1. Smooth Muscle cells contain actin and myosin, but are quite different from skeletal and cardiac myocytes.
1) Where do we find smooth muscle under involuntary control?
2) Smooth Muscle usually comes in flat sheets or tubes with cells that may or may not be connected by gap junctions.
3) The gut and artery have both circular and longitudinally arranged sheets of smooth muscle to complement each others function.
Function of circular fibers Function of longitudinal fibers
4) In smooth muscle cells the actin and myosin are not organized into sarcomeres, but more loosely attached to the plasma membrane and sarcoplasm (no striations)
5) Smooth Muscle can generate force that is sustained for a longer time and uses less ATP (its cells typically have fewer mitochondria and rely more on glycolysis)
6) Smooth muscle is stimulated by the autonomic NS at a classic synapse (Multi-unit) or via a series of varicosities from a single axon (Single-Unit) where gap junctions carry depolarization to neighboring cells.

2. Smooth muscle cells are found in blood vessels, glands, guts,and other places. SMCs contract using calcium entry/calmodulin binding as a signal to activate myosin light chain kinase (MLCK). MLCK phosphorylates myosin letting it bind actin and contract.

3. Once the myosin in smooth muscle is phosphorylated it binds actin and the cell contracts, contraction ends when Ca++ leaves the cell and MLC-phosphatase removes the phosphate from MYOSIN…leading to SMC relaxation

4. Smooth Muscle Applications:
Asthma: excess constriction of airways
Solution: promote dilation (reduce SMC contraction)
High Blood Pressure: Excess contraction of blood vessel Solution: vasodilator drugs
Low Blood Pressure: Not enough SMC tone in blood vessels. Solution: vasoconstrict blood vessels to push blood back to heart
Peristaltic Waves in intestine: propel chyme, often the contractions are not strong enough (constipation)…what is the solution?
Hypermotile Intestine (Diarrhea): Solution is to reduce intestinal SCM contractile force
Hyperactive Bladder: improve receptive relaxation