Download K +

Sensation of light
Retina
rod
cone
Bipolar cell
Horizontal cell
amacrine cell
ganglion cell
5 cell types, 50 various cells
39.
40.
Chemical and light signals
neuron
photoreceptor
NEUROTRANSMITTER
LIGHT
G protein –coupled receptor
Effector enzyme
opszin
G protein –coupled receptor
(rodopsin)
Effector enzyme
retinál
G protein
Secondary messenger
(increase of amount)
G protein
(transducin)
Secondary messenger
(decrease of amount)
Ion channel
Ion channel
(open or close)
(close)
Phototransduction
dark
41.
light
cGMP-gated
Na+- channel
Na+: INWARD
Dark current
K+: OUTWARD
K+: OUTWARD
K+- channel
K+
Na+
Na+:
K+
Na+
depolarized state
hyperpolarization
- 40 mV
-40 - -70 mV
Na+/K+ pump
light impulse
42.
Phototransduction
retinal binding site
Space between discs
George Wald
Nobel Prise, 1967
disc membrane
opsin
retinal
cytosol
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
Dark current
43.
Na+
- depolarization
Na+
cGMP-gated ion channel
Na+
Na+
Na+
rod
rhodopsin
cGMP phosphodiesterase
R
GDP
T
K+
transducin
K+
Na+
K+
K+
guanilyl cyclase
GTP
glutamate neurotransmitter
(analogous release)
cGMP
Transducin: G-protein
rodopsin: G-protein-coupled metabotropic receptor
Phototransduction
44.
Na+
- hyperpolarization
Na+
Na+
cGMP-gated ion channel
rhodopsin
Na+
cGMP phosphodiesterase
R
T
GDP
GDP
GTP
T
K+
Na+
K+
K+
LIGHT
transducin
K+
-1 photon activates 500 transducin molecules, which in turn close several
glutamate neurotransmitter hundred ionchannel, and protects the entrance of 106 Na+
- Approximately 10% of the incoming light reaches the retina
Na+