Muscle contraction

Muscle Contraction Prof. Vajira Weerasinghe Dept of Physiology Faculty 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 Pg. 401

Muscle Fiber Structure Figure 12-4: T-tubules and the sarcoplasmic reticulum Pg. 402

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)

THIN FILAMENT THICK FILAMENT Troponin Actin Myosin Tropomyosin

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
Troponin Actin 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
Troponin Actin Myosin Tropomyosin ATP

Detachment Sliding Binding Ca 2+ Troponin Actin Myosin Tropomyosin Ca 2+ Myosin binding sites Ca 2+ Ca 2+

Ca ++ Troponin Actin Myosin ATP Ca ++ binds to troponin Tropomyosin exposes actin myosin head binds to actin & cross bridge forms Filaments slide ATP is broken down New ATP comes, Ca is removed, ready to detach

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)

Produces no movement
Used in
Standing
Sitting
Postural control

Muscle contraction

by vajira54

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