This is our report for General Physiology

1. GENERAL MECHANISM OF
MUSCLE CONTRACTION
Sequential steps of initiation and
execution of muscle contraction

2. MUSCLE CONTRACTION
1. An action potential travels along a motor
nerve to its endings on muscle fibers.

3. 2. The nerve secretes a small amount of the
neurotransmitter substance –
acetylcholine at each ending.

4. 3. The acetylcholine acts on a local area
of the muscle fiber membrane to open
multiple “acetylcholine –gated” cation
channels through protein molecules
floating in the membrane.

5. 4. Opening of the acetyl-gated channels
allows large quantities of sodium ions to
diffuse to the interior of the muscle fiber
membrane . This causes a local
depolarization that in turn leads to opening
of voltage-gated sodium channels. This
initiates action potential at the membrane.

6. 5. The action potential travels along the
muscle fiber membrane in the same way
that action potentials travel along nerve
fiber membranes.

7. 6.The action potential depolarizes the
muscle membrane, and much of the action
potential electricity flows through the
center of the muscle fiber. Here it causes
the sarcoplasmic reticulum to release
large quantities of calcium ions that have
been stored within this reticulum.

8. 7. The calcium ions initiate attractive forces
between the actin and myosin
filaments, causing them to slide alongside
each other, which is the contractile
process.

9. 8. After a fraction of a second, the calcium
ion are pumped back into the
sarcoplasmic reticulum until a new muscle
action potential comes along; this removal
of calcium ions from the myofibrils causes
the muscle contraction to cease.

10. MOLECULAR
MECHANISM OF MUSCLE
CONTRACTION

11. Sliding Filament Mechanism of
Muscle Contraction
In contracted
state, actin (thin)
filaments have
been pulled
inward among
the myosin
(thick)
filaments, so
their ends
overlap one
another to their
maximum extent.

12. Sliding Filament Mechanism of
Muscle Contraction
• Muscle contraction occurs by a sliding
filament mechanism.
• When action potential travels along the
muscle fiber, sarcoplasmic reticulum
release large quantities of calcium ions
that rapidly surround the myofibrils.
• Energy is needed for the contractile
process to proceed. (ATP to ADP)

13. Molecular Characteristics of the
Contractile Filaments
• Myosin filaments is made up of 200 or
more individual myosin molecules.

14. Molecular Characteristics of the
Contractile Filaments
arm
Cross-bridges
hinges
body

15. Molecular Characteristics of the
Contractile Filaments
• Body- bundled tails of myosin
molecules.
• Arm- extends the head outward from
the body.
• Head- globular polypeptide structure.
• Cross-bridges- heads and arms
together.
• Hinges- flexible point of a cross-bridge.

16. Molecular Characteristics of the
Contractile Filaments
• The total length of each myosin filament if
uniform, almost exactly 1.6 micrometers.
• The distance between cross-bridges and
body is 0.2 micrometers.
• Myosin filament itself is twisted from the
previous pair by 120 degrees ensuring that
cross-bridges are extended in all
directions around the filament.

17. Molecular Characteristics of the
Contractile Filaments
• Myosin molecule is composed of 6 polypeptide
chains.
– 2 heavy chains
• Molecular weight of 200,000 each
• Double helix
• Tail
– Ends: head
– 4 light chains
• Molecular weight of 20,000 each
• Part of myosin head (2 at each head)
• Help control the function of the head during muscle
contraction.

18. Molecular Characteristics of the
Contractile Filaments
• ATPase activity of the Myosin Head
– The myosin head functions as an ATPase
enzyme.
• Actin Filaments are composed of
Actin, Tropomyosin, and Troponin.
– The double stranded F-actin protein molecule
• backbone of the actin filament
• wounded in a helix
• (each strand) is composed of polymerized G-actin
molecules.

19. Molecular Characteristics of the
Contractile Filaments
• G-actin molecule has a molecular weight
of about 42,000.
• One molecule of ADP is attached to each
of it.
• Each actin filament is about 1 micrometer
long.
• The bases of the actin filaments is inserted
strongly into the Z-discs.

20. Molecular Characteristics of the
Contractile Filaments

21. Molecular Characteristics of the
Contractile Filaments
• Each molecule of tropomyosin found in the
actin filament has a molecular weight of
70,000 and a length of 40 nanometers.
• These molecules are wrapped spirally
around the sides of the F-actin helix.
• In the resting stage, the tropomyosin
molecules lie on top of the active sites of
the actin strands.

22. Molecular Characteristics of the
Contractile Filaments
• Attached intermittently along the sides of
the tropomyosin molecule is the troponin
molecule.
• There are three subunits:
– Troponin 1: has strong affinity for actin
– Troponin T: for tropomyosin
– Troponin C: for calcium ions