Muscle contraction

1. RK Goit, Lecturer
Department of Physiology

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

6. Myosin Filament
composed of multiple myosin molecules (200)
myosin moleculeis 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

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-tropomyosincomplex
in the presence of large amounts of Ca
++
, the
inhibitory effect of the troponin-tropomyosinon 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

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-tropomyosincomplex, 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.

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

22. References
GanongReview of Medical Physiology, 23/E
Textbook of Medical Physiology, 12/E Guyton &
Hall

23. Thank You