Muscle Contraction

Prof. Vajira WeerasingheDept of PhysiologyFaculty of Medicine

University of Peradeniya

Objectives

• Describe the process of excitation and contraction coupling and muscle relaxation

• Practical– In a muscle tracing, identify the following

phenomena, muscle twitch, summation, tetanus, staircase phenomenon, muscle fatigue, effect of temperature on muscle contraction

Skeletal Muscle Fiber Structure

Figure 12-3b: ANATOMY SUMMARY: Skeletal Muscle

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Muscle Fiber Structure

Figure 12-4: T-tubules and the sarcoplasmic reticulum

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Muscle contraction

• Depolarisation of the muscle membrane spreads through the muscle

• Causes muscle contraction (mechanical event)

Muscle contraction

• Excitation - contraction coupling

– Excitation : electrical event– Contraction : mechanical event

Important structural details

• sarcolemma– conduct AP over the surface of the

muscle fibre

• t tubules– Invagination of sarcolemmal

membrane

– conduct AP deep into the muscle fibre

Important structural details

• sarcoplasmic reticulum (SR)– ends dilated as terminal cisternae– contains abundance of Ca++ ions bound to

calsequestrin– Release Ca++ in response to AP in t

tubules– Remove Ca++ back in to SR (resequester)

Ca++Ca++

• AP spreads through t tubule into the muscle tissue

• Close to the sarcoplasmic reticulum DHP receptor (dihydropyridine receptor) senses the membrane depolarization

• alters its conformation• activates the ryanodine receptor (RyR)• that releases Ca2+ from the SR• • Ca flows to the myoplasm in the vicinity of actin &

myosin

THIN FILAMENT (Actin)

THICK FILAMENT (Myosin)

TroponinActin

Myosin

Tropomyosin

THIN FILAMENT

THICK FILAMENT

• Actin– Composed of 3 different proteins: actin, tropomyosin & troponin– Actin has myosin binding sites– They are normally covered by tropomyosin– Troponin contains Ca++ binding sites

• Myosin– Myosin contains protein chains with bent heads which forms

the cross bridges– Myosin head contain ATPase. An ATP molecule is attached to

it. ATP is broken down during sliding

TroponinActin

Myosin

Tropomyosin

ATP

• Ca++ binds to troponin• Troponin shifts tropomyosin• Myosin binding sites in actin filament

uncovered• Myosin head binds with actin• Cross bridges form• Filaments slide with ATP being broken down• Muscle shortens

• New ATP occupies myosin head• Myosin head detaches• Filaments slide back• Cycling continues as long as Ca is available

TroponinActin

Myosin

Tropomyosin

ATP

Ca2+TroponinActin

Myosin

Tropomyosin

Ca2+Myosin binding sites

Ca2+Ca2+Detachment Sliding

Binding

Ca++

Troponin

Ca++ binds to troponin

Actin

Myosin

Tropomyosin exposes actin

myosin head binds to actin& cross bridge forms

Filaments slideATP is broken down

New ATP comes, Ca is removed, ready to detach

ATP

Walk-along theory of contraction

• Heads of myosin filament is known to forms cross bridges by attachment

• Then head bends causing actin filament to slide• Then head detaches from the actin filament and

walked to a new site in actin filament and attaches again

• This process continue to happen• As if myosin head walk-along actin filament

• Relaxation– This occurs when Ca++ is removed from myoplasm

by Ca++ pump located in the sarcoplasmic reticulum

– When Ca++ conc is decreased– Troponin returns to original state– Trpomyosin covers myosin binding sites– Cross-bridge cycling stops

Timing of Electrical & Mechanical Events

Myogram of Single Muscle Twitch

Dystrophin

• Dystrophin is a rod-shaped cytoplasmic protein, and a vital part of a protein complex that connects the cytoskeleton of a muscle fiber to the surrounding extracellular matrix through the cell membrane

• It provides an anchoring function to the muscle proteins

• Dystrophin deficiency causes rare muscle diseases known as muscular dystrophies

Muscle twitch

Muscle twitch

• A single action potential causes a brief contraction followed by relaxation

• This response is called a muscle twitch.

• The twitch starts about 2 ms after the start of depolarization of the membrane, before repolarization is complete

Muscle twitch• Duration of the twitch varies with the type of muscle

being tested

• Fast muscle fibers, primarily those concerned with fine, rapid, precise movement, have twitch durations as short as 7.5 ms

• Slow muscle fibers, principally those involved in strong, gross, sustained movements have twitch durations up to 100 ms

• The strength of twitch depends on the number of motor units activated

Summation and tetanus

Summation

• If 2 stimuli are delivered in rapid succession the second twitch will be greater than the first

• This only occurs if repolarization is not complete

• The tension developed during summation is considerably greater than that during the single muscle twitch

Tetanus

• With rapidly repeated stimulation, activation of the contractile mechanism occurs repeatedly before any relaxation has occurred

• Individual responses fuse into one continuous contraction

• Such a response is called a tetanus or tetanic contraction

Tetanus

• It is a complete tetanus when there is no relaxation between stimuli and an incomplete tetanus when there are periods of incomplete relaxation between the summated stimuli

• During a complete tetanus, the tension developed is about four times that developed by the individual twitch contractions

Staircase phenomenon (treppe)• When a series of stimuli is delivered to skeletal muscle, there is

an increase in the tension developed during each twitch until, after several contractions, a uniform tension per contraction is reached

• This phenomenon is known as treppe, or the "staircase" phenomenon

• This is the basis of “warm up”

• Treppe is believed to be due to increased availability of Ca2+ for binding to troponin C, accumulation of heat or effect of pH

• It also occurs in cardiac muscle although cardiac muscles cannot be tetanised

• It should not be confused with summation of contractions and tetanus

Staircase phenomenon (treppe)

Staircase effect, summation and tetanus

Tetanus

Isotonic contraction

• Produces movement

• Most of the time movement is of this type

• Used in – Walking – Running– Movement of a part of the body (eg. Hand

movement)

Isotonic contraction

Isometric contraction

• Muscular contraction involves shortening of the contractile elements, but because muscles have elastic and viscous elements in series with the contractile mechanism, it is possible for contraction to occur without an appreciable decrease in the length of the whole muscle

• Such a contraction is called isometric ("same measure" or length)

Isometric contraction

Isometric contraction

• Produces no movement

• Used in – Standing– Sitting– Postural control