• The retinal changes its SHAPE, which is known as ISOMERIZATION.

How do we see light

• (a) Cis-retinal

• (b) Isomerization: WHAT-retinal becomes WHAT- retinal.

• (c) WHAT & WHAT break apart. The retina becomes WHAT

How do we see light

• (a) Cis-retinal

• (b) Isomerization: CIS-retinal becomes TRANS- retinal.

• (c) RETINAL & OPSIN break apart. The retina becomes BLEACHED

Wald’s visual cycle:

• George Wald (1906- 1997) was awarded the Nobel Prize for his discovery of this cycle.

• Isomerization of WHAT activates enzymes that break down WHAT

• cGMP-gates WHAT channels close

• Inflow of Na+ slows

• WHAT receptor potential

• WHAT release is turned off, which excites the WHAT cell (ON bipolar cell)

• Isomerization of RETINAL activates enzymes that break down cGMP

• cGMP-gates Na+ channels close

• Inflow of Na+ slows

• HYPERPOLARIZING receptor potential

• GLUTAMATE release is turned off, which excites the BIPOLAR cell (ON bipolar cell)

Dark center, light surround

• Cone WHAT

• Glutamate WHAT

• Excites WHAT bipolar cell

Dark center, light surround

• Cone DEPOLARIZE

• Glutamate RELEASED

• Excites OFF bipolar cell

light center, dark surround

• Cone WHAT

• Glutamate WHAT

• Excites WHAT bipolar cell

light center, dark surround

• Cone HYPERPOLARIZED

• Glutamate RELEASE DECREASES

• Excites ON bipolar cell

Light turns on an WHAT bipolar and WHAT ganglion cell

Light turns on an ON bipolar and ON-CENTER ganglion cell

What are the three experiments used to measure the dark adaptation curve

1. Experiment measuring WHAT adaptation

2. Experiment measuring WHAT adaptation

3. Experiment measuring WHAT and WHAT adaptation

What are the three experiments used to measure the dark adaptation curve

1. Experiment measuring CONE adaptation

2. Experiment measuring ROD adaptation

3. Experiment measuring CONE and ROD adaptation

Method using all three experiments

• Observer is WHAT adapted

• Light is turned WHAT

• Once the observer is WHAT adapted, they adjust the WHAT of a test light until they can just WHAT

Method using all three experiments

• Observer is LIGHT adapted

• Light is turned OFF

• Once the observer is DARK adapted, they adjust the INTENSITY of a test light until they can just SEE IT

Experiment for cone adaptation

• Observer looks at a test light WHAT such that the test light stimulates only the WHAT in the fovea.

• Sensitivity increases for WHAT to WHAT minutes and then WHAT (green curve).

• Threshold WHAT; sensitivity WHAT

Experiment for cone adaptation

• Observer looks at a test light DIRECTLY such that the test light stimulates only the CONES in the fovea.

• Sensitivity increases for THREE to FOUR minutes and then LEVELS OFF (green curve).

• Threshold DECREASES; sensitivity INCREASES

Experiment for rod adaptation

• Must use a rod WHAT (a person with no WHAT).

• Sensitivity WHAT (i.e., threshold decreases) for about WHAT minutes and then levels off (red curve)

Experiment for rod adaptation

• Must use a rod MONOCHROMAT (a person with no CONES).

• Sensitivity INCREASES (i.e., threshold decreases) for about 25 minutes and then levels off (red curve)

Experiment for rod and cone adaptation

• Observer looks at a fixation point but pays attention to a test light to the side.

• The test light falls on the WHAT retina, which contains both WHAT and WHAT.

• Sensitivity displays a two-stage WHAT curve (the blue curve)

Experiment for rod and cone adaptation

• Observer looks at a fixation point but pays attention to a test light to the side.

• The test light falls on the PERIPHERAL retina, which contains both RODS and CONES.

• Sensitivity displays a two-stage ADAPTATION curve (the blue curve)

Experiment for rod and cone adaptation

• Sensitivity increases in WHAT stages.

• Stage one takes place for WHAT to WHAT minutes.

• Then sensitivity levels off for WHAT to WHAT minutes – the rod-cone WHAT

• Stage two shows increased sensitivity for another WHAT to WHAT minutes.

Experiment for rod and cone adaptation

• Sensitivity increases in TWO stages.

• Stage one takes place for THREE to FOUR minutes.

• Then sensitivity levels off for SEVEN to TEN minutes – the rod-cone BREAK

• Stage two shows increased sensitivity for another 20 to 30 minutes.

Explaining the two-stage dark adaptation curve

• Our sensitivity to light depends on the concentration of the WHAT (WHAT).

• The speed at which our sensitivity increases in the dark depends on the WHAT of the visual pigment (WHAT retinal becomes WHAT retinal again).

Explaining the two-stage dark adaptation curve

• Our sensitivity to light depends on the concentration of the VISUAL PIGMENT (CIS-RETINAL).

• The speed at which our sensitivity increases in the dark depends on the REGENERATION of the visual pigment (TRANS retinal becomes CIS retinal again).

Explaining the two-stage dark adaptation curve

• Cone pigment takes WHAT minutes to regenerate completely.

• Rod pigment takes more than WHAT minutes to regenerate completely.

Explaining the two-stage dark adaptation curve

• Cone pigment takes 6 minutes to regenerate completely.

• Rod pigment takes more than 30 minutes to regenerate completely.

Spectral Sensitivity

• WHAT of rods and cones to different parts of the visual spectrum

• Use monochromatic light to determine the WHAT at different WHAT.

• Threshold for light is lowest in the WHAT of the spectrum.

• WHAT = sensitivity, which produces the spectral sensitivity curve.

Spectral Sensitivity

• SENSITIVITY of rods and cones to different parts of the visual spectrum

• Use monochromatic light to determine the THRESHOLD at different WAVELENGTHS.

• Threshold for light is lowest in the MIDDLE of the spectrum.

• 1/threshold = sensitivity, which produces the spectral sensitivity curve.

Rod spectral sensitivity

• More sensitive to WHAT light

• Most sensitive at WHAT nm

Rod spectral sensitivity

• More sensitive to SHORT-WAVELENGTH light

• Most sensitivity at 500 nm

Cone spectral sensitivity

• Most sensitive at WHATnm

Cone spectral sensitivity

• Most sensitive at 560nm

Purkinje shift

Enhanced sensitivity to short wavelengths during dark adaptation when the shift from cone to rod vision occurs

Spectral Sensitivity

• Difference in spectral sensitivity is due to WHAT of visual pigments.

• Rod pigment absorbs best at WHAT nm.

• Cone pigments absorb best at WHATnm, WHATnm, and WHATnm.

• Absorption of all cones equals the peak of WHATnm in the spectral sensitivity curve

Spectral Sensitivity

• Difference in spectral sensitivity is due to ABSORPTION SEPCTRA of visual pigments.

• Rod pigment absorbs best at 500 nm.

• Cone pigments absorb best at 419nm, 531nm, and 558nm.

• Absorption of all cones equals the peak of 560nm in the spectral sensitivity curve

Spectral sensitivity was discovered by WHO

Spectral sensitivity was discovered by GEORGE WALD