Smooth muscle contraction mechanism

The primary requirement for a smooth muscle cell to contract is to increase its intracellular Calcium concentration, which triggers a series of events shown in figure 1. This increase typically happens after the arrival of an action potential from a motoneuron. In the case of the smooth muscle locate in the gastrointestinal tract, there is one more component: the myenteric plexus.

Figure 1. Smooth muscle contraction/relaxation mechanism. Smooth muscle contraction (left) requires five steps to perform: After the increase of intracellular Ca²⁺concentrations from either the extracellular fluid or the sarcoplasmic reticulum (1), these ions bind to a protein called calmodulin (2). This complex activates a protein called myosin light-chain kinase (3), which subsequently phosphorylates light chains of myosin heads, increasing the myosin ATPase activity (4). Finally, active myosin cross-bridges slide along actin and create muscle tension to contract the cell. Once the contraction finishes, some events occur to relax the cell (steps 6 to 9 on the right).