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)

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

Microscopic Anatomy of Smooth Muscle  SR less developed than in sk mus & lacks specific pattern  No T tubules  Plasma membranes have caveoli  Ca 2+ sequestered in extracellular space near caveoli  Thin and thick filaments present, but no visible striations

 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 Organization of Myofilaments in Smooth Muscle

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

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

Regulation of SM Contraction Actin

Role of Ca 2+ for Contraction  Ca 2+ 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

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

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

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

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

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

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