The visual pigment (rhodopsin in rods) consists of an 11-cis-retinal (11cRAL) chromophore covalently attached to a GPCR opsin family member via a Schiff base linkage. Upon photon absorption, 11cRAL isomerizes to all trans retinal (atRAL), changing the conformation of opsin to a form that can activate the regulatory G protein transducin (Gt). The alpha subunit of Gt activates phosphodiesterase which hydrolyses cGMP to 5'-GMP. A high level of cGMP keeps cGMP-gated cation channels open, so lower cGMP levels close these channels and hyperpolarize the cell. The hyperpolarization spreads passively to the synapse located at the opposite end of the rod, where it subsequently closes voltage-gated calcium channels. Vesicular release of the neurotransmitter glutamate subsides as the intracellular calcium levels drop. This diminution of neurotransmitter release relays the light signal to postsynaptic neurons. The events below describe activation, inactivation, recovery and regulation of the phototransduction cascade in rods (Burns & Pugh 2010, Korenbrot 2012, Smith 2010).