Chapter 12a

Muscles

About this Chapter

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

Three Types of Muscle

Figure 12-1a

NucleusMuscle 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 diskSarcomere

Thin filaments

Tropomyosin

Troponin

Actin chainG-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

TitinNebulin

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) TitinNebulin

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 junction2. Excitation-contraction coupling3. 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 bandActinZ

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 headTN

(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 Myosin

filament

Myosin hydrolyses ATP. Myosinhead rotates and binds to actin.

ATP

Actin filament movestoward M line.

Myosin releases ADP.Contraction-relaxation

Sliding filament

ADP

Power strokeRelaxed 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 Myosin

filament

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 Myosin

filament

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+

signalADP

Pi

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

TroponinTropomyosin

KEY

ActinZ 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.

TroponinTropomyosin

KEY

ActinZ 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

TroponinTropomyosin

KEY

ActinZ 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

3 4

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 plateAxonterminal

Muscle actionpotential

Time10–100 msec

Relaxationphase

Time

Neuronmembrane

potentialin mV

+30Muscle fiber

Action potentialfrom CNS

Recordingelectrodes

-70

Time

Tens

ion

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