Tachykinins are a group of biologically active peptides which exciteneurons, evoke behavioral responses, are potent vasodilatators and contract(directly or indirectly) many smooth muscles. This family includes many other peptides. Tachykinins, like most other active peptides, are synthesized as larger protein precursors, protachykinins, that are enzymatically converted to their mature forms. Tachykinins are from ten to twelve residues long.