The document summarizes the molecular mechanisms of muscle contraction. It describes how actin, tropomyosin, and troponin proteins in the thin filament regulate muscle contraction by exposing binding sites on actin for myosin cross bridges. Calcium released from the sarcoplasmic reticulum in response to an action potential triggers contraction by causing troponin and tropomyosin to move, exposing actin binding sites. Repeated cycling of myosin cross bridges binding to and detaching from actin causes the thin filaments to slide, shortening the sarcomere and producing muscle contraction via the sliding filament mechanism. ATP provides energy for cross bridge cycling.

1. Molecular Mechanism of
Muscle Contraction
Dr. Sai Sailesh Kumar G
Associate Professor
Department of Physiology
NRIIMS
Email: dr.goothy@gmail.com

2. Explain
Erlanger Grasser classification
Uses of myelination
How AP is ensured to travel in the forward direction
What do you mean by the functional unit of the system
Basic difference between skeletal and smooth muscle

3. Actin
Thin filaments consist of three proteins:
actin,
tropomyosin,
and troponin
Actin molecules, the primary structural proteins of the thin filament, are
spherical.

4. Actin
The thin filament’s backbone is formed by actin molecules joined into
two strands and twisted together, like two intertwined strings of pearls.
 Each actin molecule has a binding site for attaching with a myosin
cross bridge.
Binding of myosin and actin at the cross-bridges leads to contraction
of the muscle fiber

5. Actin
In a relaxed muscle fiber, contraction does not take place;
actin cannot bind with cross bridges because of
the way the two other types of protein—tropomyosin and troponin—are positioned
within the thin filament.
Tropomyosin molecules are threadlike proteins that lie end to end alongside the
groove of the actin spiral.
In this position, tropomyosin covers the actin sites that bind with the cross bridges,
blocking the interaction that leads to muscle contraction

6. Actin
troponin, is a protein complex made of three polypeptide units:
one binds to tropomyosin, one binds to actin, and a third can bind with
Calcium
When troponin is not bound to Ca+2, this protein stabilizes tropomyosin in
its blocking position over actin’s cross-bridge binding sites
When Ca+2 binds to troponin, the shape of this protein is changed in such a
way that tropomyosin slips away from its blocking position

7. Actin
With tropomyosin out of the way, actin and myosin can bind and
interact at the cross bridges, resulting in muscle contraction.
 Tropomyosin and troponin are often called regulatory proteins
because of their role in covering (preventing contraction) or exposing
(permitting contraction) the binding sites for cross-bridge interaction
between actin and myosin.

11. Molecular mechanism of muscle contraction
How does cross-bridge interaction between actin and myosin bring
about muscle contraction?
How does a muscle action potential trigger this contractile process?
 What is the source of the Ca+2 that physically repositions troponin
and tropomyosin to permit cross-bridge binding?

12. Molecular mechanism of muscle contraction
Cross-bridge interaction between actin and myosin brings about
muscle contraction by means of the sliding filament mechanism.

13. Sliding filament mechanism
The thin filaments on each side of a sarcomere slide
inward over the stationary thick filaments
toward the A band’s center during contraction
As they slide inward,
the thin filaments pull the Z lines to which they are attached closer
together,
so the sarcomere shortens

14. Sliding filament mechanism
As all sarcomeres throughout the muscle fiber’s length shorten
simultaneously,
 the entire fiber shortens.
This is the sliding filament mechanism of muscle contraction.

15. Sliding filament mechanism
The H zone, in the center of the A band where the thin filaments do not
reach, becomes smaller
as the thin filaments approach each other when they slide more deeply
inward.

16. Sliding filament mechanism
The I band, which consists of the portions of the thin filaments that do
not overlap with the thick filaments, narrows as the thin filaments
further overlap the thick filaments during their inward slide

17. Sliding filament mechanism
The thin filaments themselves do not change length during muscle
fiber shortening

18. Sliding filament mechanism
The width of the A band remains unchanged during contraction
because its width is determined by the length of the thick filaments,
and
the thick filaments do not change length during the shortening
process.

19. Sliding filament mechanism
Neither the thick nor the thin filaments decrease in length to shorten
the sarcomere.
Instead, contraction is accomplished by the thin filaments from the
opposite sides of each sarcomere sliding closer together between the
thick filaments

20. Sliding filament mechanism
Neither the thick nor the thin filaments decrease in length to shorten
the sarcomere.
Instead, contraction is accomplished by the thin filaments from the
opposite sides of each sarcomere sliding closer together between the
thick filaments

22. Power stroke
During contraction, with the tropomyosin and troponin “chaperones” pulled
out of the way by Ca+2,
The myosin heads or cross bridges from a thick filament can bind with the
actin molecules in the surrounding thin filaments
The two myosin heads of each myosin molecule act independently,
with only one head attaching to actin at a given time

23. Power stroke
When the binding site on an actin molecule is exposed, the myosin
molecule tilts at the hinge point on the tail,
elevating the myosin head to facilitate the binding of this cross bridge to the
nearest actin molecule
On binding, the myosin head tilts 45 degrees inward.
Bending at this neck hinge point creates a “stroking” motion that pulls
the thin filament toward the center of the sarcomere

24. Power stroke
This action is known as the power stroke of a cross bridge.
A single power stroke pulls the thin filament inward only a small
percentage of the total shortening distance.
Repeated cycles of cross-bridge binding and bending complete the
shortening.

25. Power stroke
At the end of one cross-bridge cycle, the link between the myosin
cross bridge and actin molecule breaks.
The cross-bridge returns to its original angle and binds to an actin
molecule behind its previous actin partner.
The cross-bridge tilts inward again to pull the thin filament in farther,
then detaches and repeats the cycle

26. Power stroke
Repeated cycles of cross-bridge power strokes successively pull in the
thin filaments, much like pulling in a rope hand over hand

29. Power stroke
How does muscle excitation switch on this cross-bridge cycling?
The term excitation–contraction coupling refers to the series of
events linking muscle excitation (the presence of an action
potential in a muscle fiber) to muscle contraction (cross-bridge
activity that causes the thin filaments to slide closer together to
produce sarcomere shortening)

30. Calcium is the link between excitation and contraction
Skeletal muscles are stimulated to contract by the release of acetylcholine
(ACh) at neuromuscular junctions between motorneuron terminal buttons
and muscle fiber
binding of ACh with the motor end plate of a muscle fiber brings about
permeability changes in the muscle fiber,
resulting in an action potential that is conducted over the entire
surface of the muscle cell membrane

31. Calcium is the link between excitation and contraction
The plasma membrane in muscle is sometimes called the
sarcolemma.
Two other membranous structures within the muscle fiber play
important roles in linking this excitation to contraction
transverse tubules
and sarcoplasmic reticulum

32. Calcium is the link between excitation and contraction
At each junction of an A band and I band,
the surface membrane dips into the muscle fiber to form a transverse
tubule (T tubule),
which runs perpendicularly from the surface of the muscle cell
membrane into the central portions of the muscle fiber

34. Calcium is the link between excitation and contraction
Because the T tubule membrane is continuous with the sarcolemma,
an action potential on the surface membrane spreads down into the T
tubule,
rapidly transmitting the surface electrical activity into the interior of the fiber.
The presence of a local action potential in the T tubules leads to
permeability changes in a separate membranous network within the muscle
fiber, the sarcoplasmic reticulum

35. Release of Calcium from the Sarcoplasmic Reticulum
The sarcoplasmic reticulum (SR) is a modified endoplasmic
reticulum
This membranous network encircles the myofibril throughout its length
but is not continuous.
Separate segments of SR are wrapped around each A band and each
I band

36. Release of Calcium from the Sarcoplasmic Reticulum
The ends of each segment expand to form saclike regions, the lateral
sacs (alternatively known as terminal cisternae),
which are separated from the adjacent T tubules by a slight gap.
 The lateral sacs store Ca+2
Spread of an action potential down a T tubule triggers the release of
Ca+2 from the SR into the cytosol

37. Release of Calcium from the Sarcoplasmic Reticulum
How is a change in T tubule potential linked with the release of Ca+2
from the lateral sacs?

38. Release of Calcium from the Sarcoplasmic Reticulum
T tubule membrane proteins known as dihydropyridine receptors (because
they are blocked by the drug dihydropyridine) serve as voltage sensors
 Local depolarization of the T tubules activates the dihydropyridine
receptors,
which in turn triggers the opening of directly abutting foot proteins
(alias Ca+2-release channels or ryanodine receptors) in the adjacent
lateral sacs

39. Release of Calcium from the Sarcoplasmic Reticulum
These foot proteins not only bridge the gap, but also serve as Ca+2-release
channels
and are also known as ryanodine receptors because they are locked in the
open position by the plant chemical ryanodine
When these Ca21-release channels are opened in the presence of a
local action potential in the adjacent T tubule, Ca+2 is released into the
cytosol from the lateral sacs

40. Release of Calcium from the Sarcoplasmic Reticulum
By slightly repositioning the troponin and tropomyosin molecules,
this released Ca+2 exposes the binding sites on the actin molecules
 so that they can link with the myosin cross-bridges at their
complementary binding sites
Excitation–contraction coupling

42. ATP-Powered Cross-Bridge Cycling
A myosin cross-bridge has two special sites:
an actin-binding site and an ATPase site
The latter is an enzymatic site that can bind the energy carrier adenosine
triphosphate (ATP)and split it into adenosine diphosphate (ADP) and inorganic
phosphate (Pi),
 yielding energy in the process.
The breakdown of ATP occurs on the myosin cross-bridge before the bridge ever
links with an actin molecule

43. ATP-Powered Cross-Bridge Cycling
The ADP and Pi remain tightly bound to the myosin,
and the generated energy is stored within the cross bridge to produce
a high-energy form of myosin.
To use an analogy, the cross-bridge is “cocked” like a gun, ready to be
fired when the trigger is pulled.

44. ATP-Powered Cross-Bridge Cycling
When the muscle fiber is excited,
Ca+2 pulls the troponin–tropomyosin complex out of its blocking position so that the
energized (cocked) myosin cross-bridge can bind with an actin molecule.
This contact between myosin and actin “pulls the trigger,”
causing the cross-bridge bending that produces the power stroke
Pi is released from the cross bridge during the power stroke.
 After the power stroke is complete, ADP is released

45. ATP-Powered Cross-Bridge Cycling
When the muscle is not excited and Ca+2 is not released,
troponin and tropomyosin remain in their blocking position
so that actin and the myosin cross-bridges do not bind and no power
stroking takes place

46. ATP-Powered Cross-Bridge Cycling
When Pi and ADP are released from myosin following contact with actin and the
subsequent power stroke,
the myosin ATPase site is free for attachment of another ATP molecule.
The actin and myosin remain linked at the cross-bridge until a fresh
molecule of ATP attaches to myosin at the end of the power stroke.
 Attachment of the new ATP molecule reduces the binding affinity between
the myosin head and actin, thus allowing the cross bridge to detach

47. ATP-Powered Cross-Bridge Cycling
The newly attached ATP is then split by myosin ATPase, recocking and
energizing the myosin cross bridge once again so that it is ready to
start another cycle

51. Rigor Mortis
fresh ATP must attach to myosin to permit the cross-bridge link between myosin
and actin to break at the end of a cycle,
even though the ATP is not split during this dissociation process.
The need for ATP in separating myosin and actin is amply shown in rigor mortis.
This “stiffness of death” is a generalized locking in place of the skeletal muscles
that begins 3 to 4 hours after death and completes in about 12 hours

52. Rigor Mortis
Following death, the cytosolic concentration of Ca+2 begins to rise,
 most likely because the inactive muscle cell membrane cannot keep out
extracellular Ca+2 and perhaps because Ca+2 leaks out of the lateral sacs.
This Ca21 moves troponin and tropomyosin aside, letting actin bind
with the myosin cross bridges, which were already charged with ATP
before death.

53. Rigor Mortis
Dead cells cannot produce any more ATP
so actin and myosin, once bound, cannot detach because they lack fresh
ATP
The thick and thin filaments thus stay linked by the immobilized cross
bridges, leaving dead muscles stiff
During the next several days, rigor mortis gradually subsides as the proteins
involved in the rigor complex begin to degrade

55. Relaxation
How is relaxation normally accomplished in a living muscle?

56. Relaxation
relaxation occurs when Ca21 is returned to the lateral sacs when local
electrical activity stops.
The SR has an energy-consuming carrier, the
sarcoplasmic/endoplasmic reticulum Ca+2–ATPase (SERCA) pump,
which actively transports Ca21 from the cytosol and concentrates it in
the lateral sacs

57. Relaxation
Ach esterase removes Ach from NMJ
End plate potential stops
AP stops
When a local action potential is no longer in the T tubules to trigger
release of Ca+2,
the ongoing activity of the SERCA pump returns released Ca+2 back
into the lateral sacs.

58. Relaxation
Removing cytosolic Ca+2 lets the troponin–tropomyosin complex slip
back into its blocking position,
so actin and myosin can no longer bind at the cross bridges
The muscle fiber has relaxed.

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