chapter 9 – muscle tissue

Chapter 9 – MUSCLE Chapter 9 – MUSCLE TISSUE TISSUE

Structure, FunctionStructure, Function

& Metabolism& Metabolism

Alireza Ashraf, M.D.Professor of Physical Medicine & Rehabilitation

Shiraz Medical school

SummarySummary

GeneralGeneral Types of muscle tissueTypes of muscle tissue Functional characteristicsFunctional characteristics FunctionsFunctions Structural organization of skeletal Structural organization of skeletal

musclemuscle Gross and microscopic anatomyGross and microscopic anatomy Excitation/contraction sequenceExcitation/contraction sequence Muscle metabolismMuscle metabolism

GeneralGeneral

Skeletal muscle represents about 40% of Skeletal muscle represents about 40% of our body mass, including smooth and our body mass, including smooth and cardiac muscle the figure may be as high as cardiac muscle the figure may be as high as 50%50%

Muscles transform chemical energy into Muscles transform chemical energy into mechanical energy, i.e. exert a force.mechanical energy, i.e. exert a force.

Muscles pull they do not push.Muscles pull they do not push. Skeletal and smooth muscle cells are called Skeletal and smooth muscle cells are called

fibers.fibers. Myo-, mys- and sarco- refer to musclesMyo-, mys- and sarco- refer to muscles

Types of Muscle Tissue, Table Types of Muscle Tissue, Table 9.39.3

SkeletalSkeletal

SmoothSmooth

Types of Muscle TissueTypes of Muscle Tissue

CardiacCardiac

Functional CharacteristicsFunctional Characteristics

ExcitabilityExcitability (irritability) – receive and (irritability) – receive and respond to stimuli. respond to stimuli. – Stimuli = neurotransmitters, extracellular pHStimuli = neurotransmitters, extracellular pH– Response = generation of an electrical impulse Response = generation of an electrical impulse

(AP)(AP) ContractilityContractility = ability to shorten, requires = ability to shorten, requires

energyenergy ExtensibilityExtensibility = ability to stretch, does not = ability to stretch, does not

require energyrequire energy ElasticityElasticity = ability to recoil, i.e. return to = ability to recoil, i.e. return to

resting lengthresting length

Functions of Muscle TissueFunctions of Muscle Tissue

Produces movementProduces movement– LocomotionLocomotion– PropulsionPropulsion– ManipulationManipulation

Maintains postureMaintains posture Stabilizes jointsStabilizes joints Generates heat – primarily skeletal Generates heat – primarily skeletal

m.m.

Structural Organization of Structural Organization of Skeletal Muscle, Table 9.1Skeletal Muscle, Table 9.1

Organ =Organ = Fascicles =Fascicles = Cell =Cell = MyofibrilsMyofibrils MyofilamentsMyofilaments

Skeletal Muscle Gross Skeletal Muscle Gross AnatomyAnatomy

Tissues:Tissues:– Blood vessels Blood vessels – Nerves – branches to each fiberNerves – branches to each fiber– Connective Tissue (Fig 9.2, Table 9.1)Connective Tissue (Fig 9.2, Table 9.1)

EndomysiumEndomysium –wraps each fiber –wraps each fiber PerimysiumPerimysium –wraps fibers into fascicles –wraps fibers into fascicles EpimysiumEpimysium –wraps fascicles into a muscle –wraps fascicles into a muscle All are continuous with each other and the All are continuous with each other and the

tendons.tendons.

Skeletal Muscle Microscopic Skeletal Muscle Microscopic Anatomy, fig 9.3Anatomy, fig 9.3

Cell membrane = Cell membrane = sarcolemmasarcolemma Cell interior gel = Cell interior gel = sarcoplasmsarcoplasm with with

myoglobinmyoglobin OrganellesOrganelles

– Mitochondria, multiple nuclei, etc. Mitochondria, multiple nuclei, etc. squeezed between myofibrils.squeezed between myofibrils.

– MyofibrilsMyofibrils aligned in such a way as to aligned in such a way as to produce alternating light (I) and dark (A) produce alternating light (I) and dark (A) bands or striations.bands or striations.

Skeletal Muscle MicroscopicSkeletal Muscle Microscopic Anatomy Anatomy

Skeletal Muscle Microscopic Skeletal Muscle Microscopic AnatomyAnatomy

Myofibrils – hundreds to thousands Myofibrils – hundreds to thousands per cell contain the contractile per cell contain the contractile proteins = proteins = myofilamentsmyofilaments – Actin – thin filamentsActin – thin filaments– Myosin – thick filamentsMyosin – thick filaments


BandsBands– A bandsA bands = actin & = actin &

myosin overlapmyosin overlap– I bandsI bands = actin only = actin only– H zoneH zone in A band – in A band –

myosin onlymyosin only– Z discZ disc – attachment – attachment

of actin and of actin and myosin; distance myosin; distance between Z discs = between Z discs = sarcomeresarcomere


Closer look at a Closer look at a sarcomeresarcomere


Ultrastructure of Sarcomere, fig 9.4Ultrastructure of Sarcomere, fig 9.4– MyosinMyosin – 2 globular heads whose tails are – 2 globular heads whose tails are

intertwined. Heads are the “business” end, intertwined. Heads are the “business” end, i.e. form i.e. form cross bridgescross bridges with actin. with actin.

– ActinActin – globular proteins arranged like 2 – globular proteins arranged like 2 strands of beads twisted together in a helix. strands of beads twisted together in a helix.

– TropomyosinTropomyosin – protein filaments give – protein filaments give strength and cover active sites on actin. strength and cover active sites on actin.

– TroponinTroponin – controls position of tropomyosin. – controls position of tropomyosin.


Sarcoplasmic reticulumSarcoplasmic reticulum – smooth ER, – smooth ER, regulates intracellular calcium; forms regulates intracellular calcium; forms paired terminal cisternae at A-I paired terminal cisternae at A-I junctions.junctions.

T-tubulesT-tubules – invaginations of – invaginations of sarcolemma that reach each A and I sarcolemma that reach each A and I band junction, traveling between band junction, traveling between paired terminal cisternae = paired terminal cisternae = triadtriad. . – Communicate with external environment, Communicate with external environment,

carry electrical impulses into muscle mass.carry electrical impulses into muscle mass.

Skeletal MuscleSkeletal Muscle

Contraction – Sliding Filament Contraction – Sliding Filament ModelModel

Sliding filament theorySliding filament theory– Hugh Huxley 1950’sHugh Huxley 1950’s– Contraction (shortening) - the thin Contraction (shortening) - the thin

filaments slide past the thick filaments filaments slide past the thick filaments and overlap increases. and overlap increases.

– Relaxation (lengthening) - thin filaments Relaxation (lengthening) - thin filaments return to their original position.return to their original position.

– Occurs simultaneously in sarcomeres Occurs simultaneously in sarcomeres throughout the fiber = muscle shortening.throughout the fiber = muscle shortening.

Physiology of Skeletal Muscle Physiology of Skeletal Muscle FiberFiber

Neuromuscular junction – chemical Neuromuscular junction – chemical synapse (Fig 9.7)synapse (Fig 9.7)


Neural StimulationNeural Stimulation– Motor neuron generates electrical impulse (AP) Motor neuron generates electrical impulse (AP)

that travels down the axon to the synapse.that travels down the axon to the synapse.– The impulse opens CaThe impulse opens Ca2+2+ channels, Ca channels, Ca2+2+ moves moves

in and causes vesicles, filled with ACh, to in and causes vesicles, filled with ACh, to empty the ACh into the synaptic cleft.empty the ACh into the synaptic cleft.

– ACh diffuses across the cleft to the Motor End ACh diffuses across the cleft to the Motor End Plate on the muscle fiber and binds to ACh Plate on the muscle fiber and binds to ACh receptors.receptors.

– AcetylcholinesteraseAcetylcholinesterase destroys remaining ACh destroys remaining ACh quickly to confine stimulation locally.quickly to confine stimulation locally.

Neuromuscular JunctionNeuromuscular Junction


Skeletal Muscle Excitation, fig 9.8Skeletal Muscle Excitation, fig 9.8– Sarcolemma is polarized at rest, inside Sarcolemma is polarized at rest, inside

negative relative to the outside. RMP = negative relative to the outside. RMP = -65mV -65mV

– ACh binds to ACh receptors at motor ACh binds to ACh receptors at motor end plate and opens end plate and opens LG NaLG Na+ + channelschannels, , Na ions move into the cell causing a Na ions move into the cell causing a small depolarization.small depolarization.


Excitation cont’dExcitation cont’d– At threshold potential, At threshold potential, VG NaVG Na++ channels channels

open.open.– NaNa++ rushes into the cell and an rushes into the cell and an Action Action

PotentialPotential is generated. is generated.


Excitation cont’dExcitation cont’d– AP is propagated along entire AP is propagated along entire

sarcolemma. sarcolemma. – RepolarizationRepolarization follows closely behind follows closely behind

depolarization as Na channels close and depolarization as Na channels close and VG KVG K channels open to restore the channels open to restore the membrane potential back to normal. membrane potential back to normal.

– During this period, the muscle fiber During this period, the muscle fiber cannot produce another action potential cannot produce another action potential = = Refractory PeriodRefractory Period..

Contraction Sequence, fig 9.10Contraction Sequence, fig 9.10

Latent Period (excitation/contraction Latent Period (excitation/contraction coupling)coupling)– Action potential (AP) travels across Action potential (AP) travels across

sarcolemma down the T-tubules to sarcolemma down the T-tubules to Terminal Cisternae. Terminal Cisternae.

– Terminal Cisternae – CaTerminal Cisternae – Ca2+2+ channels open channels open and release stored Caand release stored Ca2+2+ into sarcoplasm. into sarcoplasm.

Contraction SequenceContraction Sequence

ContractionContraction– CaCa2+2+ binds to troponin that then pulls binds to troponin that then pulls

tropomyosin out of groove to expose the tropomyosin out of groove to expose the active sites on actin.active sites on actin.

– As calcium levels increase, the myosin As calcium levels increase, the myosin heads are activated and alternately heads are activated and alternately attach/detach from actin filaments, attach/detach from actin filaments, moving them toward the center of the moving them toward the center of the sarcomere. The sarcomere shortens.sarcomere. The sarcomere shortens.

ContractionContraction

RelaxationRelaxation– Ca-ATPase (calcium pump) moves Ca-ATPase (calcium pump) moves

calcium back into the terminal cisternae, calcium back into the terminal cisternae, tropomyosin moves back to cover active tropomyosin moves back to cover active sites on actin.sites on actin.

– Myosin heads detach and actin filaments Myosin heads detach and actin filaments move back to resting position = move back to resting position = relaxationrelaxation


Role of ATP:Role of ATP:– Cross-bridge formation: ATPase on Cross-bridge formation: ATPase on

head hydrolyses ATP to ADP and Pi and head hydrolyses ATP to ADP and Pi and head “cocks” to attach to actin.head “cocks” to attach to actin.

– Power stroke: head rotates downward Power stroke: head rotates downward and pulls actin toward center of and pulls actin toward center of sarcomere, ADP and Pi are releasedsarcomere, ADP and Pi are released

– Cross bridge detachment – head binds a Cross bridge detachment – head binds a new ATPnew ATP

Muscle MetabolismMuscle Metabolism

Stored ATP – 4-6 seconds but is Stored ATP – 4-6 seconds but is regenerated by 3 mechanisms:regenerated by 3 mechanisms:– Direct phosphorylationDirect phosphorylation of ADP from creatine of ADP from creatine

phosphate (CP) with the help of creatine phosphate (CP) with the help of creatine kinase. SUPER FAST gives another 6-10 kinase. SUPER FAST gives another 6-10 seconds of activity.seconds of activity.

– Anaerobic glycolysisAnaerobic glycolysis – glucose is broken down – glucose is broken down into 2 pyruvate molecules and 2 ATP into 2 pyruvate molecules and 2 ATP molecules. Fast but short term – supports molecules. Fast but short term – supports another 30-40 seconds of activity. Problem - another 30-40 seconds of activity. Problem - lactic acid build-uplactic acid build-up

Muscle MetabolismMuscle Metabolism

EnergyEnergy– Aerobic glycolysisAerobic glycolysis (respiration) or Kreb’s (respiration) or Kreb’s

CycleCycle Glucose + O2 yields CO2 + HGlucose + O2 yields CO2 + H22 O + 30 ATP’s O + 30 ATP’s Pyruvate, amino acids, and fatty acids can Pyruvate, amino acids, and fatty acids can

also enter this cycle. Slow process, more also enter this cycle. Slow process, more useful for endurance exercise.useful for endurance exercise.

FatigueFatigue

Physiological inability of a muscle to Physiological inability of a muscle to contract – not enough ATP; different from contract – not enough ATP; different from psychological fatigue. ATP production lags psychological fatigue. ATP production lags behind use – contractures (no ATP to behind use – contractures (no ATP to release cross bridges.release cross bridges.

Accumulation of lactic acid decreases pH Accumulation of lactic acid decreases pH and inhibits ATP production.and inhibits ATP production.

Restoration of ionic balance of Na and K Restoration of ionic balance of Na and K requires ATP also – impaired with intense requires ATP also – impaired with intense exercise.exercise.

FatigueFatigue

Prolonged exercise leads to SR Prolonged exercise leads to SR damage and lack of control of damage and lack of control of intracellular Ca.intracellular Ca.

Oxygen debtOxygen debt = amount of oxygen = amount of oxygen needed to restore muscle anaerobic needed to restore muscle anaerobic fuel stores.fuel stores.

HeatHeat

60% of the energy released by 60% of the energy released by muscle contraction is in the form of muscle contraction is in the form of heat – 40% is in the form of work. heat – 40% is in the form of work. Shivering is muscle contraction used Shivering is muscle contraction used to warm a cold body.to warm a cold body.

When you exercise strenuously your When you exercise strenuously your body heats up. How is the heat body heats up. How is the heat dissipated?????dissipated?????

chapter 9 – muscle tissue

Documents

muscle mass

cardiac muscle

muscle tissue structure

musclestypes of muscle

smooth muscle cells

skeletal musclecontraction

myofilaments actin

actin myosin overlapi