Physiology of The MusclePhysiology of The Muscle

Week 5Week 5

Dr. Walid DaoudDr. Walid Daoud

A. ProfessorA. Professor

Skeletal MuscleSkeletal Muscle MorphologyMorphology::

--Begin and end in fibrous tendons and made Begin and end in fibrous tendons and made up of muscle fibers arranged in parallel up of muscle fibers arranged in parallel bundles between tendinous endingsbundles between tendinous endings..

--Each muscle fiber is a multinucleated unit Each muscle fiber is a multinucleated unit made up of many parallel myofibrils made up of many parallel myofibrils . .

--Each myofibril contains interdigitating Each myofibril contains interdigitating myofilaments: thick (Myosin) and thin (Actin) myofilaments: thick (Myosin) and thin (Actin)

in the sarcotubular systemin the sarcotubular system . .

Skeletal MuscleSkeletal Muscle

SarcomereSarcomere::

--Functional unit of the muscleFunctional unit of the muscle..

--Extend between 2 transverse protein sheets Extend between 2 transverse protein sheets called Z linescalled Z lines..

--Thick filaments in the middle of sarcomereThick filaments in the middle of sarcomere..

--Thin filaments arranged on both sides of Thin filaments arranged on both sides of sarcomere with one end attached to Z line sarcomere with one end attached to Z line and the other overlap with thick filamentsand the other overlap with thick filaments..

MyofilamentsMyofilamentsContractile muscle proteins: myosin & actinContractile muscle proteins: myosin & actin..

11--Thick (myosin) filamentsThick (myosin) filaments:: --Each myosin molecule has 2 heads attached Each myosin molecule has 2 heads attached

to double helix tail to double helix tail..- - Myosin head containsMyosin head contains

. . Actin-binding siteActin-binding site.. . . ATP-binding siteATP-binding site..

. . Catalytic site for ATP hydrolysisCatalytic site for ATP hydrolysis..- - Myosin head protrude out of thick filamentsMyosin head protrude out of thick filaments cross-bridges that contact actin moleculescross-bridges that contact actin molecules..

MyofilamentsMyofilaments22 - -Thin (actin) filamentsThin (actin) filaments::

Formed of 3 muscle proteinsFormed of 3 muscle proteins:: . .Actin:Actin: double helix form with active site to double helix form with active site to

combine with myosin cross bridges combine with myosin cross bridges.. . .Tropomyosin:Tropomyosin: strands cover the active strands cover the active

sites on actin molecules during rest sites on actin molecules during rest.. . .Troponin:Troponin: 3 subunits: troponin I, T and C 3 subunits: troponin I, T and C

with strong affinity for actin, tropomyosin with strong affinity for actin, tropomyosin and Ca, respectively and Ca, respectively..

Sarco-tubular SystemSarco-tubular System11 - -T-TubulesT-Tubules::

Network of transverse tubules surround each Network of transverse tubules surround each myofibril. They are invaginations of muscle myofibril. They are invaginations of muscle fiber membrane to carry wave of the action fiber membrane to carry wave of the action potential to cell interiorpotential to cell interior..

22 - -Sarcoplasmic reticulum (SR)Sarcoplasmic reticulum (SR)::Network of longitudinal channels between T-Network of longitudinal channels between T-Tubules surrounding & parallel to myofibrilsTubules surrounding & parallel to myofibrils..

It is site of Ca storage & release during muscleIt is site of Ca storage & release during muscleContraction. Expanded ends (terminal cistern)Contraction. Expanded ends (terminal cistern)..

Changes following Skeletal Changes following Skeletal Muscle StimulationMuscle Stimulation

11 - -Electrical ChangesElectrical Changes..

22 - -Excitability ChangesExcitability Changes..

33 - -Mechanical ChangesMechanical Changes..

44 - -Metabolic ChangesMetabolic Changes..

Electrical ChangesElectrical ChangesSimilar to those in nerve fiber butSimilar to those in nerve fiber but::

- -RMP in the muscle fiber is -90 mvRMP in the muscle fiber is -90 mv..

- -Action potential lasts 2-4 msec & precedes Action potential lasts 2-4 msec & precedes muscle contraction by 2 msec muscle contraction by 2 msec..

Single muscle fiber obeys All or None RuleSingle muscle fiber obeys All or None Rule..

Simple muscle twitchSimple muscle twitch::

Single action potential causes brief contraction Single action potential causes brief contraction followed by relaxation. It starts 2 msec after followed by relaxation. It starts 2 msec after start of depolarizationstart of depolarization..

Excitability ChangesExcitability Changes

Skeletal muscle fiber, like nerve fiber, is Skeletal muscle fiber, like nerve fiber, is refractory to re-stimulation during the refractory to re-stimulation during the action potentialaction potential..

Thus, as the muscle begins to contract, it Thus, as the muscle begins to contract, it has regained its excitabilityhas regained its excitability..

Mechanical ChangesMechanical ChangesExcitation-Contraction CouplingExcitation-Contraction Coupling

Steps that action potential initiates muscle contractionSteps that action potential initiates muscle contraction

11--Release of CaRelease of Ca..

22--Activation of muscle proteinsActivation of muscle proteins..

33--Generation of tension by cross bridge cyclingGeneration of tension by cross bridge cycling::

a-Bindinga-Binding..

b-Bendingb-Bending..

c-Detachmentc-Detachment..

d-Returnd-Return..

44--Relaxation: Ca pumpRelaxation: Ca pump..

Types of Skeletal Muscle ContractionTypes of Skeletal Muscle Contraction11--Isotonic contractionIsotonic contraction::

. .Muscle shortensMuscle shortens.. . . Tension is constantTension is constant..

. . Work done by muscle = Weight x DistanceWork done by muscle = Weight x Distance22--Isometric contractionIsometric contraction::

. .Muscle length remains constantMuscle length remains constant.. . . Tension is increasedTension is increased..

. . Work done by muscle = ZeroWork done by muscle = Zero.. If person lifts heavy weight, contraction starts If person lifts heavy weight, contraction starts

isometrically then completes isotonicallyisometrically then completes isotonically . .

Factors Affecting Muscle ContractionFactors Affecting Muscle Contraction

11--Types of muscle fibersTypes of muscle fibers::

Slow (red) muscle fibersSlow (red) muscle fibersRapid (pale) muscle fibersRapid (pale) muscle fibers

--Slow contraction and Slow contraction and relaxationrelaxation..

--Rich in myoglobinRich in myoglobin..

--Not easily fatiguedNot easily fatigued..

--Adapted for prolonged Adapted for prolonged weight bearing weight bearing contractions: soleus contractions: soleus

musclemuscle . .

--Rapid contraction and Rapid contraction and relaxationrelaxation..

--Poor in myoglobinPoor in myoglobin..

--Easily fatiguedEasily fatigued..

--Adapted for fine skilledAdapted for fine skilled

Movements: hand and Movements: hand and extraocular musclesextraocular muscles . .

Factors Affecting Muscle ContractionFactors Affecting Muscle Contraction

22--Types of loadTypes of load:: a-Preload:a-Preload: before start of contraction before start of contraction::

The more the initial length of muscle the The more the initial length of muscle the more will be the tension in isometric more will be the tension in isometric contraction (within limits) contraction (within limits)..

b-Afterload:b-Afterload: after contraction started after contraction started.. The more the afterload, the less will be The more the afterload, the less will be

the velocity of contraction the velocity of contraction..

Factors Affecting Muscle ContractionFactors Affecting Muscle Contraction

33--Stimulus factorsStimulus factors:: a-Stimulus strengtha-Stimulus strength::

The more the strength, the more fibers are The more the strength, the more fibers are stimulated, the more force of contraction stimulated, the more force of contraction..

b-Stimulus frequencyb-Stimulus frequency:: Low frequency: Low frequency: separate muscle twitchseparate muscle twitch..

Medium frequency:Medium frequency: clones clones.. High frequency tetanus:High frequency tetanus: maintained maintained

contraction with no relaxationcontraction with no relaxation..

Factors Affecting Muscle ContractionFactors Affecting Muscle Contraction

44--FatigueFatigue::

Repeated stimulation of muscle results in Repeated stimulation of muscle results in muscle fatigue due to muscle fatigue due to::

- - Depletion of ATP, creatine phosphate & Depletion of ATP, creatine phosphate & glycogen glycogen..

- - Consumption of acetyl choline in motor Consumption of acetyl choline in motor end plate end plate..

- - Accumulation of metabolites: lactic acidAccumulation of metabolites: lactic acid..

- - Decreased oxygen & nutrient supplyDecreased oxygen & nutrient supply..

Metabolic Changes in MuscleMetabolic Changes in MuscleMuscular contraction requires energy and Muscular contraction requires energy and has been called a machine for converting has been called a machine for converting chemical energy into mechanical workchemical energy into mechanical work..

Sources of energySources of energy::

- -Adenosine triphosphate (ATP)Adenosine triphosphate (ATP)..

- -Creatine phosphate (CP)Creatine phosphate (CP)..

- -Glycogen or glucoseGlycogen or glucose..

- -Lipids: free fatty acidsLipids: free fatty acids..

Steps of Metabolic ChangesSteps of Metabolic Changes11 - -One mol ATP is hydrolyzed to provide One mol ATP is hydrolyzed to provide

energy for muscle contraction energy for muscle contraction..

22 - -Resynthesis of ATP from ADP and CPResynthesis of ATP from ADP and CP..

33 - -At rest during light exercise muscles utilizeAt rest during light exercise muscles utilize

free fatty acidsfree fatty acids..

44 - -As exercise intensity increases, muscles As exercise intensity increases, muscles utilize glucose if O2 supply is sufficient utilize glucose if O2 supply is sufficient . .

55 - -If O2 supply is not sufficient (anaerobic If O2 supply is not sufficient (anaerobic glycolysis), glucose will give lactic acid glycolysis), glucose will give lactic acid..

Oxygen DebtOxygen Debt

It is extra O2 consumption after exercise is It is extra O2 consumption after exercise is over to remove excess lactate, replenish over to remove excess lactate, replenish ATP and CP storesATP and CP stores..

This O2 consumed is proportionate to the This O2 consumed is proportionate to the extent to which the energy demands during extent to which the energy demands during exercise exceeded the capacity of the exercise exceeded the capacity of the aerobic synthesis of energy storesaerobic synthesis of energy stores..

O2 debt is 6 times the basal O2 consumptionO2 debt is 6 times the basal O2 consumption..