Illustration of a protein (CNGA1/B1) formed of three cyclic nucleotide-gated channel alpha 1 (CNGA1) units and one cyclic nucleotide-gated channel beta 1 (CNGB1) unit bound to the calcium channel blocker drug diltiazem. CNGA1/B1 is found in human retinal rod cells. The retina is the light sensitive membrane at the back of the eye. In the absence of light, cyclic guanosine monophosphate (cGMP) binds to this transmembrane channel, causing it to open. Light activates the enzyme phosphodiesterase, which breaks down cGMP, resulting in the channel closing. This generates an electrical signal that is sent to the brain.