23476397 05 contraction of skeletal muscle

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Chapter 6

Contraction ofSkeletal Muscle



Outline

Physiological structure of skeletal muscles

Mechanism of muscle contraction

Energetics of muscular activity

Analysis of the force generated by contraction

Summation of contraction



§ 6.1 Physiological

Structure of SkeletalMuscles



The parts of a muscle

Skeletal muscle

Nuclei

Cytosole

Myofibrils

FilamentThick filament

Thin filament

Muscle fibers



Sarcomere: the basic unit of contraction



Structure of a sarcomere

1. Actin and myosin filaments(A-band,)

2. Myosin filaments only (H-zone)

3. Actin filaments only (I-band)

4. M line

5. Z line



Molecular Structure of

Myofilament

M y o f i l a m

e n t

Thick filament: Myosin

F

Head consists of 4 light,

two heavy chain

Tail consist of two

heavy chains

Thin filament

F Trponin has affinities for actin,

tropomyosin and Ca

2+

R

C

Tropomyosin

Actin has binding sites of

myosin



Actin binding site

ATP binding site



Thin filament



§ 6.2 Mechanism of contraction

When muscle shorten, the

filaments of action and myosin which make up a

sarcomere do not shorten;

rather they slide past eachother like the fingers of two

hands interdigitating.

Sliding filament theory



The Molecular Basis of

Contraction

• The crossbridge cycle is the process

of the myosin binding to the actin,

going through the Power Stroke, and

then disengaging from the Actin.

The Crossbridge Cycle



The Crossbridge Cycle

Muscle is a chemomechanical transducer.It has the ability to convert chemical energy,

stored in the terminal phosphate group of

ATP, into mechanical work.The myosin crossbridge, or myosin molecular

motor, is the site for this energy conversion.

Thus in addition to generating force and

shortening, myosin is an enzyme that hydrolyzes

ATP (i.e. ATPase).



The energetic of filament sliding

1. During crossbridge cycle, myosin converts thechemical energy of ATP into the mechanical

energy of filament sliding.

2. Each cycle of mechanical activity of the

myosin crossbridge takes about 50 ms and is

accompanied by a cycle of ATPase activity.

3. During a contraction, each myosin head

undergoes a conformational change that movesthe thin filament 5 to 15 nm during a period as

shorter as 50ms.



• Two sarcomeres, or sets of filament arrays,shortening according to the filament sliding

hypothesis. The thick filaments (orange)consist of myosin, and the thin filaments(green) consist predominantly of actin.



Biochemical Cycle for ATP

Hydrolysis • Hydrolysis occurs while myosin is detached.

• Hydrolysis of ATP primes myosin for attachment.

• Once attached to actin, the products of ATPhydrolysis are released and the myosin then

undergoes a conformational change believed to be

the force generating step or powerstroke.

• Myosin detaches from actin upon binding of ATP

to complete one cycle of the actomyosin ATPase.



§6.3 Energetics of

Muscular Activity• A single muscle fiber may contain 15

billion thick filaments • When that muscle fiber is actively

contracting, each thick filament breaks

down roughly 2500 ATP molecules per

second



Energy for Contraction

• ATP ATPase ADP + Pi + energy 50-70%

• Na/K ATPase, Ca ATPase 10-20, 10-30%

• Amount of ATP in muscle is small…. – Must be quickly re-synthesized

• Three ATP production lines:

1. Creatine Phosphate2. Rapid Glycolysis

3. Aerobic Oxidation

myosin



Three source of ATP Production



In a resting skeletal muscle

• The demand for ATP is low

• Resting muscle fibers

absorb fatty acids andglucose that are delivered by

the bloodstream

• The extra ATP is used to

build up reserves of CreatinePhosphate (CP) and glycogen



At moderate levels of activity

• the demand for ATP increases,of which demand is met by themitochondria

• As the rate of mitochondrial

ATP production rises, so doesthe rate of oxygen consumption.

• As all the ATP produced isneeded by the muscle fiber, and

no surplus is available, theskeletal muscle now reliesprimarily on the aerobicmetabolism of pyruvic acid to

generate ATP•



At peak levels of activity

• the ATP demands are

enormous andmitochondrial ATPproduction rises to amaximum

• At peak levels of exertion, mitochondrialactivity can provideonly about 1/3 of the

ATP needed.• The remainder is

produced throughglycolysis



The difference of muscle fiber



Motor Unit

•One somatic motor unit and themuscle fibers that it innervates



Motor UnitThe number of muscle

fibers per motor unit canvary from four to several

hundreds

Muscles that control finemovements (fingers, eyes)

have small motor units

Large weight-bearing

muscles (thighs, hips) have

large motor units



Muscle Tone

• Muscle tone: – Is the constant, slightly contracted state of all

muscles, which does not produce active

movements

– Keeps the muscles firm, healthy, and ready to

respond to stimulus

• Spinal reflexes account for muscle tone by:

– Activating one motor unit and then another

– Responding to activation of stretch receptors in

muscles and tendons



§6.4 Analysis of the Force

Generated by Contraction• Now that you are familiar with the basic

mechanisms of muscle contraction at the

level of the individual muscle fiber, we can begin to examine the performance of

skeletal muscles--organs of the muscular

system. In this section, we will consider thecoordinated contractions of an entire

population of skeletal muscle fibers.



When a muscle contracts against a load,

it performs work.

Energy is transferred from muscle to theexternal load.

How much work has been done by

muscle contraction? It defined by:

W(work output)＝L (load) D (distance)

Work Output During Muscle

Contraction



Isometric Contractions

the muscle as a whole does not change length, and

the tension produced never exceeds the resistance

holding a heavy weight above the ground, pushingagainst a locked door, or trying to pick up a car

These are rather unusual movements. However,

many of the reflexive muscle contractions that

keep your body upright when you stand or sitinvolve the isometric contractions of muscles that

oppose the force of gravity.



•Isometric Contraction = Muscle does not shorten

•Tension increases

Isometric Contraction



Isometric Contraction

wall

Tension increases to the muscle’s capacity,

but the muscle neither shortens nor

lengthens

Occurs if the load is greater than thetension the muscle is able to develop

Since D = 0 so, W = 0, that means

energy

tension



How can a muscle generate force

without changing its length ?• Each muscle elastic elements:

– Tendons

– Intracellular cytoskeletal proteins with elastic properties

– Contractile proteins themselves can stretch

• These are included in the term: series elasticelements.



Non-contractile and connective

tissue in muscle itself

Tendon



•Isotonic Contraction = tension does not change

•Length shortens

Isotonic Contraction



Isotonic ContractionIn isotonic contractions, the muscle changes in length

(decreasing the angle of the joint) and moves the load

Shortening occurs when the tension generated by the cross

bridge exceeds forces opposing shortening

W=DL D

load



Muscle Contraction inLiving Body

Normal daily activities therefore involve a

combination of isotonic and isometric muscular

contractions. As you sit and read this text, isometric

contractions of postural muscles stabilize your

vertebrae and maintain your upright position.

When you turn a page, the movements of your arm,forearm, hand, and fingers are mainly produced by

isotonic contractions.



Isotonic Isometric

Walking, and running involve isotonic contractions.

Standing up or pushing a heavy objective involve

isometric contraction.



Effects of initial length onforce contraction of the

whole intact muscle

W d t di ti i h



passive force (tension), is the force developed by simply stretching a muscle to different lengths(think of a rubber band), which is determined by

preload and the elasticity of the muscle itself. total forces (tension), the force developed when

a muscle is stimulated to contract at different preload

active forces (tension) is the force developedwhen the muscle contracts, determined bysubtracting the passive tension from the totaltension, representing the relation between preloadand tension generated by contraction.

We need to distinguishbetween muscle…



force

(tension)

length

no stimulation length

02550

motor

force

transducer



force

(tension)

length

passive force

no stimulation length

02550

motor

force

transducer



force

(tension)

length

passive force

total force

(active + passive)

supramaximal stimulation

length

02550

motor

force

transducer



force

(tension)

length

passive force

total force

active force

supramaximal stimulation

length

02550

motor

force

transducer



force

(tension)

length

passive force

total force

active force

physiological range



Obviously…. There is a range limitation to

generate a greater active force,

optimal preload or optimal initiallength

Why？



The active tension is maximal when there is

maximal overlap of thick and thin filaments

and maximal cross-bridge



Effects of afterload onthe contraction of muscle:

Force-Velocity relationship

Three effects of velocity



Tension transducer

35Afterload9

ExperimentThree effects of velocityof shortening:

– latent period of shortening increases with

increasing load. – duration of shorteningdecreases with increasingload.

– velocity of shortening

decreases with increasingload.

Preload

pivot

Stop



Stretch the muscle to desired length by hanging appropriateweight (preload) from the relaxed muscle. The desired preload

is determined for the passive length:force relationship

• a) Place a support beneath

the muscle to allowadditional weights

(afterload) to be added

without stretching themuscle. b) Stimulate the muscleelectrically.c) Record change in musclelength once musclegenerates sufficient

force to lift total load(preload + afterload).d) Repeat series for different afterloads

B



Because:

1. The speed of muscle shortening depends

on the speed of cross-bridge cycling.

2. The force developed depends on the

number of cross-bridge formed.

3. As the afterload on the muscle increases,

the velocity will be decreased becausecross-bridge can cycle less rapidly

against the higher resistance.

F V l i C



Force–Velocity Curve

P0:

V=0

1. When the load is very small

(the weight of the muscleonly), the velocity of

shortening is maximal.

2. When the muscle can no

longer overcome the massattached to it-- P0 is the point

where That is the isometric

force that this muscle can

produce.

3. In all cases, both phases are

present (isometric followed

by isotonic) hence these are

auxotonic twitches.

4. The larger the load the

slower the rate of

shortening.

Afterload & Power



Afterload & Power POWER = FORCE x VELOCITY

physiological power = strength of muscle

contraction x velocity of muscle contraction

P= L V 1. Muscle fiber can

shorten rapidly or develop high forces

but not at the same

time

2. Peak power occursat approximately 1/3

maximum

shortening velocity.



Effects of contractility onthe contraction of muscle



Contractility is determined by the level of intracellular

Ca2+, the activity of myosin and ATPase, etc. In vivo, the

contractility of the muscle is regulated by hormones,

drugs, and some other humoral substances.



§ 6.5 Summation of Contraction

Th F f M l



The Frequency of Muscle

Stimulation

• A twitch is a single stimulus – contraction –

relaxation sequence in a muscle fiber.

• Twitches vary in duration.



• A single twitch can be divided into a latent period ,

a contraction phase , and a relaxation phase:

2 ms 15ms 25 ms



• If a second stimulus arrives before the relaxation phasehas ended, a second, more

powerful contraction occurs.The addition of one twitchto another in this wayconstitutes the summationof twitched ,or wavesummation

• The duration of a single

twitch determines themaximum time available to

produce wave summation

Wave Summation



Incomplete Tetanus

• If the stimulation continuesand the muscle is never allowed to relax completely,

tension will rise to a peak.• A muscle producing peak tension during rapid cyclesof contraction andrelaxation is in incomplete

tetanus.



Complete Tetanus

• Complete tetanus is

obtained by increasing

the stimulation rate untilthe relaxation phase is

eliminated



Muscle tetanus offers large force of contraction than single twitch

In living body, tetanus is most pattern of muscle contraction

resulted from a continually impulse from an axon.



Summary

Force Regulation in Muscle• Types and number of motor units recruited

– More motor units = greater force

– Fast motor units = greater force• Initial muscle length

– “Ideal” length for force generation

• Nature of the motor units neural stimulation

– Frequency of stimulation

• Simple twitch, summation, and tetanus

23476397 05 contraction of skeletal muscle

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