Lecture Human anatomy and physiology - Chapter 9: Muscles and muscle tissue (part a)

Chapter 9 - Muscles and muscle tissue (part a) provides knowledge of muscle tissues and skeletal muscle. The following will be discussed in this chapter: Types of muscle tissue, special characteristics of muscle tissue, muscle functions, gross anatomy of a skeletal muscle, microscopic anatomy of a skeletal muscle fiber, sliding filament model of contraction, physiology of skeletal muscle fibers,...

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PowerPoint ® Lecture Slides

prepared by Janice Meeking, Mount Royal College

C H A P T E R

9

Muscles and Muscle

Tissue: Part A

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Three Types of Muscle Tissue

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urinary bladder, and airways

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Special Characteristics of Muscle Tissue

ability to receive and respond to stimuli

stimulated

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Muscle Functions

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Skeletal Muscle

nerve, and one or more veins

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Skeletal Muscle

surrounding entire muscle

surrounding fascicles (groups of muscle fibers)

surrounding each muscle fiber

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Bone

Perimysium

Endomysium (between individual muscle fibers)

Muscle fiber

Fascicle (wrapped by perimysium)

Epimysium Tendon

Epimysium

Muscle fiber

in middle of

a fascicle Blood vessel

Perimysium Endomysium

Fascicle

(a)

(b)

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Skeletal Muscle: Attachments

• Muscles attach:

periosteum of bone or perichondrium of cartilage

extend beyond the muscle as a ropelike tendon or sheetlike aponeurosis

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Microscopic Anatomy of a Skeletal Muscle Fiber

reticulum, and T tubules

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Myofibrils

series of dark A bands and light I bands

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Nucleus Light I band

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made of contractile proteins

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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 midregion where filaments do not

overlap

• M line: line of protein myomesin that holds adjacent

thick filaments together

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Sarcomere

H zone

Thin (actin) filament

Thick (myosin) filament

M line

(c) Small part of one myofibril enlarged to show the myofilaments

responsible for the banding pattern Each sarcomere extends from one Z disc to the next.

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.

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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 as cross bridges during contraction

• Binding sites for actin of thin filaments

• Binding sites for ATP

• ATPase enzymes

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Ultrastructure of Thin Filament

actin

attachment during contraction

bound to actin

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Flexible hinge region

Actin subunits Actin-binding sites

Thick filament Each thick filament consists of many

myosin molecules whose heads protrude

at opposite ends of the filament.

Thin filament

A thin filament consists of two strands

of actin subunits twisted into a helix plus two types of regulatory proteins (troponin and tropomyosin).

Thin filament Thick filament

In the center of the sarcomere, the thick filaments lack myosin heads Myosin heads are present only in areas of myosin-actin overlap.

Longitudinal section of filaments within one sarcomere of a myofibril

Portion of a thick filament

Portion of a thin filament

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Sarcoplasmic Reticulum (SR)

surrounding each myofibril

cross channels

levels

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T Tubules

band junction

form triads that encircle each sarcomere

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Myofibril

Myofibrils

Triad:

Tubules of the SR

M line

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Triad Relationships

fiber

intermembrane space from T tubule and SR cisternae membranes

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Contraction

fiber

cross bridges on the thin filaments exceeds

forces opposing shortening

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Sliding Filament Model of Contraction

overlap only slightly

actin, detach, and bind again, to propel the thin filaments toward the M line

sarcomeres shorten, muscle cells shorten, and the whole muscle shortens

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I Fully relaxed sarcomere of a muscle fiber

Fully contracted sarcomere of a muscle fiber

I A

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Requirements for Skeletal Muscle

Contraction

neuromuscular junction

potential along the sarcolemma

levels

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Events at the Neuromuscular Junction

motor neurons

central nervous system via nerves to skeletal muscles

enters a muscle

junction with a single muscle fiber

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Nucleus

Action potential (AP)

Myelinated axon

of motor neuron

Axon terminal of

Sarcolemma of the muscle fiber

Axon terminal

of motor neuron

Synaptic vesicle containing ACh Mitochondrion

Synaptic cleft

Fusing synaptic vesicles

1

Action potential arrives at

axon terminal of motor neuron

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Neuromuscular Junction

fiber

by a gel-filled space called the synaptic cleft

neurotransmitter acetylcholine (ACh)

ACh receptors

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Events at the Neuromuscular Junction

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Nucleus

Action potential (AP)

Myelinated axon

of motor neuron

Axon terminal of

Sarcolemma of the muscle fiber

Axon terminal

of motor neuron

Synaptic vesicle containing ACh Mitochondrion

Synaptic cleft

Junctional folds of sarcolemma

Fusing synaptic vesicles ACh

Sarcoplasm of muscle fiber

Postsynaptic membrane ion channel opens;

Acetyl-Postsynaptic membrane ion channel closed;

ions cannot pass.

1 Action potential arrives at axon terminal of motor neuron

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

3

Ca 2+ entry causes some synaptic vesicles to release their contents (acetylcholine)

by exocytosis.

4 Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma.

5 ACh binding opens ion channels that allow simultaneous passage of Na + into the muscle fiber and K + out of the muscle fiber.

6 ACh effects are terminated

by its enzymatic breakdown in the synaptic cleft by

acetylcholinesterase.

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Destruction of Acetylcholine

enzyme acetylcholinesterase

the absence of additional stimulation

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Events in Generation of an Action Potential

1 Local depolarization (end plate potential):

ion channels

sarcolemma becomes less negative

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2 Generation and propagation of an action

potential:

membrane areas

toward a critical threshold

generated

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spread, changing the permeability of the

sarcolemma

adjacent patch, causing it to depolarize to

threshold

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3 Repolarization:

channels open

refractory period until repolarization is complete

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Axon terminal

Synaptic cleft

ACh ACh

Sarcoplasm of muscle fiber

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Axon terminal

Synaptic cleft ACh

ACh

K +

1 Local depolarization: generation of the

end plate potential on the sarcolemma

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Axon terminal

Synaptic cleft ACh

1 Local depolarization: generation of the

end plate potential on the sarcolemma

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Axon terminal

Synaptic cleft

ACh ACh

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Na + channels close, K + channels open

K + channels close

Depolarization

due to Na+ entry

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Excitation-Contraction (E-C) Coupling

an AP along the sarcolemma leads to sliding

of the myofilaments

of contraction

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Events of Excitation-Contraction (E-C) Coupling

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Axon terminal

of motor neuron

Muscle fiber

Triad One sarcomere

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

Steps in E-C Coupling:

Troponin Tropomyosin

blocking active sites Myosin Actin

Active sites exposed and ready for myosin binding

Ca 2+

Terminal cisterna

of SR

Voltage-sensitive tubule protein

T tubule

Ca 2+

release channel

Myosin cross bridge

Ca 2+

Sarcolemma

Calcium ions are released.

Calcium binds to troponin and removes the blocking action of tropomyosin.

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Steps in

E-C Coupling:

Sarcolemma

1

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Steps in

E-C Coupling:

Sarcolemma

1

2

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The aftermath

3

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Contraction begins

The aftermath

3

4

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Steps in E-C Coupling:

Troponin Tropomyosin

blocking active sites Myosin Actin

Active sites exposed and ready for myosin binding

Ca 2+

of SR

Voltage-sensitive tubule protein

T tubule

Ca 2+

release channel

Ca 2+

Sarcolemma

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tropomyosin away from active sites

pumped back into the SR and contraction ends

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Cross Bridge Cycle

adequate ATP are present

head attaches to thin filament

and pulls thin filament toward M line

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Cross Bridge Cycle

myosin head and the cross bridge detaches

hydrolysis of ATP cocks the myosin head into the high-energy state

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1

Actin

Cross bridge formation.

stroke.

Cross bridge detachment.

Ca 2+

Thick filament

ATP

2 4

3

ADP Pi ADP

Pi

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Actin

Ca 2+

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The power (working) stroke.

ADP Pi

2

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ATP

3

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Cocking of myosin head.

ATP hydrolysis

ADP

P i

4

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1

Actin

stroke.

Ca 2+

Thick filament

ATP

2 4

3

ADP Pi ADP

Pi

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