Muscle Contraction DMS

8/11/2019 Muscle Contraction DMS

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Muscle and Contraction

*Blok Dermatomuscular System-K8-9*

Dept.Fisiologi FK USU


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Types of muscle

The three types ofmuscle tissue are

skeletal, cardiac,and smoothThese types differin structure,

location, function,and means ofactivation


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Summary: Comparison of Three Muscle Types

Table 12-3: Comparison of Three Muscle Types


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Skeletal Muscle Fiber Type:

Characteristics :Speed of contraction determined byspeed in which ATPases split ATP

slow and fast fibers ATP-forming pathways

Oxidative fibers use aerobic pathways

Glycolytic fibers use anaerobic glycolysisThese two criteria define threecategories slow oxidative fibers , fast

oxidative fibers, and fast glycolytic fibers


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Muscle Fiber Type: Speed ofContraction

Slow oxidative fibers contract slowly,have slow acting myosin ATPases, andare fatigue resistant

Fast oxidative fibers contract quickly,have fast myosin ATPases, and havemoderate resistance to fatigue

Fast glycolytic fibers contract quickly,have fast myosin ATPases, and are

easily fatigued


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Morphology of Skeletal Muscle


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Sarcotubular system :

Structur of membranesurounding the muslefibrils, consist of :T tubules : arecontinuous with thesarcolemmaSarcoplasmic reticulum

: functions in theregulation ofintracellular calciummovement


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Electrical phenomena & ionic fluxesElectrical characteristics of skeletal muscle :

Resting membrane potential : - 90 mVDuration of Action Potential : 2-4 ms

Speed of conduction : 5 m/sAbsolut refractory period : 1-3 ms

Ionic fluxes :Na + influx depolarizationK+ efflux repolarization


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Stimulation

Depolarization at Actionmotor end-plate potential

muscle fiber

contractile

response

Single A.P single contraction = muscle twitch

Contractile responses


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Skeletal Muscle ContractionIn order to contract, a skeletal muscle must bestimulated by a nerve ending of the somaticnervous system Axons of this neurons branch profusely as theyenter musclesEach axonal branch forms a neuromuscular

junction with a single muscle fiber

When a nerve impulse reaches the end of anaxon at the neuromuscular junction, ACh releaseto the synaptic cleft.


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Neuromuscular Junction


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Skeletal Muscle Contraction, contACh diffuses across the synaptic cleft toACh receptors on the sarcolemma

Binding of ACh to its receptors initiatesan action potential in the muscle.

The process by which depolarization ofthe muscle fiber initiates contraction iscalled Excitation-Contraction coupling .


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Molecular basis of contraction :1. Acetylcholin initiates A.P in muscle cell

propagated to entire surface of muscle cellmembrane.

2.The surface electric activity caried into themuscle fiber by the T tubules.

3. A.P in the T tubules trigers Ca 2+ release fromsarcoplasmic reticulum (SR)

4. Ca2+

bind to troponin (on actin filament) leadsto tropomyosin moved aside uncover actinscross-bridge binding sites.


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Myosin cross bridge attaches tothe actin myofilament

1

2

3

4 Working stroke the myosin head pivots andbends as it pulls on the actin filament, sliding ittoward the M line

As new ATP attaches to the myosinhead, the cross bridge detaches

As ATP is split into ADP and P i,cocking of the myosin head occurs

Myosin head(high-energy

configuration)

Thickfilament

Myosin head(low-energyconfiguration)

ADP and P i (inorganicphosphate) released

Sequential Events of Contraction

Figure 9.11

Thin filament


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Molecular basis of contraction.cont :5. Myosin cross-bridges attach to actin &

bend, producing a power stroke pullingactin filaments toward center ofsarcomere (previously, myosin have beenenergized by the splitting of ATP into ADP+Pi + energy by myosin ATPase, site onthe cross-bridge)

6. Inward sliding of all the thin filaments(actin) surounding a thick filament(myosin) shortens the sarcomere / causemuscle contraction.


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Molecular basis of contraction.cont :

7. Pi & ADP is released from the cross-bridgeafter the power stroke is complete.

8. New ATP attach permits separation of thecross- bridge return to its original conformation

9. Splitting of ATP by myosin ATPase energizes thecross-bridge once again.

10. If Ca 2+ still present so that the troponin-tropomyosincomplex remain pulled aside : the cross-bridge gothrough another cycle of binding & bending, pullingthe thin filament in even further.


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Types ofContraction

Isotonic contractionMuscle tension remains constant as the

muscle changes length.


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Isometric contraction

Muscle is prevented from shortening,tension developed at constant musclelength.


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Jenis Gerakan Otot : Concentric. Static.

Eccentric.


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Factors influence tension ofcontraction

1. The frequency of stimulation2. The length of the fiber at the onset

of contraction3. The extent of fatique4. The thickness of the fiber.


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Motor Unit: The Nerve-MuscleFunctional Unit

A motor unit is a motor neuron and all the

muscle fibers it suppliesThe number of muscle fibers per motor unitcan vary from four to several hundred

Muscles that control fine movements (fingers,eyes) have small motor units


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Motor Unit: The Nerve-Muscle FunctionalUnit

Figure 9.12 (a)


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Stimulation Strength

Threshold stimulus the stimulus strength atwhich the first observable muscle contraction

occursBeyond threshold, muscle contracts morevigorously as stimulus strength is increased

Force of contraction is precisely controlled bymultiple motor unit summationThis phenomenon, called recruitment, brings

more and more muscle fibers into play


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StimulusIntensity and

Muscle Tension

Figure 9.15 (a, b)


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The effect of frequency of stimulationSummation of contraction ;repeated stimulation (before relaxation has

occurred) additional activation of thecontractile elements greater tensiondeveloped.

1


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Tetanic contraction ;rapidly repeated stimulation, no relaxation hasoccurred continuous contraction.

Complete tetanusIncomplete tetanus

Treppe ; an increase in the tension developedduring each twitch until, after several

contractions, a uniform tension percontraction is reached.


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Treppe: The Staircase EffectStaircase increased contraction in responseto multiple stimuli of the same strength

Contractions increase because:There is increasing availability of Ca 2+ in thesarcoplasm

Muscle enzyme systems become more efficient because heat is increased as muscle contracts


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Relation between muscle length, tension &velocity of contraction

Maximum tension produce if length of thefiber at the onset of contraction is normal(resting length)If the muscle is stretched (longer) or shorter,the active tension & total tension willreduced.The velocity of muscle contraction is maximalat the resting length, & declines if the muscleget shorter or longer.

(see fig. 3.11 in Ganong)

2


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Source of energy for muscle contraction

Muscle use ATP for :Cross-bridge binding & separationPumping Ca 2+ from sarcoplasma into SRPumping Na + & K + through the cell

membrane

ATP is sufficient to maintain full contraction for1-2 s (ATP ADP+Pi+energy)

Several source of energy for rephosphorylation is:1. Phosphorylcrea tine2. Glycogen3. Oxidative metabolism


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Muscle FatigueMuscle fatigue the muscle is in a state ofphysiological inability to contract

Muscle fatigue occurs when:ATP production fails to keep pace with ATP useThere is a relative deficit of ATP, causingcontracturesLactic acid accumulates in the muscleIonic imbalances are present

3


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Rigor

When muscle fiber are completelydepleted of ATP &Phosphorylcreatine, relaxation doesnot occur a state rigidity.

When this occur after death rigormortis.


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The thickness of the fiberThe force of contraction is affected by:

The number of muscle fibers contracting themore motor fibers in a muscle, the strongerthe contraction

The relative size of the muscle the bulkierthe muscle, the greater its strength

4


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Heat Production During Muscle Activity

Only 40% of the energy released inmuscle activity is useful as work

The remaining 60% is given off asheatDangerous heat levels areprevented by radiation of heat fromthe skin and sweating


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Thermodynamically, the energy supplied to amuscle must equal its energy output. The energyoutput appears in work done by the muscle, inenergy-rich phosphate bonds formed for lateruse, and in heat. The overall mechanicalefficiency of skeletal muscle (work done/totalenergy expenditure) ranges up to 50% whilelifting a weight during isotonic contraction and isessentially 0% during isometric contraction.Energy storage in phosphate bonds is a smallfactor. Consequently, heat production is

considerable. The heat produced in muscle can bemeasured accurately with suitable thermocouples.


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Resting heat , the heat given off at rest, isthe external manifestation of basalmetabolic processes. The heat producedin excess of resting heat during

contraction is called the initial heat . Thisis made up of activation heat , the heatthat muscle produces whenever it iscontracting, and shortening heat , which isproportionate in amount to the distance

the muscle shortens. Shortening heat isapparently due to some change in thestructure of the muscle during shortening.


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Following contraction, heat production in excess ofresting heat continues for as long as 30 minutes. Thisrecovery heat is the heat liberated by the metabolicprocesses that restore the muscle to its precontractionstate. The recovery heat of muscle is approximatelyequal to the initial heat; ie, the heat produced duringrecovery is equal to the heat produced duringcontraction.

If a muscle that has contracted isotonically is restoredto its previous length, extra heat in addition torecovery heat is produced ( relaxation heat ). Externalwork must be done on the muscle to return it to itsprevious length, and relaxation heat is mainly amanifestation of this work.


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Developmental Aspects: Age RelatedWith age, connective tissue increases andmuscle fibers decreaseMuscles become stringier and more sinewy

By age 80, 50% of muscle mass is lost(sarcopenia)Regular exercise reverses sarcopeniaAging of the cardiovascular system affects

every organ in the bodyAtherosclerosis may block distal arteries,leading to intermittent claudication andcausing severe pain in leg muscles


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Muscle Contraction DMS

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