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Phototransduction describes the process of light being converted into an action potential.
Phototransduction takes place in the outer segment of the photoreceptor layer (as discussed in retinal layers). Rod cells contain the pigment rhodopsin which is made of retinal (vitamin A derivative) and opsin. Retinal exists in two forms – 11-cis-retinal which when struck by a photon becomes all-trans-retinal. All-trans-retinal cannot bind opsin, so in effect the photon causes the opsin molecule to be freed from rhodopsin. The opsin molecule activates a protein in the rod disc called Transducin. The activated transducing activates phosphodiesterase. The phosphodiesterase converts cyclic GMP to GMP.
Guanylate Cyclase converts GTP to cGMP. cGMP is usually bound to cGMP gated ion channels of the rod cells. The channels when activated allow Na+ and Ca2+ in and push K+ out making the rod cell more positive. When a photon of light frees opsin from rhodopsin the level of cGMP within the rod decreases. This shuts off the cGMP gated ion channels, reducing the inflow of Na+ and Ca2+ and outflow of K+ resulting in the rod cell membrane potential becoming more negatively charged i.e. hyperpolarised - this is called an Inhibitory Post Synaptic Potential. This closes voltage gated calcium channels at the rod / bipolar cell synapse reducing the release of glutamate into the synapse. The lack of glutamate in the synapse results in the bipolar cell membrane potential becoming more positive i.e. an Excitatory Post Synaptic Potential. This counter-intuitively results in more glutamate being released into the bipolar / ganglion cell synapse which stimulates the ganglion cell finally resulting in an action potential which travels down the optic nerve to the brain.
The horizontal cells release GABA in response to glutamate from the rod cells. GABA inhibits the rod cell as a negative feedback loop. This aids the rod cell to respond to light levels and adjust to bright light.
The amacrine cell function is less well known but it responds to glutamate at the bipolar / ganglion cell synapse and releases glycine, GABA, dopamine and acetycholine to modulate the output of the ganglion cell.
Now think about what happens in the absence of light.
When there is no light the level of cGMP increases so the cGMP gated Na+/Ca2+ channels open. The rod cell membrane potential becomes more positively charged (Excitatory Post Synaptic Potential) which closes voltage gated calcium channels at the rod/bipolar cell synapse increasing the amount of glutamate released into the synapse. The increase of glutamate results in the bipolar cell membrane potential becoming more negative i.e. Inhibitory Post Synaptic Potential. This results in less glutamate released into the bipolar/ganglion cell synapse which inhibits the ganglion cell preventing an action potential down the optic nerve.