Copyright © 2010 Pearson Education, Inc. Three Types of Muscle Tissue 1.Skeletal muscle tissue:...

Copyright © 2010 Pearson Education, Inc.

Three Types of Muscle Tissue

1. Skeletal muscle tissue:

• Attached to bones and skin

• Striated

• Voluntary (i.e., conscious control)

• Powerful

• Primary topic of this chapter



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


Special Characteristics of Muscle Tissue

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

• Contractility: ability to shorten when stimulated

• Extensibility: ability to be stretched

• Elasticity: ability to recoil to resting length


Muscle Functions

1. Movement of bones or fluids (e.g., blood)

2. Maintaining posture and body position

3. Stabilizing joints

4. Heat generation (especially skeletal muscle)


Skeletal Muscle

• Each muscle is served by one artery, one nerve, and one or more veins


Skeletal Muscle

• Connective tissue sheaths of skeletal muscle:

• Epimysium: dense regular connective tissue surrounding entire muscle

• Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers)

• Endomysium: fine areolar connective tissue surrounding each muscle fiber

Copyright © 2010 Pearson Education, Inc. Figure 9.1

Bone

Perimysium

Endomysium(between individualmuscle fibers)

Muscle fiber

Fascicle(wrapped by perimysium)

Epimysium

Tendon

Epimysium

Muscle fiberin middle ofa fascicle

Blood vessel

Perimysium

Endomysium

Fascicle(a)

(b)


Skeletal Muscle: Attachments

• Muscles attach:

• Directly—epimysium of muscle is fused to the periosteum of bone or perichondrium of cartilage

• Indirectly (more common) —connective tissue wrappings extend beyond the muscle as a ropelike tendon or sheetlike aponeurosis


Microscopic Anatomy of a Skeletal Muscle Fiber

• Surrounded by sarcolemma (plasma membrane)

• Long (huge) cylindrical cells (up to 30 cm!!!)

• Multiple nuclei

• Many mitochondria (Why So Many?)

• Glycosomes (for glycogen storage) & myoglobin (for O2 storage)

• Also contain myofibrils, sarcoplasmic reticulum, and T tubules


Myofibrils

• Densely packed, rodlike elements (100’s to 1000’s per muscle fiber)

• Makes up to 80% of muscle cell volume

• Exhibit striations: perfectly aligned repeating series of dark A bands and light I bands


NucleusLight I bandDark A band

Sarcolemma

Mitochondrion

(b) Diagram of part of a muscle fiber showing the myofibrils. Onemyofibril is extended afrom the cut end of the fiber.

Myofibril


Sarcomere

• Smallest contractile unit (functional unit) of a muscle fiber

• The region of a myofibril between two successive Z discs

• Composed of thick and thin myofilaments made of contractile proteins responsible for muscle contraction


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 anchors the thin filaments and connects myofibrils to one another

• H zone: lighter mid-region on either side of the M line. Only seen in resting muscle fibers

• M line: Found in the center of the H zone, it is a line of protein myomesin (M for middle)

Copyright © 2010 Pearson Education, Inc. Figure 9.2c, d

I band I bandA bandSarcomere

H zoneThin (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 sarcomere extends 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 the myosin heads on the thick filaments.


Thick Filament(Myosin)

• Composed of the protein myosin

• Myosin tails contain:

• 2 interwoven, heavy polypeptide chains

• Myosin heads contain:

• The “Business End” that act as cross bridges during contraction

• Binding sites for actin of thin filaments

• Binding sites for ATP

• ATPase enzymes (split ATP to generate energy)


Thin Filament(Actin)

• Composed mostly of protein actin

• Bears active sites for the cross-bridges (heads of myosin) during contraction

• Contains tropomyosin and troponin: regulatory proteins bound to actin

• Elastic filament (made of titan protein) allows muscle to spring back into place


Flexible hinge region

Tail

Tropomyosin Troponin Actin

Myosin 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 heads are 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


Sarcoplasmic Reticulum (SR)

• Network of smooth endoplasmic reticulum surrounding each myofibril

• Pairs of terminal cisternae (reservoirs) form perpendicular cross channels

• Regulates calcium - stores and releases Ca+ for contraction (we’ll talk about Ca+ later)


T Tubules

• Continuous with the sarcolemma

• Penetrate the cell’s interior at each A band–I band junction

• Associate with the paired terminal cisternae to form triads that encircle each sarcomere

***NOTE: A skeletal muscle is very long. T-tubules allow the electrical stimulus and ECF to come in contact with deep regions which makes muscle reaction occur quicker


Myofibril

Myofibrils

Triad:

Tubules ofthe SR

Sarcolemma

Sarcolemma

Mitochondria

I band I bandA band

H zone Z discZ disc

Part of a skeletalmuscle fiber (cell)

• T tubule• Terminal

cisternaeof the SR (2)

M line


CHECK POINT!!!!!

1.) Which myofilaments have heads that form cross-bridges that are important during contraction?

Thick filaments2.) What surrounds the myofibril and regulates Ca+ needed for contraction?

Sarcoplasmic Reticulum


You have 7 minutes from the time the bell rings…. If you don’t turn it in on time, you don’t get the credit!

Briefly explain how actin and myosin work together in the sliding filament model

Explain the major role of the sarcoplasmic reticulum (SR), especially the terminal cisternae.

What key substance provides the final “go” signal for contraction? Hint: pg 282

What structure works close with the SR and forms a triad relationship?


Contraction

• The generation of force

• Shortening occurs when tension from the cross bridges on the thin filaments pulls the thin filament toward the M line ultimately contracting or shortening the muscle fiber


Sliding Filament Model of Contraction

• In the relaxed state, thin and thick filaments only overlap slightly

• During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line

• As H zones shorten and disappear, sarcomeres and muscle cells shorten, thus, the whole muscle shortens


I

Fully relaxed sarcomere of a muscle fiber

Fully contracted sarcomere of a muscle fiber

IA

Z ZH

I IA

Z Z

1

2

Sarcomere Contraction Animation


So now we know how a muscle fiber contracts…but what causes it to

contract?1. Activation: neural stimulation at a

neuromuscular junction

2. Excitation-contraction coupling:

• Creation and increase of an action potential (electrical current) along the sarcolemma

• Final trigger: a brief rise in intracellular Ca+ levels


Step One- The Activation Step: Takes place at the

Neuromuscular Junction • Skeletal muscles are stimulated by somatic motor neurons

• Axons of motor neurons travel from the central nervous system (brain or spinal cord) via nerves to skeletal muscles

• Each axon forms several branches as it enters a muscle

• Each axon ending forms a neuromuscular junction with a single muscle fiber


The axon of each motor neuron divides profusely and forms a neuromuscular junction

at each muscle fiber


Nucleus

Actionpotential (AP)

Myelinated axonof motor neuron

Axon terminal ofneuromuscular junction

Sarcolemma ofthe muscle fiber

Ca2+Ca2+

Axon terminalof motor neuron

Synaptic vesiclecontaining ACh

MitochondrionSynapticcleft

Fusing synaptic vesicles

1 Action potential arrives ataxon terminal of motor neuron.

2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal.

Figure 9.8


Neuromuscular Junction

• Situated midway along the length of a muscle fiber

• Axon terminal and muscle fiber are separated by a gel-filled space called the synaptic cleft

• Synaptic vesicles within the axon terminal contain the neurotransmitter acetylcholine (ACh)

• Junctional folds of the sarcolemma contain ACh receptors


Events at the Neuromuscular Junction

• Nerve impulse arrives at axon terminal

• Voltage sensitive Calcium channels open and release Calcium into the axon terminal

• Due to increased Calcium levels, ACh is released and binds with receptors on the sarcolemma which triggers electrical events

• These electrical events lead to the creation of an action potential (electrical current) which spreads down the sarcolemma


Destruction of Acetylcholine

• ACh effects are quickly stopped by the enzyme acetylcholinesterase which is located in the synaptic cleft

• Prevents continued muscle fiber contraction by breaking down Ach into basic “non-stimulating” components


Nucleus

Actionpotential (AP)

Myelinated axonof motor neuron

Axon terminal ofneuromuscular junction

Sarcolemma ofthe muscle fiber

Ca2+Ca2+


Synaptic vesiclecontaining AChMitochondrionSynapticcleft

Junctionalfolds ofsarcolemma

Fusing synaptic vesicles

ACh

Sarcoplasm ofmuscle fiber

Postsynaptic membraneion channel opens;ions pass.

Na+ K+

Ach–

Na+

K+

Degraded ACh

Acetyl-cholinesterase

Postsynaptic membraneion channel closed;ions cannot pass.

1 Action potential arrives ataxon terminal of motor neuron.

2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal.

3 Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine)by exocytosis.

4 Acetylcholine, aneurotransmitter, diffuses across the synaptic cleft and binds to receptors 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 terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase.


Na+

Na+

Open Na+

Channel

Closed Na+

Channel

Closed K+

Channel

Open K+

Channel

Action potential++++++

+++++

+

Axon terminal

Synapticcleft

ACh

ACh

Sarcoplasm of muscle fiber

K+

2 Generation and propagation ofthe action potential (AP)

3 Repolarization

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

K+

K+Na+

K+Na+

Wave ofde

po

lari

zatio

n

Copyright © 2010 Pearson Education, Inc. Figure 9.9, step 2

Na+

Na+

Open Na+

ChannelClosed K+

Channel

K+

Na+ K+Action potential

++++++

+++++

+

Axon terminal

Synapticcleft

ACh

ACh

Sarcoplasm of muscle fiber

K+

Generation and propagation of the action potential (AP)

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

2

1

Wave ofde

po

lari

zatio

n


Excitation-Contraction (E-C) Coupling

• Sequence of events by which transmission of an AP along the sarcolemma leads to sliding of the myofilaments

• Occurs during latent period:

• Time between AP initiation and the beginning of contraction


Events of Excitation-Contraction (E-C) Coupling

• AP is spread along sarcolemma to the T tubules

• Voltage-sensitive proteins stimulate the SR to release Ca+

• Ca+ is necessary for contraction



Muscle fiberTriad

One sarcomere

Synaptic cleft

Setting the stage

Sarcolemma

Action potentialis generated

Terminal cisterna of SR ACh

Ca2+


Action potential is propagated alongthe sarcolemma and down the T tubules.

Steps in E-C Coupling:

Troponin Tropomyosinblocking active sites

Myosin

Actin

Active sites exposed and ready for myosin binding

Ca2+

Terminal cisterna of SR

Voltage-sensitivetubule protein

T tubule

Ca2+

releasechannel

Myosincross bridge

Ca2+

Sarcolemma

Calcium ions are released.

Calcium binds to troponin andremoves the blocking action oftropomyosin.

Contraction begins

The aftermath

1

2

3

4


Steps inE-C Coupling:



T tubule

Ca2+

releasechannel

Ca2+

Sarcolemma

Action potential ispropagated along thesarcolemma and downthe T tubules.

1


Steps inE-C Coupling:



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



Actin


Ca2+

Calcium binds totroponin and removesthe blocking action oftropomyosin.

The aftermath

3



Actin


Ca2+

Myosincross bridge

Calcium binds totroponin and removesthe blocking action oftropomyosin.

Contraction begins

The aftermath

3

4


Action potential is propagated alongthe sarcolemma and down the T tubules.

Steps in E-C Coupling:

Troponin Tropomyosinblocking active sites

Myosin

Actin


Ca2+



T tubule

Ca2+

releasechannel

Myosincross bridge

Ca2+

Sarcolemma

Calcium ions are released.

Calcium binds to troponin andremoves the blocking action oftropomyosin.

Contraction begins

The aftermath

1

2

3

4


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 Ca+ concentrations:

• Ca+ binds to troponin

• Troponin changes shape and moves tropomyosin away from active sites

• Events of the cross bridge cycle occur

• When nervous stimulation stops, Ca+ is pumped back into the SR and contraction ends


Cross Bridge Cycle

Continues as long as the Ca+ signal and adequate ATP are present

1. Cross bridge formation—Energized myosin head attaches to actin on the thin filament forming a “cross bridge”

2. Working (power) stroke—ADP and Pi are released and the myosin head pivots and bends (low energy shape), pulling the thin filament toward M line


Cross Bridge Cycle

3. Cross bridge detachment—ATP attaches to myosin head weakening the link and the cross bridge detaches

4. “Cocking” of the myosin head—energy from hydrolysis of ATP back to ADP and Pi 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+

Myosincross bridge

Thick filament

Thin filament

ADP

Myosin

Pi

ATPhydrolysis

ATP

ATP

24

3

ADP

Pi

ADPPi


Actin


Ca2+

Myosincross bridge

Thick filament

Thin filament

ADP

Myosin

Pi

1


The power (working) stroke.

ADP

Pi

2



ATP

3


Cocking of myosin head.

ATPhydrolysis

ADPPi

4


1

Actin


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


Ca2+

Myosincross bridge

Thick filament

Thin filament

ADP

Myosin

Pi

ATPhydrolysis

ATP

ATP

24

3

ADP

Pi

ADPPi

Cross bridge animation

Copyright © 2010 Pearson Education, Inc. Three Types of Muscle Tissue 1.Skeletal muscle tissue:...

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