Smooth muscle forms the walls of many hollow organs and passageways. Its specialized structure supports slow, sustained contractions that move substances through the body and regulate the diameter of organs and blood vessels.
Smooth muscle is named for its lack of visible striations. It is found in the walls of hollow organs—including the stomach, intestines, urinary bladder, and uterus—and in passageways such as blood vessels and the respiratory, urinary, and reproductive tracts. In the eyes, it changes the size of the iris and the shape of the lens. In the skin, it can make hairs stand erect in response to cold or fear.
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What are the defining features of a smooth muscle fiber?
It is a short, spindle-shaped, nonstriated cell, usually with one nucleus. It contains actin and myosin but does not organize them into sarcomeres.
What are dense bodies, and what do they do?
Dense bodies anchor thin filaments. They are functionally analogous to Z-discs and help transmit force through intermediate filaments and the sarcolemma.
How does calcium initiate smooth muscle contraction?
Calcium binds calmodulin. The calcium–calmodulin complex activates MLCK, which phosphorylates myosin heads so they can form cross-bridges with actin.
Where does the calcium that triggers smooth muscle contraction come from?
Most enters from extracellular fluid through calcium channels in the sarcolemma. Additional calcium can be released from the sarcoplasmic reticulum.
What are latch-bridges, and why are they useful?
Latch-bridges are long-lasting links between myosin and actin. They help smooth muscle sustain tone for extended periods while using little ATP.
Quiz questions
How do autonomic neurons commonly signal smooth muscle without forming skeletal-muscle-type neuromuscular junctions?Answer: Axonal varicosities release neurotransmitters into nearby tissue.
What allows single-unit smooth muscle to contract in a coordinated manner?Answer: Gap junctions allow electrical activity to spread between cells.
What is the stress-relaxation response of visceral smooth muscle?Answer: Stretch triggers contraction followed by relaxation, allowing a hollow organ to accommodate filling.
What role can pacesetter cells have in some visceral smooth muscle?Answer: They spontaneously generate electrical activity that can trigger action potentials and contractions.
What does hyperplasia mean in smooth muscle?Answer: An increase in tissue size through an increase in cell number.
Based on Anatomy and Physiology 2e by J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.