A model for Limulus excitation. The cascade is initiated by the isomerization of rhodopsin to metarhodopsin by light. Metarhodopsin catalyzes exchange of GTP for GDP on multiple G proteins (Gq). Gq-GTP binds and activates phospholipase C (PLC). This complex cleaves phosphatidyl inositol-4,5-bisphosphate (PIP2) producing InsP3. InsP3 opens Ca2+ ion channels in the endoplasmic reticulum (ER) leading to the release of Ca2+ into the cytosol. Ca2+ release activates GC. A rise in cGMP opens cyclic nucleotide-gated ion channels (CNCG) in the plasma membrane.