Chapter 10

Copyright © John Wiley and Sons, Inc. All rights reserved.
Chapter 10
Muscular
Tissue
Lecture slides prepared by Curtis DeFriez, Weber State University

Likenervoustissue, musclesareexcitableor "irritable”
they havetheability to respond to astimulus
Unlikenerves, however, musclesarealso:
Contractible (they can shorten
in length)
Extensible (they can extend or
stretch)
Elastic (they can return to their
original shape)
Functions of Muscular Tissue

Musclemakesup alargepercentageof thebody’sweight
Their main functionsareto:
Create motion– muscleswork with nerves, bones, and
jointsto producebody movements
Stabilizebody positionsand maintain posture
Storesubstanceswithin thebody using sphincters
Movesubstancesby peristaltic contractions
Generateheat through thermogenesis
Functions of Muscular Tissue

Three Types of Muscular Tissue

(b) Cardiac muscle (c) Visceral smooth muscle
(a) Skeletal muscle
Three Types of Muscular Tissue

Skeletal Muscle

Skeletal musclefibersarevery long “cells” - next to
neurons(which can beover ameter long),
perhapsthelongest in thebody
TheSartoriousmusclecontains
singlefibersthat areat least
30 cm long
A single skeletal muscle fiber
Skeletal Muscle

Sarcolemma
Motor neuron
Skeletal Muscle
Theterminal processesof amotor
neuron in closeproximity to the
sarcolemmaof askeletal musclefiber

Theepimysium, perimysium, and
endomysium all arecontinuouswith
theconnectivetissuesthat form
tendonsand ligaments(attach
skeletal muscleto bone) and muscle
fascia (connect musclesto other
musclesto form groupsof muscles)
Organization of Muscle Tissue

Organization of asinglemusclebelly
Epimysium
Perimysium

Organization of afasciculus

Organization of amusclefiber

A muscle, afasciculus, and afiber all visualized

In groupsof musclesthe
epimysium continuesto
becomethicker, forming
fasciawhich coversmany
muscles
Thisgraphic showsthe
fascia lata enveloping the
entiregroup of quadriceps
and hamstring musclesin
thething

Organization
of Muscle
Tissue
Many largemuscle
groupsareencased in
both asuperficial
and adeep fascia
Real Anatomy, John Wiley and Sons

An aponeurosisis
essentially athick
fasciathat connectstwo
musclebellies. This
epicranial aponeurosis
connectsthemuscle
belliesof theoccipitalis
and thefrontalisto form
“one” muscle: The
occipitofrontalis
Epicranial aponeurosis
Frontal belly of the
occipitofrontalism.

Veins, arteries, and
nerves are located in the
deep fascia between
muscles of the thigh.

Beneath theconnectivetissueendomysium isfound
theplasmamembrane(called thesarcolemma) of an
individual skeletal musclefiber
Thecytoplasm (sarcoplasm) of skeletal musclefibers
ischocked full of
contractileproteins
arranged in myofibrils
The Skeletal Muscle Fiber

You should learn thenamesof theinternal structuresof the
musclefiber
Sarcolemma
Sarcoplasm
Myofibril
T-tubules
Triad (with
terminal cisterns
Sarcoplasmic reticulum
Sarcomere

Increasing thelevel of magnification, themyofibrilsareseen
to becomposed
of filaments
Thick filaments
Thing filaments

A scanning electron micrograph of asarcomere
Thebasic functional unit of skeletal musclefibersisthe
sarcomere: An arrangement of thick and thin filaments
sandwiched between two Z discs

The“Z line” isreally aZ disc when considered in 3
dimensions. A sarcomereextendsfrom Z disc to Z disc.
Musclecontraction occursin thesarcomeres

Myofibrilsarebuilt from threegroupsof proteins
Contractile proteins generateforceduring contraction
Regulatory proteins help switch thecontraction process
on and off
Structural proteins keep thethick and thin filamentsin
proper alignment and link themyofibrilsto the
sarcolemmaand extracellular matrix
Muscle Proteins

Thethin filamentsarecomprised mostly of thestructural
protein actin, and thethick filamentsarecomprised mostly
of thestructural protein myosin
However, in both typesof filaments, therearealso other
structural and regulatory proteins
Muscle Proteins

In thethin filamentsactin proteinsarestrung together likea
bead of pearls
In thethick filamentsmyosin proteinslook likegolf clubs
bound together
Muscle Proteins

In thisfirst graphic, themyosin binding siteson theactin
proteinsarereadily visible.
Theregulatory proteinstroponin and tropomyosin have
been added to thebottom graphic: Themyosin binding sites
havebeen
covered
Muscle Proteins

In thisgraphic thetroponin-tropomyosin complex hasslid
down into the“gutters” of theactin moleculeunblocking
themyosin binding site
Thetroponin-tropomyosin complex can slideback and forth
depending on thepresenceof Ca2+
Myosin binding siteexposed
Muscle Proteins

Ca2+bindsto troponin which changestheshapeof the
troponin-tropomyosin complex and uncoversthe
myosin binding siteson actin
Muscle Proteins

• Besidescontractileand regulatory proteins, musclecontains
about adozen structural proteinswhich contributeto the
alignment, stability, elasticity, and extensibility of
myofibrils
• Titanisthethird most plentiful protein in muscle, after
actin and myosin - it extendsfrom theZ disc and
accountsfor much of theelasticity of myofibrils
• Dystrophinisdiscussed later asit relatesto thedisease
of muscular dystrophy
Muscle Proteins

With exposureof themyosin binding siteson actin (thethin
filaments)—in thepresenceof Ca2+ and ATP—thethick and
thin filaments“slide” on oneanother and thesarcomereis
shortened
The Sliding-Filament Mechanism

The“sliding” of actin on myosin (thick filamentson thin
filaments) can bebroken down into a4 step process

Step 1: ATPhydrolysis
Step 2: Attachment

Step 3: PowerStroke
Step 4: Detachment

Contraction and Movement
Overview
Interactions Animation
Contraction and Movement
You must be connected to the internet to run this animation.

Limited contact
between actin
and myosin
Compressed
thick
filaments
Length-Tension Relationship
Sarcomereshortening producestension within amuscle

Excitation-Contraction Coupling
Wewill comeback to theterm excitation-contraction
coupling in alittlebit
Beforewecan describethe
entireprocess, from
thinking of moving a
muscleto actual contraction
of sarcomeres, wemust
first exploretheprocesses
that occur at theneuromuscular junction

Excitation-Contraction coupling (EC coupling) involves
eventsat thejunction between amotor neuron and askeletal
musclefiber
Neuromuscular Junction

An enlarged view of theneuromuscular junction
Thepresynaptic membraneison theneuron whilethe
postsynaptic membraneisthemotorend plate on the
musclecell. Thetwo membranesare
separated by aspace,
or “cleft”

Consciousthought (to moveamuscle) resultsin activation of
amotor neuron, and releaseof theneurotransmitter
acetylcholine (AcCh) at theNM junction
Theenzyme
acetylcholinesterase
breaksdown AcCh
after ashort period
of time

Theplasmamembraneon the“far side” of theNMJbelongsto
themusclecell and iscalled themotorend plate
Themotor end plateisrich in chemical (ligand) - gated
sodium channelsthat respond to AcCh. Another way to say
this: Thereceptorsfor AcCh areon theligand-gated sodium
channelson themotor end plate

Thechemical events at theNMJtransmit theelectrical
eventsof aneuronal action potential into theelectrical
events of a muscle action potential

Neuromuscular Junctions

ThemuscleAPispropagated over thesurfaceof themuscle
cell membrane(sarcolemma) viavoltage(electrical)-gated
Na+
and K+
channels
Muscle Action Potential

By placing amicropipetteinsideamusclecell, and then
measuring theelectrical potential acrossthecell membrane,
thephasesof an
action potential
(AP) can be
graphed (asin this
figure)

Thebehavior of theNa+
and K+
channels, at various
pointsin theAP, areseen
in thisgraphic
Na+
gatesopen during the
depolarization phase
K+
gatesopen during the
repolarization phase

Generating An Action Potential
Theflow of ionsthrough cell amembranelooksalot likea
"piece" of electricity flowing through awire(but not asfast)
Generating an APon themusclemembraneinvolvesthe
transfer of information from an electrical signal (down the
neuron), to achemical signal (at theNMJ), back to an
electrical signal (depolarization of thesarcolemma)
Thisadded complexity (changing from electrical to
chemical back to electrical signals) providesnecessary
control of theprocess

EC coupling involvesputting it all together
Thethought processgoing on in thebrain
TheAParriving at theneuromuscular junction
Theregeneration of an APon themusclemembrane
Releaseof Ca2+from thesarcoplasmic reticulum
Sliding of thick on thin filamentsin sarcomeres
Generation of muscletension (work)

Thebrain
Themotor neuron
Acetylcholine(ACh)
Acetylcholinesteraseenzyme
Ach receptorson the
motor endplate
Na+-K+ channelson the
sarcolemma
Na+
flow
K+ flow
RegenerateAP
TheT-tubules
TheSR
Ca2+
release
Troponin/Tropomyosin
ATP
Myosin binding
Filamentsslide
Musclescontract
Role Players in E-C coupling

Contraction of Sarcomere
Contraction of aSarcomere

Sources of Muscle Energy
Stored ATP
3 seconds
Energy transferred from stored creatine phosphate
12 seconds
Aerobic ATPproduction
Anaerobic glucose use
30-40 seconds

Sources of Muscle Energy

In a state of homeostasis, muscleuseof O2 and
nutrientsisbalanced by theproduction of manageable
levelsof wasteproductslike
CO2
Heat - 70-80% of theenergy used by musclesislost as
heat - muscleactivity isimportant for maintaining body
temperature
Lactic acid (anaerobic)
Skeletal Muscle Metabolism

Oxygen Debt, or "ExcessPost-ExerciseOxygen
Consumption" (EPOC) istheamount of O2 repayment
required after exercisein skeletal muscleto:
Replenish ATPstores
Replenish creatinephosphateand
myoglobin stores
Convert lactic acid back into pyruvate
so it can beused in theKrebscycleto replenish ATP

Muscle Metabolism
MuscleMetabolism

Cardiac and Smooth Muscle
Metabolism
In responseto asingleAP, cardiac musclecontracts10-15
timeslonger than skeletal muscle, and must continueto do
so, without rest, for thelifeof theindividual
To meet thisconstant demand, cardiac musclegenerally
usestherich supply of O2 delivered by theextensive
coronary circulation to generateATPthrough aerobic
respiration

Cardiac and Smooth Muscle
Metabolism
Likecardiac muscle, smooth muscle(in your deep organs)
isautorhythmic and isnot under voluntary control (your
heart beatsand your stomach digestswithout you thinking
about it).
Unlikecardiac (and skeletal muscle) however, smooth
musclehasalow capacity for generating ATPand doesso
only through anaerobic respiration (glycolysis)

Motor Unit iscomposed of amotorneuron plus all of the
muscle cells it innervates
 High precision
• Fewer musclefibersper neuron
• Laryngeal and extraocular muscles(2-20)
 Low precision
• Many musclefibersper neuron
• Thigh muscles(2,000-3,000)
The Motor Unit

Florescent dyeisused to show theterminal processesof asingle
neuron which terminateon afew musclefibers
The Motor Unit

Activitiesrequiring extremeprecision (likethesubtleand rapid
movementsof theeye) involvemuscleswith very small motor units
(1-4 musclefibers/neuron)
The Motor Unit

All-or-none principle of muscle contraction
When an individual musclefiber isstimulated to
depolarization, and an action potential ispropagated along
itssarcolemma, it must contract to it’sfull force—it can’t
partially contract
Also, when asinglemotor unit isrecruited to contract, all
themusclefibersin that motor unit must all contract at the
sametime
The Motor Unit

Skeletal musclefibersarenot all alikein appearanceor
function. By appearance:
Red musclefibers(thedark meat in chicken legs) havea
high myoglobin content, moremitochondria, moreenergy
stores, and agreater blood supply
Whitemusclefibers(thewhitemeat in chicken breasts)
havelessmyoglobin, mitochondria, and blood supply
Skeletal Muscle Fiber Types

Slow oxidative fibers(SO) aresmall, appear dark red, are
theleast powerful type. They arevery fatigue resistant
Used for endurancelikerunning amarathon
Fast oxidative-glycolytic fibers(FOG) areintermediatein
size, appear dark red, and aremoderately resistant to fatigue.
Used for walking
Fast glycolytic fibers(FG) arelarge, white, and powerful
Suited to intenseanaerobic activity of short duration

Most skeletal musclesareamixtureof all threetypesof
skeletal musclefibers; about half thefibersin atypical
skeletal muscleareslow oxidative(SO) fibers
Within aparticular motor unit all theskeletal musclefibers
arethesametype
Thedifferent motor unitsin amusclearerecruited in a
specific order depending on thetask being performed (fast
anaerobic activity for maximal force, etc.)

Thereisabrief delay called thelatent period astheAP
sweepsover thesarcolemmaand Ca2+
ionsarereleased from
thesarcoplasmic reticulum (SR)
During thenext phasethefiber isactively contracting
Thisisfollowed by relaxationastheCa2+
ionsarere-
sequestered into theSR and myosin binding sitesarecovered
by tropomyosin
Temporary lossof excitability iscall the refractory period –
All musclefibersin amotor unit will not respond to a
stimulusduring thisshort time
Tension in a Muscle

A twitchisrecorded when astimulusthat resultsin
contraction (force) of asinglemusclefiber ismeasured over
avery brief millisecond timeframe
Tension in a Muscle

Applying increased numbersof action potentialsto amuscle
fiber (or afascicle, amuscle, or amusclegroup) resultsin
fusion of contractions(tetanus) and theperformanceof
useful work
Tension in a Muscle

Two motor units, onein green, theother in purple,
demonstratetheconcept of progressiveactivation of amuscle
known asrecruitment
Recruitment allowsamuscleto accomplish increasing
gradationsof contractilestrength
Tension in a Muscle

Muscle Tension
Control of MuscleTension

Muscle Contraction
Isotonic contractions resultsin movement
Concentric isotonic isatypeof musclecontraction in
which themuscleshorten whilegenerating force
Eccentric isotonic isacontraction in which muscle
tension islessthan theresistance(themusclelengthens)
Isometric contractions resultsin no movement
Muscleforceand resistanceareequal
Supporting objectsin afixed position and posture

Muscle Contraction

Exercise-induced muscle damage
After intenseexerciseelectron micrographsreveal
considerablemuscledamageincluding torn sarcolemmas
and disrupted Z-discs
Blood levelsof proteinsnormally confined only to
muscle(including myoglobin and theenzymecreatine
kinase) increaseasthey arereleased from damaged
muscle
Imbalances of Homeostasis

Spasm
A sudden involuntary contraction of asinglemusclewithin
alargegroup of muscles– usually painless
Cramp
Involuntary and often painful musclecontractions
Caused by inadequateblood flow to muscles(such asin
dehydration), overuseand injury, and abnormal blood
electrolytelevels

DiseaseStatesand Disorders
Fibrosis (myofibrosis)
 Replacement of musclefibersby excessiveamountsof
connectivetissues(fibrousscar tissue)
Myosclerosis
 Hardening of themusclecaused by calcification
Both myosclerosisand musclefibrosisoccur asaresult
of traumaand variousmetabolic disorders

Aging
In part dueto decreased levelsof physical activity, with
aging humansundergo aslow, progressivelossof
skeletal musclemassthat isreplaced largely by fibrous
connectivetissueand adiposetissue
Musclestrength at 85 isabout half that at age25
Compared to theother two fiber types, therelative
number of slow oxidativefibersappearsto increase

Chapter 10

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Chapter 10

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