smooth muscle contraction
Summary
Smooth muscle contraction is the slow, usually involuntary shortening of nonstriated muscle. Ca²⁺ binds calmodulin, which activates myosin light-chain kinase to phosphorylate myosin, so it lacks the troponin step used in skeletal muscle.
Detail
Meaning: Smooth muscle is nonstriated tissue in the walls of hollow organs such as the gut, blood vessels, and airways. It is generally controlled involuntarily by the autonomic nervous system, hormones, and local signals. It still slides actin past myosin, but it has no troponin. Rising cytosolic Ca²⁺ (from outside the cell and from the sarcoplasmic reticulum) binds calmodulin. Ca²⁺-calmodulin activates myosin light-chain kinase, which phosphorylates myosin heads so they can cross-bridge with actin using ATP. When Ca²⁺ falls, myosin light-chain phosphatase removes the phosphate and the muscle relaxes.
On the MCAT: Expect a table comparing skeletal, cardiac, and smooth muscle (striations, nuclei, control, Ca²⁺ sensor). Passages may ask what happens if calmodulin or myosin light-chain kinase is inhibited (less contraction), or if extracellular Ca²⁺ is removed. Many smooth muscle sheets are linked by gap junctions and contract as a unit, and contraction is slow but can be sustained.
Common confusion: Students carry over the skeletal mechanism, where Ca²⁺ binds troponin and tropomyosin shifts to uncover binding sites. In smooth muscle, Ca²⁺ acts through calmodulin, and the myosin is switched on by phosphorylation. Smooth muscle still uses sliding filaments; it just lacks organized sarcomeres.
Sources
- Campbell Biology
- Human Physiology: An Integrated Approach
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.