Figure 4
From: Human depression: a new approach in quantitative psychiatry
Ligand reaches the receptor and guanosine-5'-triphosphate (GTP) reaches the protein. Cell membrane proteins coupled to cell surface receptors bind to GTP upon stimulation of the receptor by an extracellular signalling molecule (as a hormone or neurotransmitter) to form an active complex that mediates an intracellular event (for example, activation of adenylate cyclase).