Muscle Contraction III

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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

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Muscle Contraction III