Smooth Muscle Spindle-shaped cells 2-10m across & ~100m 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 Ca2+ 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 [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

Regulation of SM Contraction

Actin

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

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