Part 3 Smooth Muscles Smooth muscle cells are

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Part 3: Smooth Muscles

Part 3: Smooth Muscles

§ Smooth muscle cells are small, spindle shaped cells with one central nucleus, and

§ Smooth muscle cells are small, spindle shaped cells with one central nucleus, and lack the coarse connective tissue coverings of skeletal muscle. § Smooth muscle cells are usually arranged into sheets of opposing fibers, forming a longitudinal layer and circular layer.

§ Contraction of the opposing layers of muscle leads to a rhythmic form of

§ Contraction of the opposing layers of muscle leads to a rhythmic form of contraction, called peristalsis, which propels substances through the organs. § Have no striations, no sarcomeres, a lower ratio of thick to thin filaments compared with skeletal muscle.

§ Mechanism and characteristics of contractions § Smooth muscle fibers exhibit slow, synchronized contractions

§ Mechanism and characteristics of contractions § Smooth muscle fibers exhibit slow, synchronized contractions due to electrical gap junctions

§ Like skeletal muscle, actin and myosin interact by the sliding filament mechanism. The

§ Like skeletal muscle, actin and myosin interact by the sliding filament mechanism. The final trigger for contraction is a rise in intracellular calcium level, and the process is energized by ATP. § Smooth muscle contracts more slowly and consumes less ATP than skeletal muscle.

§ Regulation of contraction § Autonomic nerve endings release either acetylcholine or norepinephrine, which

§ Regulation of contraction § Autonomic nerve endings release either acetylcholine or norepinephrine, which may result in excitation of certain groups of smooth muscle cells, and inhibition of others. § Hormones and local factors, such as lack of oxygen, histamine, excess carbon dioxide, or low p. H, act as signals for contraction.

§ Special features of smooth muscle contraction § Smooth muscle initially contracts when stretched,

§ Special features of smooth muscle contraction § Smooth muscle initially contracts when stretched, but contraction is brief, and then the cells relax to accommodate the stretch. § Smooth muscle stretches more and generates more tension when stretched than skeletal muscle

§ Hyperplasia, an increase in cell number through division, is possible in addition to

§ Hyperplasia, an increase in cell number through division, is possible in addition to hypertrophy, an increase in individual cell size.

§ Types of smooth muscle § Single-unit smooth muscle, called visceral muscle, is the

§ Types of smooth muscle § Single-unit smooth muscle, called visceral muscle, is the most common type of smooth muscle. It contracts rhythmically as a unit, is electrically coupled by gap junctions, and exhibits spontaneous action potentials. § Multiunit smooth muscle is located in large airways to the lungs, large arteries, arrector pili muscles in hair follicles, and the iris of the eye.

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