Question

Draw and/or describe the process of phototransduction from the molecular change in retinal to the change in ion flow through cGMP-gated ion channels. What would happen if a photoreceptor had a mutation that made the phosphodiesterase enzyme constantly active?

Answer 1

light abosrption converts cis retinal to trans retinal and activating rhodopsin, which catatlyseses the replacement of GDP by GTP on transduction(T). Talpha -GTP activates the cGMP phosphodiestrase and converts cGMP to GMP.Thus reduction of cGMP which kept the cation channels in rod cells. Reduction in cGMP causes closure of cation channels and prevent the Na+ and Ca+ influx and the membarne is hyperpolarised and thus inhibiting the relaese of inhibitory neurotransmitter from rod cells.So there is no inhibitory neurotransmitter from rod cells to the bipolar cells, so bipolar cells relaese excitatory neurotransmitter to the gannglion cells and generate action potential in its axon which is a part of optic nerve.

In dark,the cGMP level will be high enough to keep the cation channels of rod cells and the rod cells will be depolarised state and it relaese inhibitory signals to the bipolar cells and there will be no release of neurotransmitters from bipolar cells and ganglion cells wont be stimulated.

If the phosphodiestrase is constantly active,the cGMP level will be very low in cell. loss of cation channels will lead to hyperpolarised rod cells,and there is no inhibitory neurotransmitter to the bipolar cell and the depolarised bipolar cells relaese excitatory neurotransmitter stimulates ganglion. The ganglion cells will keep actiavted even in dark when phosphodiestrase is constantly active.