MUSCLE PHYSIOLOGY Sliding Filament Model of Contraction Skeletal Muscle Contraction Nerve
Muscle contraction
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Transcript of Muscle contraction
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RK Goit, Lecturer
Department of Physiology
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contraction occurs by a sliding filament mechanism
Z discs have been pulled by the actin filaments up to the ends of the myosin filaments
caused by forces generated by interaction of the cross-bridges with the actin filaments
when an action potential travels along the muscle fiber, this causes SR to release Ca++
Ca++ activate forces between myosin & actin filaments
energy comes from ATP molecule
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Myosin Filament
composed of multiple myosin molecules (200)
myosin molecule is composed of six polypeptide
chains—two heavy chains, & four light chains
two heavy chains wrap spirally around each other
to form a double helix, called a myosin tail
one end of each of these chains is folded bilaterally,
called a myosin head
four light chains are also part of the myosin head
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tails of the myosin molecules bundled together to
form the body of the filament
part of the body of each myosin molecule hangs to
the side along with the head- arm
protruding arms & heads together are called cross-
bridges
each cross-bridge is flexible at two points called
hinges
where the arm leaves the body of the myosin filament
where the head attaches to the arm
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ATPase Activity of the Myosin Head
myosin head functions as an ATPase enzyme
this property allows the head to cleave ATP
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Actin Filament
composed of: actin, tropomyosin, & troponin
Actin
actin filament is a double stranded F-actin protein molecule
F-actin helix is composed of polymerized G-actin molecules
attached to each one of the G-actin molecules is one molecule of ADP
these ADP molecules are the active sites
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Tropomyosin
molecules are wrapped spirally around the sides of
the F-actin helix
in the resting state, lie on top of the active sites of
the actin strands
Troponin
are actually complexes of three loosely bound
protein subunits
troponin I for actin
troponin T for tropomyosin
troponin C for Ca++
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active sites on the normal actin filament of the relaxed muscle are inhibited or physically covered by the troponin-tropomyosin complex
in the presence of large amounts of Ca++, the inhibitory effect of the troponin-tropomyosin on the actin filaments is itself inhibited
when Ca++ combine with troponin C, the troponin complex undergoes a conformational change
this “uncovers” the active sites of the actin, thus allowing these to attract the myosin cross-bridge heads & cause contraction to proceed
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“Walk-Along” Theory (ratchet theory) of Contraction
when a head attaches to an active site, causes the
head to tilt toward the arm (power stroke) & to drag the
actin filament along with it
↓
head automatically breaks away from the active site
↓
it combines with a new active site
↓
then the head tilts again
↓
pulling the ends of two successive actin filaments
toward the center of the myosin filament
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ATP as the source of energy for contraction
the heads of the cross-bridges bind with ATP
ATPase activity of the myosin head immediately
cleaves the ATP
Ca++ binds with troponin-tropomyosin complex, active
sites on the actin filament are uncovered
bond between head of the cross-bridge & the active
site of the actin filament
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once the head of the cross-bridge tilts, this allows
release of the ADP & phosphate ion
a new molecule of ATP binds
binding of new ATP causes detachment of the head
from the actin
the new molecule of ATP is cleaved to begin the
next cycle
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The process by which depolarization of the muscle
fiber initiates contraction is called excitation-
contraction coupling.
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myofibrils surrounded by T tubule–sarcoplasmic
reticulum system
penetrate all the way from one side of the muscle
fiber to the opposite side
they communicate with the extracellular fluid
when an action potential spreads over a muscle
fiber membrane, a potential change also spreads
along the T tubules to the deep interior of the
muscle fiber
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sarcoplasmic reticulum composed of 2 major parts:
large chambers called terminal cisternae
long longitudinal tubules
muscle contraction continues as long as the Ca++
remain in high concentration
a continually active calcium pump located in the
walls of the sarcoplasmic reticulum pumps Ca++
away from the myofibrils back into the sarcoplasmic
tubules
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References
Ganong Review of Medical Physiology, 23/E
Textbook of Medical Physiology, 12/E Guyton &
Hall
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Thank You