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General Shape of a Smooth Muscle Cell
high S/V
no obvious banding
nucleus
knp
Smooth Muscle Motor Units?
Contraction Patterns in Smooth Muscles
Force
0
time
knp
Phasic (quick, SkMus-like) and tonic contractions
Force
Skeletal
Muscle
Twitch
Smooth Muscle
Twitch (can be shorter
or longer)
knp
Time
EC Coupling in Smooth Muscle
Force
Em
Time
Force
tonus level
hyperpolarized
Em
Time
knp
add one
drug here
Force
Em
add another
type of drug
here
"normal" tonus
roughly constant Em
knp
time
Thin and Thick Filament
Arrangements in Smooth Muscle
Caldesmon
Caldesmon In Relaxed Fibers
Regulation of Smooth Muscle Contraction
Thick Filament Regulation (on myosin light chain):
-- Modulation by MLCK/MLCP
-- Other Mechanisms
Thin Filament Regulation (?):
-- Does caldesmon have a regulatory role?
-- Other mechanisms
Calcium and the Control of MLCK
Leading to Control of the Myosin
Light Chain
Ca++ + Calmodulin
Calmodulin-- Ca ++
MLCK
ATP + Myosin * ADP * Pi
MLCK -Calmodulin -Ca++
MLCK -Calmodulin -Ca ++
P--Myosin * ADP * Pi + ADP
Contracted SM Mus
There’s always a reverse process:
P--Myosin --Actin
MLCP (Phosphatase)
Myosin --Actin + Pi
MLCP phosphatase is inhibited by a Ca++
dependent pathway (but in any case, its
maximum activity is lower than MLCK)
SM Contraction and Relaxation: Ca++
and the Regulation of Phosphorylation
of the Myosin Light Chain
MLCK (inactive)
add Ca ++
(from plasma membrane)
Calmodulin
Calmodulin Ca++
ATP
remove Ca ++
(via plasma membrane)
MLCK-- Calmodulin -- Ca ++
Myosin
(after a diagram
in Berne and Levy,
Physiology , Mosby,
1983)
ADP
Myosin-Pi
Pi
MLCP
Phosphorylated
Crossbridge
Regulation via the PKC Pathway
Latch Mechanism?