1. GENERAL MECHANISM OF MUSCLE CONTRACTION Sequential steps of
initiation and execution of muscle contraction
2. MUSCLE CONTRACTION1. 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
channelsallows large quantities of sodium ions todiffuse to the
interior of the muscle fibermembrane . This causes a
localdepolarization that in turn leads to openingof voltage-gated
sodium channels. Thisinitiates action potential at the membrane. 6.
5. The action potential travels along themuscle fiber membrane in
the same waythat action potentials travel along nervefiber
membranes. 7. 6.The action potential depolarizes themuscle
membrane, and much of the actionpotential electricity flows through
thecenter of the muscle fiber. Here it causesthe sarcoplasmic
reticulum to releaselarge quantities of calcium ions that havebeen
stored within this reticulum. 8. 7. The calcium ions initiate
attractive forcesbetween the actin and myosinfilaments, causing
them to slide alongsideeach other, which is the contractileprocess.
9. 8. After a fraction of a second, the calciumion are pumped back
into thesarcoplasmic reticulum until a new muscleaction potential
comes along; this removalof calcium ions from the myofibrils
causesthe muscle contraction to cease. 10. MOLECULARMECHANISM OF
MUSCLE CONTRACTION 11. Sliding Filament Mechanism ofMuscle
ContractionIn contractedstate, actin (thin)filaments havebeen
pulledinward amongthe myosin(thick)filaments, sotheir endsoverlap
oneanother to theirmaximum extent. 12. Sliding Filament Mechanism
of Muscle Contraction Muscle contraction occurs by a
slidingfilament mechanism. When action potential travels along
themuscle fiber, sarcoplasmic reticulumrelease large quantities of
calcium ionsthat rapidly surround the myofibrils. Energy is needed
for the contractileprocess to proceed. (ATP to ADP) 13. Molecular
Characteristics of theContractile Filaments Myosin filaments is
made up of 200 ormore individual myosin molecules. 14. Molecular
Characteristics of theContractile Filaments arm Cross-bridges
hinges body 15. Molecular Characteristics of theContractile
Filaments Body- bundled tails of myosinmolecules. Arm- extends the
head outward fromthe body. Head- globular polypeptide structure.
Cross-bridges- heads and armstogether. Hinges- flexible point of a
cross-bridge. 16. Molecular Characteristics of theContractile
Filaments The total length of each myosin filament ifuniform,
almost exactly 1.6 micrometers. The distance between cross-bridges
andbody is 0.2 micrometers. Myosin filament itself is twisted from
theprevious pair by 120 degrees ensuring thatcross-bridges are
extended in alldirections around the filament. 17. Molecular
Characteristics of theContractile Filaments Myosin molecule is
composed of 6 polypeptidechains. 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 theContractile Filaments ATPase
activity of the Myosin Head The myosin head functions as an
ATPaseenzyme. Actin Filaments are composed ofActin, 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-actinmolecules. 19. Molecular Characteristics of
theContractile Filaments G-actin molecule has a molecular weightof
about 42,000. One molecule of ADP is attached to eachof it. Each
actin filament is about 1 micrometerlong. The bases of the actin
filaments is insertedstrongly into the Z-discs. 20. Molecular
Characteristics of theContractile Filaments 21. Molecular
Characteristics of theContractile Filaments Each molecule of
tropomyosin found in theactin filament has a molecular weight
of70,000 and a length of 40 nanometers. These molecules are wrapped
spirallyaround the sides of the F-actin helix. In the resting
stage, the tropomyosinmolecules lie on top of the active sites
ofthe actin strands. 22. Molecular Characteristics of
theContractile Filaments Attached intermittently along the sides
ofthe tropomyosin molecule is the troponinmolecule. There are three
subunits: Troponin 1: has strong affinity for actin Troponin T: for
tropomyosin Troponin C: for calcium ions
