The document summarizes the structure and function of skeletal muscle myosin and actin filaments during muscle contraction. It describes how myosin filaments are composed of myosin molecules that form cross-bridges with heads that can interact with actin filaments. Actin filaments contain actin, tropomyosin and troponin. Calcium released from the sarcoplasmic reticulum activates contraction by allowing the myosin heads to bind to active sites on actin. The myosin heads then tilt and pull the actin filaments towards the center in a ratcheting motion, powered by ATP hydrolysis.

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 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

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

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.

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
 Ganong Review of Medical Physiology, 23/E
 Textbook of Medical Physiology, 12/E Guyton &
Hall

23. Thank You