Smooth muscle

Smooth muscle is involuntary, nonstriated muscle tissue found in the walls of hollow organs and vessels. Its contractions move substances and regulate the diameter of structures such as blood vessels and airways.

Calmodulin: A calcium-binding protein that regulates many cellular enzymes and signaling pathways. Calcium-bound calmodulin activates the kinase that starts smooth-muscle contraction.
Actin: A protein that polymerizes into filaments and supports cell structure, movement, and muscle contraction. Smooth-muscle actin filaments anchor to dense bodies and interact with myosin.
Vasoconstriction: Narrowing of blood vessels caused by contraction of their smooth-muscle walls. Vascular smooth muscle changes vessel diameter and influences blood pressure.
Skeletal muscle: Striated muscle attached to bones that is usually controlled voluntarily. Unlike smooth muscle, skeletal muscle has sarcomeres and supports deliberate movement.
Myosin light-chain kinase: An enzyme that phosphorylates myosin regulatory light chains in response to calcium-calmodulin. It enables myosin to interact with actin and generate smooth-muscle force.
Myosin: A family of motor proteins that use ATP to move along actin filaments. Smooth-muscle myosin generates force after its regulatory light chains are phosphorylated.
Bronchoconstriction: Narrowing of the airways caused by contraction of bronchial smooth muscle. Airway smooth muscle regulates airflow and can constrict during asthma.
Cardiac muscle: Striated, involuntary muscle tissue that forms the contractile walls of the heart. Cardiac muscle is involuntary like smooth muscle but striated and specialized for pumping blood.
Myosin light-chain phosphatase: An enzyme that removes phosphate groups from myosin regulatory light chains. Its activity relaxes smooth muscle and helps determine contraction strength.
Intermediate filament: A family of durable cytoskeletal proteins that provides cells with mechanical strength. Intermediate filaments connect dense bodies and help distribute contractile force.

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