23476397 05 contraction of skeletal muscle
TRANSCRIPT
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 1/71
Chapter 6
Contraction ofSkeletal Muscle
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 2/71
Outline
Physiological structure of skeletal muscles
Mechanism of muscle contraction
Energetics of muscular activity
Analysis of the force generated by contraction
Summation of contraction
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 3/71
§ 6.1 Physiological
Structure of SkeletalMuscles
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 4/71
The parts of a muscle
Skeletal muscle
Nuclei
Cytosole
Myofibrils
FilamentThick filament
Thin filament
Muscle fibers
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 5/71
Sarcomere: the basic unit of contraction
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 6/71
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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 7/71
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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 8/71
Actin binding site
ATP binding site
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 9/71
Thin filament
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 10/71
§ 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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 11/71
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 12/71
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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 13/71
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).
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 14/71
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 15/71
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.
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 16/71
• 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.
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 17/71
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.
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 18/71
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 19/71
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 20/71
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 21/71
§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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 22/71
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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 23/71
Three source of ATP Production
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 24/71
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 25/71
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 26/71
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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 27/71
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•
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 28/71
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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 29/71
The difference of muscle fiber
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 30/71
Motor Unit
•One somatic motor unit and themuscle fibers that it innervates
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 31/71
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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 32/71
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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 33/71
§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.
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 34/71
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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 35/71
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.
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 36/71
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 37/71
•Isometric Contraction = Muscle does not shorten
•Tension increases
Isometric Contraction
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 38/71
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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 39/71
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.
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 40/71
Non-contractile and connective
tissue in muscle itself
Tendon
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 41/71
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 42/71
•Isotonic Contraction = tension does not change
•Length shortens
Isotonic Contraction
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 43/71
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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 44/71
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.
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 45/71
Isotonic Isometric
Walking, and running involve isotonic contractions.
Standing up or pushing a heavy objective involve
isometric contraction.
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 46/71
Effects of initial length onforce contraction of the
whole intact muscle
W d t di ti i h
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 47/71
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…
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 48/71
force
(tension)
length
no stimulation length
02550
motor
force
transducer
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 49/71
force
(tension)
length
passive force
no stimulation length
02550
motor
force
transducer
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 50/71
force
(tension)
length
passive force
total force
(active + passive)
supramaximal stimulation
length
02550
motor
force
transducer
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 51/71
force
(tension)
length
passive force
total force
active force
supramaximal stimulation
length
02550
motor
force
transducer
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 52/71
force
(tension)
length
passive force
total force
active force
physiological range
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 53/71
Obviously…. There is a range limitation to
generate a greater active force,
optimal preload or optimal initiallength
Why?
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 54/71
The active tension is maximal when there is
maximal overlap of thick and thin filaments
and maximal cross-bridge
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 55/71
Effects of afterload onthe contraction of muscle:
Force-Velocity relationship
Three effects of velocity
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 56/71
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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 57/71
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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 58/71
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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 59/71
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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 60/71
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.
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 61/71
Effects of contractility onthe contraction of muscle
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 62/71
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.
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 63/71
§ 6.5 Summation of Contraction
Th F f M l
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 64/71
The Frequency of Muscle
Stimulation
• A twitch is a single stimulus – contraction –
relaxation sequence in a muscle fiber.
• Twitches vary in duration.
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 65/71
• A single twitch can be divided into a latent period ,
a contraction phase , and a relaxation phase:
2 ms 15ms 25 ms
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 66/71
• 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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 67/71
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.
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 68/71
Complete Tetanus
• Complete tetanus is
obtained by increasing
the stimulation rate untilthe relaxation phase is
eliminated
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 69/71
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.
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 70/71
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
7/28/2019 23476397 05 Contraction of Skeletal Muscle
http://slidepdf.com/reader/full/23476397-05-contraction-of-skeletal-muscle 71/71