physiology (muscle)
DESCRIPTION
i've used this note before this for my first year medicine in egypt. Fot those who taking this course(medicine) , I hope it'll give some ideas to you to study about this subject. Goodluck :) !TRANSCRIPT
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Muscle Physiology
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Skeletal muscle
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The muscle is formed of bundles of
muscle fibres.
- Each fibre is a single cylindrical elongated multinucleated cell 10-80 microns in diameter.
- It is surrounded by inner plasma
membrane and outer sarcolemma.
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- The fibre contain cytoplasm (sarcoplasm) that contains large number of mitochondria, K+, Mg++, enzymes and energy sources as glycogen, ATP & creatine phosphate also contain sarcoplasmic reticulum.
- Each fibre contains several hundred myofibrils.
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Each myofibril consisted of several filaments.
-These filaments are made up of the contractile proteinsof muscle (actin and myosin) and regulatory proteins of muscle (troponin and tropomyosin).
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The arrangement of thesecontractile proteins in the filament shows cross striations with alternative dark (A) and light (I) bands.
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- The dark band:
.Called A band.
.Contain myosin filaments and ends of actin filaments.
.Has lighter central H-zone (has no actin filaments).
.M-line in middle of H-zone due to central bulge on thick myosin filament.
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The light band:
. Called I band (Isotropic to light).
. Contain thin actin filaments + troponin.
. Contain central Z-line (site of attachment of thin filaments).
-Longitudinal striations are due to longitudinal arrangement of the muscle fibres and transverse striations are due to alternating light and dark bands.
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Z line Z line
-The area between two Z-lines is called sarcomere which is the contractile unit of the muscle.
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Sarcotubular system:It is a system of tubules present in the
sarcoplasm of the muscle fibre and made of:
1) T- tubules (transverse tubules):
It is a deep infolding of the sarcolemma as an extension of the extracellular space (at junction between A & I bands), this infolding is perpendicular on the myofibril so it is called transverse tubule.
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T-tubule membrane contains a voltage sensitive receptor that open the Ca++
channels on the sarcoplasmic reticulum
- Through this tubule the action potential from the cell membrane is rapidly spread to all myofibrils.
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2) The sarcoplasmic reticulum:
- It is formed of irregular longitudinal tubules that run parallel and surround the myofibrils to be a source of Ca++ ions, via Ca+2
channels.
- At site of contact to T-tubules it dilate to form terminal cisterns.
Each 2 terminal cisterns with adjacent T-tubules is called triad.
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How does sarcotubular system work?
Stimulation of the sarcolemma stimulation of the T-tubule receptors It will open Ca+2 channels present in the membrane of cisterns release of calcium required for muscle contraction.
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* Muscle proteins:[A] Contractile proteins:
1- Myosin:
- Myosin is complex protein with M.W. 480,000.
- Composed of 6 polypeptide chains (2 heavy chains and 4 light chains).
- The 2 heavy chains wrap spirally around each other as double helix forming long tail, while the terminal part combine with the 4 light chains forming 2 globular heads, the head contains actin-binding sites and ATP-ase enzyme (help ATP hydrolysis).
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-Cross bridges is formed of the head with arm of 2 flexible points called hinges (one between arm and tail and the other between the arm and heads) to bind to actin.
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2- Actin:
- It is small globular protein with M.W. 42,000.
- The globules attached to each other to form filamentous structure arranged in two chains as long double helix.
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H Band
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[B] Regulatory protein:
1- Tropomyosin:
It is long filament of two polypeptide chains twisting on each other and located between the 2 chains of actin covering its active sites which combine to myosin.
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2- Troponin:
- Small globules located at intervals along tropomyosin.
- Of 3 subunits with MW 18,000-25,000.
1-Troponin T:
binds troponin to tropomyosin.
2-Troponin I:
inhibits binding of actin & myosin.
3-Troponin C:
binds Ca+2 ions contraction.
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