Phototransduction

In the dark

Dark Current

A dark current is produced when the cells are at rest, i.e. there is no light. When they are not stimulated by light, the photoreceptors have a high intracellular level of cGMP. This activates cyclic-nucleotide gated channels (CNGs) which causes a constant influx of sodium ions into the cell, which depolarizes the membrane.

 This current is slightly apposed by the presence of:

·         Na+, K+, Ca2+ exchanger

·         Delayed-rectifier K+ current (Ik)

Both of these contribute to a membrane potential of ~ -30 – 40 mV. The depolarisation and therefore, increased intracellular concentrations of Ca2+ causes vesicles containing the neurotransmitter Glutamate to be fuse with the cell membrane and release it into the synaptic cleft. This causes two things:

•  Hyperpolarisation of on-centre Bipolar neurons

•  Depolarises of off centre bipolar cells

In the light

A light photon interacts with the retinal present in the photoreceptor. This retinal then undergoes a conformational change from 11-cis retinal to all-trans retinal. This change means it can no longer bind to its opsin binding site. This then activates the regulatory protein, transducin which swaps its bound GDP for GTP. The activated alpha subunit then stimulates phosphodiesterases which converts cGMP to 5-GMP.

This causes CNG channels to close which prevents the large influx of Na+ ions in to the cell. However, the exchanger is still working; these both cause the membrane potential to become even more negative which causes hyperpolarization. This causes less calcium to enter the cell which inhibits the amount of neurotransmitter released.

A decrease in the amount of glutamate released causes two things:

•     Depolarization of on centre Bipolar cells

•     Hyperpolarization of off centre bipolar cells

        Flow chart to show the phototransduction pathway in the presence of light.