Describe the steps in phototransduction.
- 1)cGMP in photoreceptors gates a cation channel and when open this
- channel allows Na+ to flow into the cell. cGMP concentration is high in the
- dark, maintains channels in an open state (causes the “dark current”) and cell is relatively
- depolarized (RMP -40 mV).
2) Light stimulates cGMP phosphodiesterase and cGMP concentration decreases.
3) As cGMP concentration falls, channels close and the photoreceptor hyperpolarizes.
- 4) Decreased release of either excitatory or inhibitory NT from synaptic terminals of
- photoreceptor leads to either a depolarization or hyperpolarization of bipolar and horizontal cells.
light leads to formation of Metarhodopsin II => activation of transducin (G protein) => activation of phosphodiesterase => decrease in cGMP => cation channels close (sodium influx stops) => photoreceptor hyperpolarizes