1. Sensation of light

2. Retina 39. Bipolar cell Horizontal cell amacrine cell ganglion cell
rod
cone
Bipolar cell
Horizontal cell
amacrine cell
ganglion cell
5 cell types, 50 various cells

3. Chemical and light signals
40.
Chemical and light signals
neuron photoreceptor
NEUROTRANSMITTER
LIGHT
G protein –coupled receptor (rodopsin)
G protein –coupled receptor
Effector enzyme
Effector enzyme
opszin
retinál
G protein
G protein
(transducin)
Secondary messenger
(increase of amount)
Secondary messenger
(decrease of amount)
Ion channel
(open or close)
Ion channel
(close)

4. Phototransduction 41. dark light depolarized state hyperpolarization
cGMP-gated
Na+- channel
Na+: INWARD
K+: OUTWARD
Na+:
K+: OUTWARD
Dark current
K+- channel K+ K+
Na+
Na+
depolarized state
hyperpolarization
- 40 mV mV
Na+/K+ pump
light impulse

5. Phototransduction 42. rhodopsin = opsin + retinal retinal binding site
Space between discs
George Wald
Nobel Prise, 1967
disc membrane
opsin
cytosol
retinal
cytoplasmic tail
cis-retinal
Light absorption
curved cytoplazmic tail
rhodopsin = opsin + retinal
retinal: isomerization
(G-protein-coupled
metabotropic receptor)
trans-retinal
opsin: structural change
non-curved cytoplazmic tail

6. glutamate neurotransmitter
Dark current
- depolarization
43.
Na+
Na+
cGMP-gated ion channel
Na+
Na+
Na+
cGMP phosphodiesterase
rod
rhodopsin R
cGMP T
GDP I + K+
transducin K+
Na+ I + K+ K+ + I
guanilyl cyclase
GTP
cGMP
Transducin: G-protein
rodopsin: G-protein-coupled metabotropic receptor
glutamate neurotransmitter
(analogous release)

7. + + + + + + I I I LIGHT I I I Phototransduction 44.
- hyperpolarization
44.
Na+
Na+
Na+
Na+
cGMP-gated ion channel
Na+
cGMP phosphodiesterase
rhodopsin R
cGMP T T I +
GMP I +
GDP
GTP
GTP
GDP K+
transducin I +
Na+ K+
LIGHT K+ I + K+ I + I +
1 photon activates 500 transducin molecules, which in turn close several
hundred ionchannel, and protects the entrance of 106 Na+
- Approximately 10% of the incoming light reaches the retina
glutamate neurotransmitter