Gated ion channel Serpentine reeeptor Opens or clases in External ligand binding response to concentration to receptor (R) activates an of signal ligand (S) intracellular gtP- binding or membrane potential protein(G), which regulates an enzyme(Enz) that generates an intracellular Ion second messenger, X R Plasma Receptor enzyme membrane Ligand binding to extracellular domain stimulates enzyme activity in intracellular Steroid receptor DNA Steroid binding to a nuclear receptor protein allows the mRNA receptor to regulat Nuclear the expression of envelope Protein specific genes