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Muscle contraction
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Introduction
Living muscle : is highly specialized tissuethat is capable of converting chemicalenergy into mechanical energy through its
contraction .Muscles are positioned and attached to the
skeleton in such a way that their contractionand relaxation lead to movement andlocomotion .
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Introduction
The ability to contract and relax is lostwhen muscle is converted to meat.
The biochemical processes that provide
energy for muscle function in the livinganimal are the same processes that cause
lactic acid production and loss of waterholding capacity during the postmortemperiod.
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Muscle how to contract and generate
force and perform work
At first, a muscle contraction is initiated bya stimulusthat arrives at the surface of themuscle fiber.
This stimulus is coming from the brain andspinal cord and is transmitted to muscle via
a nerveMembrane potential
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Membrane potentialDivided into two, one is the restingmembrane potential and another one isaction membrane potential.
A electrical potential always exists betweenthe inside and outside of the cell.
Vary from 10-100 millivolts.
Nerve and muscle fibers exhibit membranepotential
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The resting membrane potential
Generally, a slight excess of negative ionsaccumulates in the intracellular fluid alongthe inner surface of the membrane and a
slight excess of positive ions is presentalong the extracellular surface of the
membrane. This condition is exist in thenormal resting nerve fiber
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The resting membrane potentialIs the result of
The active transport of iron through themembrane.
Te selective permeability characteristics ofthe membrane to the diffusion of ions andsmall molecular.
The unique ionic composition of theintracellular and extracellular fluids.
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The ionic changeExtracellular fluid contains high concen. Of Na+
and Cl- ,but very low concen. Of K+andnondiffusible negative ions, in contrast, K+and
nondiffusible negative ions are very high inintracellular fluid and Na+and Cl- are quite low.
Concentration gradient, maintained by active
transport. Need a Na+and K +pump which islocated in the plasma membrane.
Energy required for pump action
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Action potentials : the stimuliAn electrical impulse, can along muscle and nervemembrane surface.An action potential is transferred from a motornerve to muscle fibers.
Is a wave of reversing electrical polarization thatresults from chemical changes in membrane-electro-chemical process.
Is initiated by a several thousandfold increase inthe sodium ion permeability of the membrane .
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Action potentials : the stimuliIf Na+ permeability increase to a higher value thanthat which exists for K+ , the high concentrationof Na+ in the extracellular fluid cause s a more
rapid rate of diffusion of Na+ into the cell, than ofthe K+ out of the cell.
This results in an excess of positive charge inside
the cell membrane and negative charges on itsoutside, thus reversing the resting membranepotential.
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Myoneural Junction
The stimulus that initiates musclecontraction in transferred from the nervefiber to the muscle fiber at the myoneural
junction .Structure as figure
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Action potential -- chemicalA special mechanismamplifies the electronicsignal and transfers it to the muscle fiber.
The action potential arrives at the end of the
myoneural junction , it causes a chemicaltransmitter -acetycholine ( ) to bereleased.
Acetycholine will act on sarcolemma then makemore permeable to Na+ , its polarity reverses andan action potential is propagated along its length.
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Acetycholine
Acts on the sarcolemma for only a fewmilliseconds , then destroyed after it releaseby cholinesterase.
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Action potentials in muscle
That occurs in muscle fibers whenacetylcholine contacts the sarcolemma isnearly identical to that which occurs in
nerve fiber.The entire sequence of events in the
transmission of an action potential past anypoint onthe muscle fiber requires about5-10 millisecond.
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Contraction of skeletal muscleInvolve four myofibrillar proteins : actin, myosin,
tropomyosin and troponin.Contractile protein: actin and myosin
Crossbridges formed between the filaments-contractile force
No crossbridges between actin and myosin
filaments-relaxed state.If a permanent crossbridges form and prevent thesliding of these filaments-rigor mortis
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Contraction of skeletal muscleRelax state: Calcium concentration is lessthan 10-7 moles/liter. Total of Ca+2
concentration is about 1000 times 10-7
moles/liter( about 10-4
moles/liter). Needpump into SR, so need higher level ofenergy-ATP
Most of ATP is found in Mg-ATP, must bepresent in order to prevent interaction ofactin and myosin (crossbridges).
hibi b id (C i
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Inhibit crossbridge (Contraction
of skeletal muscle)
Two factors :
sarcoplasma Ca+2 concentration is low (lessthan 10-7 moles/liter).
the Mg-ATP concentration is high.
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Contraction of skeletal muscle
The shift by tropomyosin allow the myosinhead s to form crossbridges between themyosin and actin filaments= a contractile
force : pulled toward the center of thesarcomere. (figure) The change of the
sarcomere (distance of two Z line).Crossbridges=actomyosin
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Energy-ATP
Muscle contraction required energy, whichis derived from ATP by a reaction catalyzedby the enzyme myosin ATPase.
ATP ADP +Pi
High Ca+2 concentration also can increase
ATP splitting .
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Relaxation of skeletal muscle
sarcoplasma Ca+2 concentration is low (lessthan 10-7 moles/liter).
the Mg-ATP concentration is high.
As the troponin loss these Ca+2 , it is againable to inhibit the formation of crossbridge
then relaxation state will be occurred .
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Source of energyMuscle glycogen
Blood glucose
Aerobic metabolism
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How to supply energy
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Energy , lactic acid and heat
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The end of this chapter