Transcript Smooth Muscle

Smooth Muscle
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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
 Ca2+ sequestered in extracellular space near caveoli
 Thin and thick filaments present, but no visible
striations
Organization of Myofilaments in Smooth Muscle
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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
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
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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