lecture1 muscle physiology-3
TRANSCRIPT
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Muscle Physiology
Modul: Nervesystemet ogbevægeapparatet
Learning Outcomes
1. Describe the relationship between structureand function of the skeletal muscle fibre.
2. Describe the events leading to muscle fibrecontraction.
3. Describe the biochemistry of forcegeneration.
4. Describe the length-tension relationship.
5. Describe the role of the intramuscularconnective tissue framework.
Muscle types Smooth Cardiac Skeletal / Striated
Limb muscles generally have 2 attachmentpoints – origin & insertion
Outer connective tissue layer of muscle(epimysium) is continuous with the tendonwhich in turn attaches to the fibrousconnective tissue of bone (periosteum)
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Skeletal Muscle
Machines that convertchemical energy tomechanical work & heat
Muscles withgreater pennationangles have moresarcomeres inparallel and fewerin series.
Thus better able togenerate force butnot so greatshorteningvelocities..ie. Nogood for speed
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Single muscle fiber enclosed
by endomysium – anextension of the sarcolemma
One muscle fiber is approxas thick as a strand of yourhair!
Under the microscope it hasa “striated” appearance
Many fibers (up to 150)bundled together = fasiculus
Fasiculus enclosed byperimysium
Many fasicles bundledtogether & enclosed byepimysium = one muscle
Basic Structure
Basic Structure
Important
All the connective tissue – epimysium,perimysium, andendomysium iscontinuous with thetendon
Thus any tensiondeveloped in onemuscle cell istransmitted to thetendon.
Under theMicroscope
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Myofibrils – what you cannotsee
approx 100’s - 1000’smyofibrils = one musclefiber
1/100th size of a strandof your hair
Contain the apparatusthat contracts themuscle cell
approx 4500sarcomeres per cm ofmuscle
More of what you cannotsee:
The smallest functionalunit is the sarcomere
2.2microm
Actin & Myosin filaments(proteins) arrangedlongitudinally
Actin6nm diameter – thin
filaments – double helix
Myosin16nm diameter (1/10,000
of hair strand)
Cross bridges protrudefrom myosin molecule
Sacroplasmic reticulum & T-tubules
SR – intricate system oftubules – terminates asvesicles close to Z-lines(line btw 2 sacromeres)
Calcium ions stored invesicles
Regulation of calciumregulates muscularcontraction
T-tubules run to 2 SRvesicles
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The Motor Unit
A motor neuron + allthe muscle fibers it
innervates =MOTOR UNIT
The Motor Unit
So what happens when a nerve axon
excites the muscle fibers of a motorunit???
The NeuromuscularJunction
ACh diffuses acrosssynaptic cleft &attaches toreceptors onsarcolemma
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At SR & T-Tubules:
Initiation of muscleaction potential (MAP)
The MAP depolarizestransverse tubules at A-I junction of sarcomere
Calcium release fromlateral sacs ofsarcoplasmic reticulum
Ca2+ ions bind to troponin-tropomyosin in actin filamentsthus freeing actin to bind withmyosin
Actin combines with myosin- ATP. It also activates mysosin – ATPase which splits ATP.Energy thus released producesthe movements of the myosincrossbridge, thus tension iscreated.
ATP binds to myosin bridge
breaking actin-myosin bond,thus actin-myosin slide pasteach other (muscle shortens)
This process is repeated whileCa2+ concentration is high
The slidingfilament theory
a muscle shortens orlengthens because thethick (myosin) and thin(actin) filaments slidepast each other withoutactually changing length
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Sarcomere: number of attached cross-bridges, degree of overlap
Length-Tension Relationship
the force a muscle can produce depends on the amount ofactin-myosin overlap – ie. the muscle length
Types ofmuscle
contraction
Types ofmuscle
contraction
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ATP – The energy currency
Adenine: nitrogencontaining base
Ribose: a 5 carbonsugar
Most important arethe 3 phosphategroups!
Removal of 1 byhydrolysis = ADP
Removal of anotherone = AMP
ATP – The energy currency
ATP is ESSENTIAL if we are to performany movement
Where then does it come from?
Some is stored directly in muscle cell but
this is not much [5mmol], thus we haveto constantly produce ATP!
How?
Biological Energy Systems
The replenishment of ATP in human skeletalmuscle is accomplished by 3 basic energysystems:
– Phosphagen System
– Glycolytic System
– Oxidative System
All 3 are active at any one time, BUT, theextent to which each is used DEPENDS intensity of activity & duration of activity
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Phosphagen System
Active at start ofactivity regardless ofintensity
Provides ATPprimarily for short-term, high-intensityactivities
Limited amounts of ATP & creatinekinase are stored!
Phosphagen System
ADP concentration = activity of Creatine Kinase
ATP concentration =
activity of Creatine Kinase
Immediately at start of exercise this system is used, thus: ATPproduced & ADP concentration, thus activity of CreatineKinase. IF exercise stops or is at a low enough level for othersystems to kick in then ATP concentration & activity ofCreatine Kinase
Phosphagen System
Provides an immediate source of ATP
AMP stimulates glycolysis!!!!!
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Glycolytic System
Glycolysis = breakdown ofcarbohydrates to produce ATP
– Glycogen stored in muscle, or
– Glucose delivered in blood
Multiple reactions involved - nxt pg
Glycolysis – fast vs slow
LACTATE There is no such thing as LACTIC ACID! only
LACTATE – this is what you measure whenyou take blood samples! It is NOT linked tofatigue!!!!
Clearance of lactate from blood reflects aperson’s ability to recover
– Both aerobically & anaerobically trained individualsclear lactate FASTER than untrained
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Biological Energy Systems
Biological Energy Systems
Substrate Depletion & Repletion
Energy substrates – molecules that providestarting materials for bioenergetic reactions –phospagens (ATP, creatine phosphate),glucose, glycogen lactate etc etc
Depletion = ability to produce energy
Fatigue associated with substrate depletion
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Substrate Depletion & Repletion
Phosphagens Depletion:
– Creatine phospate depleted by 70% during first 30s ofintense exercise
– ATP never fully depleted – 60% during high intensityexercise
Repletion: – CP within 8 mins post exercise – ATP within first 3-5 mins
Training effect: – storage within muscle cell? - uncertain – muscle mass = storage within whole muscle – resistance
training but also sprint training
Substrate Depletion & Repletion
Glycogen Depletion:
– Limited stores in muscles & liver – these can be controlledby diet
– Rate of depletion depends on exercise intensity – liverglycogen rarely depleted
– High intensity, intermittent exercise e.g. resistance trainingcan cause substantial in muscle glycogen stores
Repletion:
– Related to post exercise carbohydrate ingestion. – Approx 24hrs required
Training effect: – storage through anaerobic training e.g. sprinting &
resistance but also aerobic training
Metabolic specificity of training
Appropriate exercise INTENSITIES & REST intervals can permitthe “selection” of specific energy systems during training forspecific athletic events.
Most sports have various “metabolic” demands i.e. they arecomprised of a series of high intensity, constant or nearconstant effort interspersed with rest periods
Pure aerobic training is NOT effective!
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The end