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RK Goit, Lecturer
Department of Physiology
 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
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
 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
ATPase Activity of the Myosin Head
 myosin head functions as an ATPase enzyme
 this property allows the head to cleave ATP
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
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++
 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
“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
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
 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
The process by which depolarization of the muscle
fiber initiates contraction is called excitation-
contraction coupling.
 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
 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
References
 Ganong Review of Medical Physiology, 23/E
 Textbook of Medical Physiology, 12/E Guyton &
Hall
Thank You

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Muscle contraction

  • 2.
  • 3.
  • 4.  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
  • 5.
  • 6. 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
  • 7.  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
  • 8. ATPase Activity of the Myosin Head  myosin head functions as an ATPase enzyme  this property allows the head to cleave ATP
  • 9.
  • 10. 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
  • 11. 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++
  • 12.  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
  • 13. “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
  • 14.
  • 15. 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
  • 16.  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
  • 17. The process by which depolarization of the muscle fiber initiates contraction is called excitation- contraction coupling.
  • 18.
  • 19.  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
  • 20.  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
  • 21.
  • 22. References  Ganong Review of Medical Physiology, 23/E  Textbook of Medical Physiology, 12/E Guyton & Hall