Smooth muscle cells may respond to injury by undergoing mitosis. In addition, smooth muscle contains regularly replicating populations of cells. Smooth muscle in the uterus proliferates  FOLDER 11.5 Functional Considerations: Comparison of the Three Muscle Types 337 Cardiac muscle shares structural and functional charac-teristics with skeletal muscle and smooth muscle. In both cardiac and skeletal muscle, the contractile elements— thick and thin filaments—are organized into sarcomeres surrounded by sER and mitochondria. Both cardiac and smooth muscle cells retain their individuality, although both are in functional communication with their neighbors through gap junctions. In addition, cardiac and smooth muscle cells have a spontaneous beat that is regulated but not initiated by autonomic or hormonal stimuli. Both have centrally located nuclei and perinuclear organelles. These common characteristics suggest that cardiac mus-cle may have evolved in the direction of skeletal muscle from the smooth muscle of primitive circulatory systems. A summary of major characteristics of all three muscle types is provided in the table below. Comparison of the Three Muscle Types Skeletal Cardiac Smooth Structural features Muscle cell Large, elongate cell, 10–100 m in diameter, up to 100 cm in length (sartorius m.) Short, narrow cell, 10–15 m in diameter, 80–100 m in length Short, elongate, fusiform cell, 0.2–2 m in diameter, 20–200 m in length Location Muscles of skeleton visceral striated (e.g., tongue, esophagus, diaphragm) Heart, superior and inferior vena cava, pulmonary veins Vessels, organs, and viscera Connective tissue components Epimysium, perimysium, endomysium Endomysium (subendocardial and subpericardial connective tissue) Endomysium, sheaths, and bundles Fiber Single skeletal muscle cell Linear branched arrangement of several cardiac muscle cells Single smooth muscle cell Striation Present Present None Nucleus Many peripheral Single central, surrounded by juxtanuclear region Single central T tubules Present at A-I junction (triad: with two terminal cisternae), two T tubules/sarcomere Present at Z lines (diad: with small terminal cisternae), one T tubule/sarcomere None, well-developed sER, many invaginations and vesicles similar to caveolae Cell-to-cell junctions None Intercalated discs containing 1. Fasciae adherentes 2. Macula adherens (desmosome) 3. Gap junctions Gap junctions (nexus) Special features Well-developed sER and T tubules Intercalated discs Dense bodies, caveolae, and cytoplasmic vesicles Functions Type of innervation Voluntary Involuntary Involuntary Efferent innervation Somatic Autonomic Autonomic Type of contraction “AlI or none” (type I and type II fibers) “All or none” rhythmic (pacemakers, conductive system of the heart) Slow, partial, rhythmic, spontaneous contractions (pacemakers of stomach) Regulation of contraction By binding of Ca2to TnC, causes tropomyosin movement and exposes myosin-binding sites on actin filaments By binding of Ca2to TnC, causes tropomyosin movement and exposes myosin-binding sites on actin filaments By phosphorylation of myosin light chain by myosin light chain kinase in the  presence of Ca2-calmodulin complex Growth and regeneration Mitosis None None (in normal condition) Present Response to demand Hypertrophy Hypertrophy Hypertrophy and hyperplasia Regeneration Limited (satellite cells and myogenic cells from bone marrow) None (in normal condition) Present chapter 11 Muscle Tissue SMOOTH M USCLE 337 during the normal menstrual cycle and during pregnancy; both activities are under hormonal control. The smooth muscle cells of blood vessels also divide regularly in the adult, presumably to replace damaged or senile cells; the smooth muscle of the muscularis externa of the stomach and colon regularly replicates and may even slowly thicken during life.