A1258149715GCpUk

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Muscle Physiology
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Striated muscle tissuex 400

• Muscle histology

Notice the fine stripes or striations along
each muscle cell.
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same view different stain
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Higher magnification showing dark and light bands
on 5 cells/fibres.
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The Muscle Cell
Skeletal muscle cells (fibres) are long,
cylindrical and multinucleate
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T.E.M. showing light I bands made of
Actin And broader dark A bands made of Myosin
A Band
I Band
MYOSIN
ACTIN
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The Muscle Cell
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Obvious Z-Line
dark A Band
Is this muscle relaxed or contracted?
light I Band
mitochondria
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The Contractile Proteins Myosin and Actin
Actin and myosin are protein molecules that form
the myofilaments
Each thin filament is a strand of actin
molecules twisted into a helix
Each actin molecule has a myosin-binding site
where myosin heads can attach
Each thick filament consists of about 200
molecules of the protein myosin
Each myosin molecule is a long fibrous protein
with a head at one end.
Myosin heads attach to the actin molecules and
form cross-bridges
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The Banding Pattern of Skeletal Muscle
Under a light microscope the striated nature of
skeletal muscle can be observed
This is seen as a regular alternation of light
and dark bands
This banding pattern is due to the arrangement of
the thick and thin filaments within the myofibrils
Dark bands (A bands) appear where thick myosin
filaments are located
The edges of the A bands are very dark as thick
and thin filaments are present together
Light bands (I bands) appear where there are
thin filaments only
H ZONE
Across the middle of each I-band is a dark line
called the Z line The section of myofibril
between these Z lines is the SARCOMERE
The centre of the A band contains thick filaments
only and is slightly lighter (H Zone)
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A Detailed View Of A Skeletal Muscle Fibre
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Following the arrival of a nerve impulse at the
neuromuscular junction, a wave of depolarisation
spreads throughout the sarcolemma and inwards
via the transverse tubules
The inward spread of the impulse depolarises
the sarcoplasmic reticulum
The inward spread of depolarisation triggers the
release of calcium ions from the
sarcoplasmic reticulum
Calcium ion concentrations in the vicinity of the
thin filaments increase
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Z-Line
Sarcolemma
H Band
A Band
I Band
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Motor end plate
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The Sliding Filament Theory
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Calcium ions now bind to TROPONIN
TROPONIN changes shape allowing TROPOMYOSIN to
move away from the myosin binding sites
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Calcium ions now bind to TROPONIN
TROPONIN changes shape allowing TROPOMYOSIN to
move away from the myosin binding sites
The myosin binding sites on the actin molecules
are EXPOSED
Myosin heads make contact with the actin filaments
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Calcium ions now bind to TROPONIN
TROPONIN changes shape allowing TROPOMYOSIN to
move away from the myosin binding sites
The myosin binding sites on the actin molecules
are EXPOSED
Myosin heads make contact with the actin filaments
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THE SLIDING FILAMENT MECHANISM IS TRIGGERED INTO
ACTION AND MUSCLE CONTRACTION TAKES PLACE
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Notice that it is the H-Band which shortens
THE SLIDING FILAMENT MECHANISM IS TRIGGERED INTO
ACTION AND MUSCLE CONTRACTION TAKES PLACE
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Both the below are very good simulations of the
slilding filament muscle contraction http//www.sc
i.sdsu.edu/movies/actin_myosin_gif.html http//ms
jensen.education.umn.edu/1135/Links/Animations/Fla
sh/0008-swf_sarcomere_shor.swf Good animation of
pairs of muscle contracting http//biology.clc.uc
.edu/courses/bio105/muscles.htm An excellent
animation on structure and function of muscle
http//entochem.tamu.edu/MuscleStrucContractswf/in
dex.html
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Summary Structure Of Skeletal Muscle
Each myofibril is composed of a regular
arrangement of myofilaments known as the thick
and thin filaments
SARCOMERE