Airway smooth muscle cell

The bronchi and bronchioles are the tubes that air moves through on its way in and out of the lungs, and they are not rigid tubes: a layer of smooth muscle wraps around them, the involuntary kind that works without being asked. Its cells do not look striped under the microscope the way the muscle in your arm does, because their filaments are not arranged in sarcomeres, the repeating units that give that striated look. Its job in the respiratory system is one specific thing: to regulate the diameter of the bronchioles. The width of the tube is not set by the tube; it is set by the muscle wrapped around it.

To contract, this cell needs calcium to come in. Calcium binds calmodulin, a protein that senses it; calmodulin activates an enzyme called myosin light chain kinase; and that enzyme attaches a phosphate to a regulatory light chain of myosin, the motor that pulls on the fiber. That is where contraction begins.

In asthma this is exactly what happens. Mast cells, defense cells loaded with chemicals, release histamine and other mediators; that muscle contracts, and the airway narrows.