Skeletal muscle, cardiac muscle and smooth muscle are the main types of muscles in vertebrates. Smooth muscle is found in various systems, such as gastrointestinal system, arteries and veins of cardiovascular system or urinary and respiratory system. A rise in cytosolic Ca²⁺concentration initiates muscle contraction. Unlike skeletal muscle myosin that elevates Ca²⁺ binding to troponin to regulate myosin and actin attachment, in smooth muscle, elevated Ca²⁺ binds to calmodulin to induce contraction which critically regulated by protein kinases and phosphatases. Multiple pathways coexist to induce smooth muscle contraction and relaxation, as spatially separate signal compartments are coordinated by a multitude of scaffolding proteins, either an increase or decrease in actomyosin activation or crossbridge cycling. Smooth muscle contraction and relaxation are important physiological process contributing to homeostasis. Smooth muscle is responsible for blood pressure regulation, food move in the digestive tract, urine excretion and parturition. Smooth muscle shows phasic and tonic contraction ability. The ununiform and widely distribution characteristics of smooth muscle push us to research more to understand the mechanisms of smooth muscle contraction and relaxation in physiological or pathological conditions in all the smooth muscle tissues.

The aim of this collection is to cover tools and methods that help to understand the smooth muscle contraction and relaxation mechanisms in cells, tissue or organ levels.