physiology of skeletal muscle by dr. roomi
TRANSCRIPT
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Mechanism of contraction of
Skeletal musclein the light of its structure
By
Dr. Mudassar Ali Roomi (MBBS, M. Phil)
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Muscle Tissue
Skeletal Muscle
Cardiac Muscle
Smooth Muscle
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Skeletal Muscle
Long cylindrical cells
Many nuclei per cell
Striated
Voluntary
Rapid contractions
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Cardiac Muscle
Branching cells
One or two nuclei per cell
Striated
Involuntary
Medium speed contractions
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Smooth Muscle
Fusiform cells
One nucleus per cell
Nonstriated
Involuntary
Slow, wave-like contractions
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Skeletal muscle
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SKELETAL MUSCLE
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Z line Z line
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THICK AND THIN FILAMENTS
From surface of thickfilaments projectionsarisecross-bridges.
In centre of sarcomere,
thick filaments have noprojections (H zone).
The thin & thick filamentscontain contractile proteins:
The thick filaments contain
myosin protein. The thin filaments contain
actin, tropomyosin &troponin proteins.
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Myosin protein: in thick filaments
In 1 thick filament 200myosin molecules.
Molecular wt. of eachmyosin molecule = 480,000.
Each myosin molecule has 6polypeptide chains: 2 heavychains & 4 light chains.
2 heavy chains are coiledtogether double helix.
At 1 end two heavy chainsare folded head portion.In head portion 4 lightchains.
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Myosin protein: in thick filaments (cont)
3 parts of myosin molecule:
Head
Arm / Neck
Body / Tail
There are 2 points in myosinmolecule at which molecule ishighly flexibleHINGES:
i) Between head & arm / neck
ii) Between arm & body / tail
Tail/body is present in thickfilaments.
Arm & head protrude out fromsurface of filament as cross bridges.
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Myosin protein: in thick filaments (cont)
Cross bridges are absent in
centre.
In the centre of filament is tail
only, while cross bridges areformed by arm & head at
periphery as cross bridges.
In myosin head there are 2important sites:
Actin binding site.
Catalytic site.
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Thin filaments
3 contractile proteins are present
here:
1) ACTIN: Consist of 2 F-actin
strands. Each strand consist of
polymerized G actin molecules. Attached to each G actin
molecule is a molecule of ATP, &
point of attachment is active
site on actin strand.
Active sites are present at every2.7 nm.
Each G actin has molecular wt.
42,000.
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Thin filaments (cont)
2) TROPOMYOSIN:
Consist of 2 strands, with 70,000
molecular wt.
Tropomyosin strands at rest
physically cover active sites onactin filaments.
3) TROPONIN:
Attached to tropomyosin at
intervals.
It has 3 components: Troponin C, Troponin T, Troponin
I.
Molecular wt. 18,000 35,000.
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Thin filaments (cont)
Troponin C Affinity for calcium ions.
Troponin T Affinity for tropomyosin.(through which troponin complex isattached to tropomyosin)
Troponin I Affinity for actin strands.
It is the bond between troponin I &Actin, which keeps tropomyosin strandsin such a position that these physicallycover active sites of actin filaments.
During muscle contraction this bondis broken.
Tropomyosin-troponin complex =relaxing protein (keeps muscle relaxedby covering physically the active sites).
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Components of Troponin
(C,T,I)
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Molecular Mechanism of skeletal muscle
contraction:
Muscle is first excited ordepolarized and then contratcs(EXCITATION-CONTRACTIONCOUPLING).
Action potential enters deep into
muscle fiber from T-Tubulesaround which are terminalcisternae.
So depolarization spreads from TTubules terminal cisternae.
Membrane of terminal cisternaeis depolarized opening ofvoltage gated calcium channels calcium ions move out of theterminal cisternae.
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SKELETAL MUSCLE
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When it is in sarcoplasm, calcium is utilized by
troponin C to initiate muscle contraction
(excitation-contraction coupling).
4 calcium ions can bind with 1 molecule of
troponin C it breaks the bond between
troponin I & Actin tropomyosin strands
become loose they reach a deeper position active sites on actin are uncovered.
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Muscle contraction involves power strokes.
Before contraction, a molecule of ATP becomesattached to myosin head.
It is hydrolyzed to ADP to liberate energy stored inmyosin head.
When active site is uncoveredmyosin head bindswith active site on actin.
With stored energy, there is power stroke. At hinges, myosin molecule moves & carries along
actin / thin filaments.
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With energy of 2nd molecule of ATP,it detaches & move back to originalposition 2nd power stroke aseries of power strokes slidingof actin over myosin so that power
stroke is towards centre ofsarcomere shortening ofsarcomere or contraction ofmuscle.
Each cross bridge operates
independently.
Greater the number of crossbridges coming in contact withmyosin head greater is force ofcontraction.
When muscle is stretchedmorenumber of cross bridges attachedwith actin filaments increasedcontraction force.
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Binding Site Tropomyosin
Troponin
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Myosin
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FRANK-STARLING LAW:
Greater the initial length ofmuscle, greater is force ofcontraction up to certainlimits.
Cardiac muscle also obeysthis law ( increased venousreturn increased lengthof cardiac muscle
increased fillingincreasedemptying by contraction ofventricle.
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Contraction is initiated bycalcium ions.
As long as calcium ion issufficient in sarcoplasm
muscle contraction continues. Normally in the wall of
longitudinal tubule, there iscalcium pump.
Calcium is released fromterminal cisternae but ispumped back by calciumpump & when calcium is lowin sarcoplasmmusclerelaxes.
So, even to produce musclerelaxation, we need ATPbecause calcium pump needsATP.
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Sarcomere Relaxed
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Sarcomere Partially Contracted
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Sarcomere Completely Contracted
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SLIDING FILAMENT MODEL OF MUSCLE CONTRACTION
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Histological changes during muscle
contraction:
RELAXED MUSCLE:
2-2.5 m length of sarcomere.
AFTER CONTRACTION:
1-1.5 m length of sarcomere.
Length of A band constant.
Length of I band constant. Z Membranes become closer.
H zone decreases / disappear
Sliding of thin over thickfilaments.
Length of individual filamentsremain the same.
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