deadrick chap 9 handouts part 1

8/2/2019 Deadrick Chap 9 Handouts Part 1

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Copyright 2010 PearsonEducation,Inc.

Three Types of Muscle Tissue

1. Skeletal muscle tissue: Attached to bones and skin Striated Voluntary (i.e., conscious control) Powerful



2. Cardiac muscle tissue: Only in the heart Striated Involuntary More details in Chapter 18



3. Smooth muscle tissue: In the walls of hollow organs, e.g.,

stomach, urinary bladder, and airways

Not striated Involuntary


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Copyright 2010 PearsonEducation,Inc. Table 9.3


Special Characteristics of Muscle Tissue

1. Excitability: (responsiveness or irritability):ability to receive and respond to stimuli

2. Contractility: ability to shorten whenstimulated

3. Extensibility: ability to be stretched4. Elasticity: ability to recoil to resting length


Muscle Functions

1.Movement of bones or fluids (e.g., blood)2.Maintaining posture and body position3.Stabilizing joints4.Heat generation (especially skeletal muscle)


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Skeletal Muscle Anatomy

A muscle is an organ(In general) Each muscle is served by one

artery, one nerve, and one or more veins


Skeletal Muscle - Anatomy

Connective tissue sheaths of skeletal muscle: Epimysium: dense regular connective tissue

surrounding entire muscle

Perimysium: fibrous connective tissue surroundingfascicles (groups of muscle fibers)

Endomysium: fine areolar connective tissuesurrounding each muscle fiber

***muscle fiber = muscle cell


Skeletal Muscle - Anatomy

Muscle Structure Associated Connective Tissue

Entire muscle Surrounded by epimysium

Fascicles (a bundle of muscle cells) Surrounded by perimysium

Muscle cell (muscle fiber) Surrounded by endomysium


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Copyright 2010 PearsonEducation,Inc. Figure 9.1

Bone

Perimysium Muscle fiber

Fascicle

(wrapped by perimysium)

Tendon

Epimysium

Muscle fiber

in middle ofa fascicle

Blood vessel

Endomysium

Fascicle(a)

(b)


Skeletal Muscle: Attachments

Muscles attach:

Directlyepimysium of muscle is fused to theperiosteum of bone orperichondrium of

cartilage

Indirectlyconnective tissue wrappingsextend beyond the muscle as a ropelike

tendon or sheetlike aponeurosis

Copyright 2010 PearsonEducation,Inc. Table 9.1


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Microscopic Anatomy of a Skeletal Muscle

Fiber

Cylindrical cell 10 to 100 m in diameter, upto 30 cm long

Multiple, peripheral nuclei (multinucleate)Many mitochondriaGlycosomes for glycogen storage,

myoglobin for O2 storage

Also contain myofibrils, sarcoplasmicreticulum, and T tubules


Myofibrils


NucleusLight I bandDark A band

Sarcolemma

Mitochondrion

(b) Diagram ofpart of a muscle fibershowing the myofibrils. One

myofibril is extended from the cut end of the fiber.


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Sarcomere

The organized, functional unit of a musclefiber

Smallest contractile unit of a muscle fiberThe region of a myofibril between two

successive Z discs

Composed ofthick and thinmyofilaments(made of contractile proteins)


Features of a Sarcomere

Thick filaments: run the entire length of an A band Thin filaments: run the length of the I band and

partway into the A band

Z disc: coin-shaped sheet of proteins that anchorsthe thin filaments and connects myofibrils to oneanother

H zone: lighter midregion where filaments do notoverlap

M line: line of protein myomesin that holds adjacentthick filaments together

Copyright 2010 PearsonEducation,Inc. Figure 9.2c, d

I band I bandA bandSarcomere

H zone

Thin (actin)filament

Thick (myosin)filament

Z disc Z disc

M line

(c)Small part of one myofibril enlarged to show the myofilamentsresponsible for the banding pattern. Each sarcomereextends fromone Z disc to the next.

Z disc Z discM line

Sarcomere

Thin (actin)filament

Thick(myosin)filament

Elastic (titin)filaments

(d)Enlargement of one sarcomere (sectioned lengthwise). Notice themyosin heads on the thick filaments.


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Ultrastructure of Thick Filament

Composed of the protein myosin

Myosin tails contain: 2 interwoven, heavy polypeptide chains

Myosin heads contain: 2 smaller, light polypeptide chains that act ascross

bridges during contraction

Binding sites for actin of thin filaments Binding sites for ATP ATPase enzymes


Ultrastructure of Thin Filament

Twisted double strand of fibrous protein Factin

F actin consists of G (globular) actin subunitsG actin bears active sites for myosin head

attachment during contraction

Tropomyosin and troponin: regulatoryproteins bound to actin


Flexible hinge region

Tail

Tropomyosin Troponin ActinMyosin head

ATP-bindingsite

Heads Active sitesfor myosinattachment

Actinsubunits

Actin-binding sites

Thick filamentEach thick filament consists of manymyosin molecules whose heads protrude

at opposite ends of the filament.

Thin filamentA thin filament consists of two strandsof actin subunits twisted into a helix

plus two types of regulatory proteins(troponin and tropomyosin).

Thin filamentThick filament

In the center of the sarcomere, the thickfilaments lack myosin heads. Myosin headsare present only in areas of myosin-actin overlap.

Longitudinal section of filamentswithin one sarcomere of a myofibril

Portion of a thick filamentPortion of a thin filament

Myosin molecule Actin subunits


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Sarcoplasmic Reticulum (SR)

Network ofsmooth endoplasmic reticulumsurrounding each myofibril

Pairs ofterminal cisternae formperpendicular cross channels

Functions in the regulation of intracellularCa2+levels


T Tubules

Continuous with the sarcolemmaPenetrate the cells interiorat each A band

I band junction

Associate with the paired terminal cisternaeto form triads that encircle each sarcomere

Carry the signal for contraction deep intothe sarcoplasm


Myofibril

Myofibrils

Triad:

Tubules of

the SR

Sarcolemma

Sarcolemma

Mitochondria

I band I bandA band

H zone Z discZ disc

Part of a skeletal

muscle fiber (cell)

T tubule Terminal

cisternaeof the SR (2)

M line


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Triad Relationships

T tubules conduct impulses deep intomuscle fiber

Integral proteins protrude into theintermembrane space from T tubule and SR

cisternae membranes

T tubule proteins: voltage sensorsSR foot proteins: gated channels that

regulate Ca2+ release from the SR cisternae


Contraction

The generation of forceDoes not necessarily cause shortening of the

fiber

Shortening occurs when tension generated bycross bridges on the thin filaments exceeds

forces opposing shortening


Sliding Filament Model of Contraction

In the relaxed state, thin and thick filamentsoverlap only slightly

During contraction, myosin heads bind toactin, detach, and bind again, to propel the

thin filaments toward the M line

As H zones shorten and disappear,sarcomeres shorten, muscle cells shorten,

and the whole muscle shortens


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IFully relaxed sarcomere of a muscle fiber

Fully contracted sarcomere of a muscle fiber

IAZ ZH

I IAZ Z

1

2


Requirements for Skeletal Muscle

Contraction

1. Activation: neural stimulation at aneuromuscular junction

2. Excitation-contraction coupling: Generation and propagation of an action

potential along the sarcolemma

Final trigger: a brief rise in intracellular Ca2+levels


Events at the Neuromuscular Junction

Skeletal muscles are stimulated by somaticmotor neurons (somatic efferents)

Axons of motor neurons travel from thecentral nervous system via nerves to skeletal

musclesEach axon forms several branches as it

enters a muscle

Each axon ending forms a neuromuscularjunction with a single muscle fiber


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Neuromuscular Junction

Situated midway along the length of a musclefiber

Axon terminal and muscle fiber are separatedby a gel-filled space called the synaptic cleft

Synaptic vesicles of axon terminal contain theneurotransmitteracetylcholine (ACh)

Junctional folds of the sarcolemma containACh receptors


Events at the Neuromuscular Junction

Nerve impulse arrives at axon terminalACh is released and binds with receptors on

the sarcolemma

Electrical events lead to the generation of anaction potential


Nucleus

Action

potential (AP)Myelinated axon

of motor neuronAxon terminal of

neuromuscularjunction

Sarcolemma ofthe muscle fiber

Ca2+ Ca2+

Axon terminalof motorneuron

Synaptic vesicle

containing AChMitochondrion

Synapticcleft

Junctional

folds ofsarcolemma

Fusing synaptic vesicles

ACh

Sarcoplasm of

muscle fiberPostsynaptic membrane

ion channel opens;ions pass.

Na+ K+

AchNa+

K+

DegradedACh

Acetyl-cholinesterase

Postsynaptic membraneion channel closed;ions cannot pass.

1 Action potential arrives ataxon terminal of motor neuron.

2 Voltage-gated Ca2+ channelsopen and Ca2+ enters the axonterminal.

3Ca2+ entry causes somesynaptic vesicles to release

their contents (acetylcholine)by exocytosis.4 Acetylcholine, aneurotransmitter, diffuses acrossthe synaptic cleft and binds toreceptors in the sarcolemma.

5 ACh binding opens ionchannels that allow simultaneous

passage of Na+ into the musclefiber and K+ out of the muscle

fiber.

6 ACh effects are terminatedby its enzymatic breakdown inthe synaptic cleft byacetylcholinesterase.


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Destruction of Acetylcholine

ACh effects are quickly terminated by theenzyme acetylcholinesterase

Prevents continued muscle fiber contraction inthe absence of additional stimulation


Events in Generation of an Action Potential

1. Local depolarization (end plate potential): ACh binding opens chemically (ligand)

gated ion channels

Simultaneous diffusion of Na+ (inward) andK+ (outward)

More Na+ diffuses, so the interior of thesarcolemma becomes less negative

Local depolarization = end plate potential


Events in Generation of an Action Potential

2. Generation and propagation of an actionpotential:

End plate potential spreads to adjacentmembrane areas

Voltage-gated Na+ channels open Na+ influx decreases the membrane voltage

toward a critical threshold

If threshold is reached, an actionpotential is generated


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Copyright 2010 PearsonEducation,Inc. Figure 9.9, step 1

Na+

Na+

Open Na+

Channel

Closed K+

Channel

K+

Na+ K+Action potential++++

++++++++

Axon terminal

Synaptic

cleft

ACh

ACh

Sarcoplasm of muscle fiber

K+

1 Local depolarization: generation of theend plate potential on the sarcolemma1

Waveofdepolarization


Na+

Na+

Open Na+

Channel

Closed K+

Channel

K+

Na+ K+Action potential++++

++++++++

Axon terminal

Synapticcleft

ACh

ACh


K+

Generation and propagation of theaction potential (AP)

1 Local depolarization: generation of theend plate potential on the sarcolemma

2

1

Waveofdepolarization


Na+

Closed Na+

Channel

Open K+

Channel

K+

Repolarization3


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Na+

Na+

Open Na+

Channel

Closed K+

Channel

Action potential++++++

++++++

Axon terminal

Synaptic

cleft

ACh

ACh


K+

2Generation and propagation ofthe action potential (AP)

3 Repolarization

1 Local depolarization:generation of the endplate potential on thesarcolemma

K+

K+Na+

K+Na+

Wave

of

dep

ola

riza

tion

Closed Na+

Channel

Open K+

Channel


Na+ channels

close, K+ channels

open

K+ channels

close

Repolarization

due to K+ exit

Threshold

Na+

channels

open

Depolarization

due to Na+ entry


Excitation-Contraction (E-C) Coupling

Sequence of events by which transmission ofan AP along the sarcolemma leads to sliding

of the myofilaments

Latent period: Time when E-C coupling events occur Time between AP initiation and the

beginning of contraction


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Events of Excitation-Contraction (E-C)

Coupling

AP is propagated along sarcomere to Ttubules

Voltage-sensitive proteins stimulate Ca2+release from SR

Ca2+ is NECESSARY for contraction


Axon terminal

of motor neuron

Muscle fiberTriad

One sarcomere

Synaptic cleft

Setting the stage

Sarcolemma

Action potential

is generated

Terminal cisterna of SR

ACh

Ca2+


Action potentialis propagatedalongthe sarcolemma anddown the Ttubules.

Steps in E-C Coupling:

Troponin Tropomyosinblockingactivesites

Myosin

Actin

Activesitesexposedandreadyformyosin binding

Ca2+

TerminalcisternaofSR

Voltage-sensitivetubuleprotein Ttubule

Ca2+releasechannel

Myosincrossbridge

Ca2+

Sarcolemma

Calcium ionsare released.

Calcium bindsto troponin andremovesthe blocking action oftropomyosin.

Contraction begins

The aftermath

1

2

3

4


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Steps inE-C Coupling:

Terminalcisternaof SR

Voltage-sensitivetubule protein

T tubule

Ca2+releasechannel

Ca2+

Sarcolemma

Action potential ispropagated along thesarcolemma and down

the T tubules.

1


Steps inE-C Coupling:

Terminalcisternaof SR

Voltage-sensitivetubule protein

T tubule

Ca2+releasechannel

Ca2+

Sarcolemma

Action potential ispropagated along thesarcolemma and downthe T tubules.

Calciumions arereleased.

1

2


Troponin Tropomyosinblocking active sitesMyosin

Actin

Ca2+

The aftermath


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Troponin Tropomyosin

blocking active sitesMyosin

Actin

Active sites exposed andready for myosin binding

Ca2+

Calcium binds totroponin and removesthe blocking action oftropomyosin.

The aftermath

3


Troponin Tropomyosinblocking active sitesMyosin

Actin

Active sites exposed andready for myosin binding

Ca2+

Myosincrossbridge

Calcium binds totroponin and removesthe blocking action oftropomyosin.

Contraction begins

The aftermath

3

4


Action potentialis propagatedalongthe sarcolemma anddown the Ttubules.

Steps in E-C Coupling:

Troponin Tropomyosinblockingactivesites

Myosin

Actin

Activesitesexposedandreadyformyosin binding

Ca2+

TerminalcisternaofSR

Voltage-sensitivetubuleprotein Ttubule

Ca2+releasechannel

Myosincrossbridge

Ca2+

Sarcolemma

Calcium ionsare released.

Calcium bindsto troponin andremovesthe blocking action oftropomyosin.

Contraction begins

The aftermath

1

2

3

4


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Role of Calcium (Ca2+) in Contraction

At low intracellular Ca2+ concentration: Tropomyosin blocks the active sites on actin Myosin heads cannot attach to actin Muscle fiber relaxes


Role of Calcium (Ca2+) in Contraction

At higher intracellular Ca2+ concentrations: Ca2+ binds to troponin Troponin changes shape and moves

tropomyosin away from active sites

Events of the cross bridge cycle occur When nervous stimulation ceases, Ca2+ is

pumped back into the SR and contraction

ends


Cross Bridge Cycle

Continues as long as the Ca2+ signal andadequate ATP are present

Cross bridge formationhigh-energymyosin head attaches to thin filament

Working (power) strokemyosin headpivots and pulls thin filament toward M line


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Cross Bridge Cycle

Cross bridge detachmentATP attaches tomyosin head and the cross bridge detaches

Cocking of the myosin headenergy fromhydrolysis of ATP cocks the myosin head into

the high-energy state


1

Actin

Cross bridge formation.

Cocking of myosin head. The power (working)stroke.

Cross bridge

detachment.

Ca2+

Myosin

cross bridge

Thick

filament

Thin filament

ADP

Myosin

Pi

ATP

hydrolysis

ATP

ATP

24

3

ADP

PiADP

Pi


Actin


Ca2+

Myosin

cross bridgeThick filament

Thin filament

ADP

Myosin

Pi

1


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The power (working) stroke.

ADP

Pi

2


Cross bridge detachment.

ATP

3


Cocking of myosin head.

ATP

hydrolysis

ADP

Pi

4


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Actin


Cocking of myosin head. The power (working)stroke.

Cross bridge

detachment.

Ca2+

Myosin

cross bridgeThick

filament

Thin filament

ADP

Myosin

Pi

ATP

hydrolysis

ATP

ATP

24

3

ADP

PiADPPi

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