Smooth Muscle - Exam 1

smooth muscle monocyte

single nucleus - centrally located, elongated/spindle shaped

each cell surrounded by reticular fibers, tissue rich in BVs and nerves

lack striations (not standard sarcomere)

involuntary

surrounds most hollow organs - uterus, UB, GI

smooth muscle myofilament arrangement

myofilaments - arranged in various orientations

thin filaments - actin, tropomyosin, NO troponin

thick filaments - myosin

dense bodies - anchor actin in association with intermediate filaments

contraction of smooth muscle

dependent upon phosphorylation of myosin

• when myosin is phosphorylated it binds to actin and muscle shortens

• phosphorylation of myosin caused by and increase in intracellular Ca

  • Ca binds to calmodulin (activation)

  • calcium-calmodulin phosphorylates myosin light-chain kinase (MLCK)

  • MLCK phosphorylates myosin and phosphorylated myosin binds actin

  • myosin phosphatase reclaims the phosphate to cause relaxation

smooth muscle contraction characteristics

cellular events that lead to smooth muscle contraction take longer

contraction is slower and more sustained

many smooth muscle organs maintain constant tone - some level of contraction

smooth muscle innervation

involuntary

under conrol of autonomic nervous system

unitary smooth muscle - signal a few cells and activate whole organ (stomach)

• cells linked by gap junctions

• all cells in organ act in unit

multi-unit smooth muscle (iris)

• no gap junctions between cells

• individual cells get their own innervation (fewer muscle cells)

• allows for precise contraction