Mechanism of photoreception
As the light strikes the photoreceptors, transformation of photo pigment occurs with the resultant HYPERPOLARIZATION of receptor cells.
During darkness, the Na+ channel are opened / remain open on rod and cone cell membrane.
Na+ leak into rods and lower the membrane potential. As light strikes the rods retinal is released from rhodopsin and causes closure of many Na+ channels.
This causes HYPERPOLARIZATION of the receptor cell membrane and a decrese in the transmitter released at the synapse with the bipolar cells.
Breakage of rhodopsin to RETINAL + OPSIN by light is temporary and Rhodopsin is resynthesized afterwork.
Activity of photoreceptors
In dark
The cell membrane of the photoreceptors contains chemical messenger-gated Na+ channels. They respond to the second messenger cGMP.
The Na+ channels are open when the cGMP is bound to them. In absence of light, the concentration of cGMP is high; therefore it is bound with the Na+ channels, keeping them open.
The inward leak of the Na+ ions depolarizes the photoreceptors. This in turn keeps the Ca 2+ channels opened, which triggers the release of the neurotransmitter from synaptic terminal.
In Light
There are other biochemical cascades that take place in the light. The concentration of the cGMP is reduced.
The reduction in cGMP is by the cascades as follows: The retinene absorbs light and changes the conformation and activates the photopigment.
The rods and cones contain G-protein called transducin, activates the enzyme phosphodiesterase. which degrades the cGMP causes the closure of Na+ channels. As the Na+ channels closes, the photoreceptor cell gets hyperpolarized which in turn reduced the release of neurotransmitter.