lecture1 muscle physiology-3

8/9/2019 Lecture1 Muscle Physiology-3

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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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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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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


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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lecture1 muscle physiology-3

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