Chapter 12a

Muscles

About this Chapter

• Skeletal muscle• Mechanics of body movement• Smooth muscle• Cardiac muscle

Three Types of Muscle

Figure 12-1a

Nucleus

Muscle fiber(cell)

Striations

(a) Skeletal muscle

Three Types of Muscle

Figure 12-1b

Muscle fiber

Nucleus

Striations

(b) Cardiac muscle

Intercalated disk

Three Types of Muscle

Figure 12-1c

Muscle fiber

Nucleus

(c) Smooth muscle

Skeletal Muscle

• Usually attached to bones by tendons• Origin: closest to the trunk• Insertion: more distal• Flexor: brings bones together• Extensor: moves bones away• Antagonistic muscle groups: flexor-extensor

pairs

Antagonistic Muscle Groups

Figure 12-2a

(a) Flexion

Biceps musclecontracts (flexor)

Tricepsmusclerelaxes

Antagonistic Muscle Groups

Figure 12-2b

(b) Extension

Triceps musclecontracts (extensor)

Bicepsmusclerelaxes

Organization of Skeletal Muscle

Figure 12-3a (1 of 2)

Skeletal muscle

Muscle fascicle:bundle of fibers

Muscle fiber

Connective tissue

Connectivetissue

Tendon Nerve andblood vessels

Nucleus

(a)

Organization of Skeletal Muscle

Figure 12-3a (2 of 2)

Ultrastructure of Muscle

Figure 12-3b-f

T-tubules

(c)

(d)

(b)

Sarcoplasmicreticulum

Sarcolemma

Mitochondria

Myofibril

Myofibril

Thickfilament

Thinfilament

A band

Z disk

Z disk

(f)

Z disk

I bandM line H zone

Z disk

Sarcomere

Thin filaments

Tropomyosin

Troponin

Actin chain

G-actin molecule

Myosin tail

Myosin heads

Myosin molecule

Thick filaments

Nucleus

Hingeregion

(e)Titin

Nebulin

Titin

M line Myosincrossbridges

M line

ULTRASTRUCTURE OF MUSCLE

ANATOMY SUMMARY

Figure 12-3b

Ultrastructure of Muscle

T-tubules

Sarcoplasmicreticulum

Sarcolemma

Mitochondria

Myofibril

Thickfilament

ThinfilamentNucleus

ULTRASTRUCTURE OF MUSCLE

(b)

Figure 12-3c

Ultrastructure of Muscle

(c)

Myofibril

A bandZ disk

I bandM line H zone

Z diskSarcomere

Figure 12-3d

Ultrastructure of Muscle

(d)Z disk Z disk

Titin

M line Myosincrossbridges

Figure 12-3e

Ultrastructure of Muscle

(e)

Myosin tail

Myosin heads

Myosin molecule

Thick filaments

Hingeregion

M line

Figure 12-3f

Ultrastructure of Muscle

(f)

Thin filaments

Tropomyosin

Troponin

Actin chain G-actin molecule

Titin

Nebulin

Ultrastructure of Muscle

Figure 12-3c-f

(c)

(d)

Myofibril

A bandZ disk

Z disk

(f)

Z disk

I bandM line H zone

Z diskSarcomere

Thin filaments

Tropomyosin

Troponin

Actin chain G-actin molecule

Myosin tail

Myosin heads

Myosin molecule

Thick filaments

Hingeregion

(e)Titin

Nebulin

Titin

M line Myosincrossbridges

M line

T-Tubules and the Sarcoplasmic Reticulum

Figure 12-4

Sarcolemma

Thin filament

Thick filament

Triad Terminalcisterna

T-tubule brings actionpotentials into interiorof muscle fiber.

Sarcoplasmic reticulumstores Ca2+

The Two- and Three-Dimensional Organization of a Sarcomere

Figure 12-5

Sarcomere

(a)

(b)

A band I band H zone I band

Z diskZ disk M line

Thick filament

Thin filament

Z disk Z disk

(c)

I bandthin filaments

only

H zonethick filaments

only

Outer edgeof A band

thick and thinfilaments overlap

M linethick filaments

linked withaccessory proteins

Anatomy Review Animation

PLAY Interactive Physiology® Animation: Muscular System: Anatomy Review: Skeletal Muscle Tissue

Muscle Contraction

• Muscle tension: force created by muscle• Load: weight that opposes contraction• Contraction: creation of tension in muscle• Relaxation: release of tension• Steps leading up to muscle contraction:

1. Events at the neuromuscular junction

2. Excitation-contraction coupling

3. Contraction-relaxation cycle

Summary of Muscle Contraction

Figure 12-7

Events at the Neuromuscular Junction

PLAY Events at the Neuromuscular Junction

PLAY Interactive Physiology® Animation: Muscular System: Events at the Neuromuscular Junction

Changes in a Sarcomere During Contraction

Figure 12-8

Z

Z

Zline

Zline

Mline

Musclerelaxed

Musclecontracted

Sarcomere shortenswith contraction

I band

A bandActin

Z

M

Z

Z

Myosin

Half ofI band

Half ofI band

H zone

H

H

I

A band constant

IH zone and I band both shorten

Z

A band

Half ofI band

Sliding Filament Theory

PLAY Interactive Physiology® Animation: Muscular System: Sliding Filament Theory

The Molecular Basis of Contraction

Figure 12-9a

ADP

Troponin G-Actin

Tropomyosinblocks binding

site on actin

Myosin head

TN

(a) Relaxed state. Myosin head cocked.

Pi

The Molecular Basis of Contraction

Figure 12-9b

Actinmoves

Cytosolic Ca2+

Tropomyosin shifts,exposing bindingsite on actin

TN

Power stroke

(b) Initiation of contraction

Ca2+ levels increasein cytosol.

Ca2+ binds to troponin (TN).

Troponin-Ca2+

complex pulls tropomyosin away from actin’smyosin-binding site.

Myosin binds to actin and completes power stroke.

Actin filament moves.

Pi

1

2 3

4

5

5

4

3

2

1

ADP

The Molecular Basis of Contraction

Figure 12-10

G-actin molecule

Tight binding in the rigor state

ATP binds to myosin.Myosin releases actin.

Myosinbinding sites

Myosinfilament

Myosin hydrolyses ATP. Myosinhead rotates and binds to actin.

ATP

Actin filament movestoward M line.

Myosin releases ADP.Contraction-relaxation

Sliding filament

ADP

Power stroke

Relaxed state with myosin heads cocked

Ca2+

signal

ADPPiPi

1

2

3

4

The Molecular Basis of Contraction

Figure 12-10, step 0

G-actin molecule

Tight binding in the rigor state

Myosinbinding sites

Myosinfilament

The Molecular Basis of Contraction

Figure 12-10, steps 0–1

G-actin molecule

Tight binding in the rigor state

ATP binds to myosin.Myosin releases actin.

Myosinbinding sites

Myosinfilament

ATP

1

The Molecular Basis of Contraction

Figure 12-10, steps 1–2

1 ATP binds to myosin.Myosin releases actin.

Myosin hydrolyses ATP. Myosin head rotates and binds to actin.

ATP

Relaxed state with myosin heads cocked

ADPPi

2

The Molecular Basis of Contraction

Figure 12-10, steps 2–3

2 Myosin hydrolyses ATP. Myosin head rotates and binds to actin.

Actin filament movestoward M line.

Power stroke

Relaxed state withmyosin heads cocked

Ca2+

signal

ADPPi

Pi

3

The Molecular Basis of Contraction

Figure 12-10, steps 3–4

Actin filament movestoward M line.

Myosin releases ADP.

ADP

Power stroke

Pi

43

Excitation-Contraction Coupling

Figure 12-11a

Myosin head

Myosin thick filament

M line

Axon terminal ofsomatic motor neuron

Motor end plate

Sarcoplasmic reticulum

ACh

DHP

DHP = dihydropyridine L-type calcium channel

RyR

RyR = ryanodine receptor-channel

T-tubule

Muscle fiber

(a) Initiation of muscle action potential

Somatic motor neuron releases ACh at neuromuscular junction.

Net entry of Na+ through ACh receptor-channel initiates a muscle action potential

Troponin

Tropomyosin

KEY

Actin

Z disk

Na+

Ca2+

1

2

1

2

Excitation-Contraction Coupling

Figure 12-11a, step 1

Myosin head

Myosin thick filament

M line

Axon terminal ofsomatic motor neuron

Motor end plate

Sarcoplasmic reticulum

ACh

DHP

DHP = dihydropyridine L-type calcium channel

RyR

RyR = ryanodine receptor-channel

T-tubule

Muscle fiber

(a) Initiation of muscle action potential

Somatic motor neuron releases ACh at neuromuscular junction.

Troponin

Tropomyosin

KEY

Actin

Z disk

Ca2+

1 1

Excitation-Contraction Coupling

Figure 12-11a, steps 1–2

Myosin head

Myosin thick filament

M line

Axon terminal ofsomatic motor neuron

Motor end plate

Sarcoplasmic reticulum

ACh

DHP

DHP = dihydropyridine L-type calcium channel

RyR

RyR = ryanodine receptor-channel

T-tubule

Muscle fiber

(a) Initiation of muscle action potential

Somatic motor neuron releases ACh at neuromuscular junction.

Net entry of Na+ through ACh receptor-channel initiates a muscle action potential

Troponin

Tropomyosin

KEY

Actin

Z disk

Na+

Ca2+

1

2

1

2

Excitation-Contraction Coupling

Figure 12-11b

DHP = dihydropyridine L-type calcium channel RyR = ryanodine receptor-channel

Ca2+ released

(b) Excitation-contraction couplingDistance actin moves

Myosin thick filament

Action potential in t-tubule altersconformation of DHP receptor.

DHP receptor opens RyR Ca2+

release channels in sarcoplasmic reticulum and Ca2+ enters cytoplasm.

Ca2+ binds to troponin, allowing actin-myosin binding.

Myosin heads execute powerstroke.

Actin filament slides toward center of sarcomere.

KEY

3

4

5

6

7

34

5

6

7

Electrical and Mechanical Events in Muscle Contraction

• A twitch is a single contraction-relaxation cycle

Figure 12-12

Muscle fibermembrane

potentialin mV

Developmentof tension

during onemuscle twitch

+30

-70 2msec

Latentperiod

Contractionphase

Motorend plate

Axonterminal

Muscle actionpotential

Time

10–100 msec

Relaxationphase

Time

Neuronmembrane

potentialin mV

+30Muscle fiber

Action potentialfrom CNS

Recordingelectrodes

-70

Time

Ten

sio

n

Phosphocreatine

Figure 12-13

1. Creatine phosphate2. Glycolysis3. Krebs cycle

Locations and Possible Causes of Muscle Fatigue

Figure 12-14

Causes of Muscle Fatigue During Exercise

• Extended submaximal exercise • Depletion of glycogen stores

• Short-duration maximal exertion • Increased levels of inorganic phosphate• May slow Pi release from myosin• Decrease calcium release

• Maximal exercise• Potassium (K+) leaves muscle fiber, leading to

increased concentration that is believed to decrease Ca2+

Skeletal Muscle Metabolism During Fatiguing Submaximal Exercise

Question 12-1

Fast-Twitch Glycolytic and Slow-Twitch Oxidative Muscle Fibers

Figure 12-15

Fast-Twitch Glycolytic and Slow-Twitch Oxidative Muscle Fibers

Table 12-2

Length-Tension Relationships in Contracting Skeletal Muscle

Figure 12-16