The state of myosin light chain (MLC) phosphorylation in cavernosal smooth muscle (SM) is regulated by MLC kinase and MLC phosphatase. MLC kinase is activated by increased intracellular levels of calcium [Ca²⁺]_i; Ca²⁺ binds to calmodulin to activate the kinase. In the active form, MLC phosphatase promotes SM relaxation, leading to penile erection, but when MLC phosphatase is phosphorylated (and hence inactive), cavernosal SM remains contracted, and the penis is not erect. Thus, by regulating the phosphorylation of MLC phosphatase, the RhoA/Rho-kinase pathway exerts regulatory control over the erectile process.