Smooth Muscle

1. Smooth Muscle • Spindle-shaped cells 2-10m across & ~100m long • Have a thin endomysium • Organized into longitudinal and circular layers • Found in walls of contractile organs (except the heart)

2. Innervation of Smooth Muscle • Lacks neuromuscular junctions • Innervating nerves form varicosities • Varicosities release neurotransmitters into synaptic clefts called diffuse junctions

3. Microscopic Anatomy of Smooth Muscle • SR less developed than in sk mus & lacks specific pattern • No T tubules • Plasma membranes have caveoli • Ca2+ sequestered in extracellular space near caveoli • Thin and thick filaments present, but no visible striations

4. Organization of Myofilaments in Smooth Muscle • Intermediate filaments attached at “dense bodies” • More thick filaments per thin filament • Thick filaments have heads along their entire length • No troponin complex • Myofilaments arranged diagonally

5. Contraction of Smooth Muscle • Synchronized contraction • sheets of smooth muscle • Gap junctions • electrically couple cells • action potentials transmitted cell to cell • Some smooth muscle cells: • Act as pacemakers and set the contractile pace for whole sheets of muscle • Are self-excitatory and depolarize without external stimuli

6. Contraction Mechanism • Actin and myosin interact via to the sliding filament mechanism • Resting State • Actin bound by caldesmon • Myosin light chain (MLC) dephosphorylated • Intracellular [Ca2+] low • Rise in intracellular Ca2+ triggers contraction • Ca2+ enters from the extracellular space (some from SR) • Increase in [Ca2+ ]caused by: • Excitation by neurotransmitters (extracellular Ca) • Hormonal signaling via receptors & 2nd messengers (SR Ca) • Ca flux can be inhibited by various inhibitory stimuli – hormonal & neuronal

7. Regulation of SM Contraction Actin

8. Role of Ca2+ for Contraction • Ca2+ binds to calmodulin (Cam) • Ca-Cam binds to & activates myosin light chain kinase (MLCK) • CaCam binds to & inactivates caldesmon • Active caldesmon binds actin • CaCam-inactivated caldesmon can’t bind actin • The free actin is available for myosin to bind • Activated MLCK phosphorylates myosin head region allowing cross bridges with actin to form

9. Special Features of Smooth Muscle Contraction • Unique characteristics of smooth muscle include: • Smooth muscle tone • Slow, prolonged contractile activity • Low energy requirements • Response to stretch

10. Response to Stretch • Smooth muscle exhibits a phenomenon called stress-relaxation response in which: • Smooth muscle responds to stretch only briefly, and then adapts to its new length • The new length, however, retains its ability to contract • This enables organs such as the stomach and bladder to temporarily store contents

11. Hyperplasia • Certain smooth muscles can divide and increase their numbers by undergoing hyperplasia • estrogen’s effect on the uterus • During pregnancy, estrogen stimulates uterine growth to accommodate growing fetus

12. Types of Smooth Muscle: Single Unit • The cells of single-unit smooth muscle, commonly called visceral muscle: • Contract rhythmically as a unit • electrically coupled via gap junctions • exhibit spontaneous action potentials • arranged in opposing sheets • exhibit stress-relaxation response

13. Types of Smooth Muscle: Multiunit • Multiunit smooth muscles are found: • airways of lungs • arteries • arrector pili muscles • internal eye muscles • characteristics include: • Rare gap junctions • Infrequent spontaneous depolarizations • Structurally independent muscle fibers • rich nerve supply, 1/motor units • Graded contractions in response to neural stimuli

14. Muscular Dystrophies • Group of inherited muscle degeneration diseases • Muscle fibers atrophy • Muscle tissue replaced by connective tissues (scar tissue) • Duchenne Muscular Dystrophy (DMD) • sex-linked recessive inheritance (gene on X chromosome) • 1/3500 live male births • caused by a defect in the gene encoding the protein dystrophin • dystrophin helps maintain muscle cell integrity during use