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Chapter 12b
Muscles
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Summation of Contractions
Figure 12-17a
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Summation of Contractions
Figure 12-17b
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Summation of Contractions
Figure 12-17c
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Summation of Contractions
Figure 12-17d
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Motor Units
Figure 12-18
SPINAL CORD
Neuron 1Neuron 2Neuron 3
Motornerve
Muscle fibers
Motor unit 2
Motor unit 1
Motor unit 3
One muscle may havemany motor units ofdifferent fiber types.
KEY
PLAYInteractive Physiology® Animation: Muscular System: Contraction of Motor Units
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Mechanics of Body Movement
• Isotonic contractions create force and move load• Concentric action is a shortening action• climbing
• Eccentric action is a lengthening action• Downhill skiing, going down stairs
• Isometric contractions create force without moving a load• Series elastic elements; sarcomeres shorten
while elastic elements stretch resulting in little change in overall length
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Isotonic Contraction
Figure 12-19a
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Isometric Contraction
Figure 12-19b
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Series Elastic Elements in Muscle
Figure 12-20
Contractilecomponents
Elasticcomponents
Tricepsmuscle
Bicepsmuscle
Elasticelement
Sarcomeres
Schematic of the series elastic elements
Muscleat rest
Isotonic contraction:Sarcomeres shortenmore but, because elasticelements are alreadystretched, the entiremuscle must shorten.
Isometric contraction:Muscle has not shortened.Sarcomeres shorten, generatingforce, but elastic elementsstretch, allowing muscle lengthto remain the same.
Mu
scle
len
gth
1 2 3
1 2 3
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The Arm is a Lever and Fulcrum System
Figure 12-21a
Lever
Load
Fulcrum
Biceps muscle
(a)
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The Arm is a Lever and Fulcrum System
Figure 12-21b
F2 = 2 kg
F1
5 cm
15 cm
(b)
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The Arm is a Lever and Fulcrum System
Figure 12-21c
D1
D2
5 cm
25 cm
A 7-kg load is addedto the hand 25 cmfrom the elbow.
(c)
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The Arm Amplifies Speed of Movement of the Load
Figure 12-22
Lever
Fulcrum 1 cm
5 cm
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Load-Velocity Relationship in Skeletal Muscle
Figure 12-23
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Muscle Disorders
• Muscle cramp: sustained painful contraction• Overuse• Disuse/Atrophy• Acquired disorders• Inherited disorders• Duchenne’s muscular dystrophy • Dystrophin
• McArdle’s disease • Myophosphorylase deficiency glycogenosis –
glycogen not converted to glucose 6-phosphate
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Duration of Muscle Contraction in the Three Types of Muscle
Figure 12-24
Skeletal
Cardiac Smooth
Time (sec)
0 1 2 3 4 5
Ten
sio
n
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Smooth Muscle
• Contraction and relaxation slower• Uses less energy• Maintains force for long periods• Low oxygen consumption
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Smooth Muscle
• Smooth muscle is not studied as much as skeletal muscle because• It has more variety• Anatomy makes functional studies difficult• It is controlled by hormones, paracrines, and
neurotransmitters• It has variable electrical properties• Multiple pathways influence contraction and
relaxation
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Types of Smooth Muscle
Figure 12-25a
Smallintestine
Autonomic neuronvaricosity
Neuro-transmitter
Receptor
Gapjunctions
Smooth musclecell
(a) Single-unit smooth muscle cells
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Smooth Muscle
• Much smaller than skeletal muscle fibers• Has longer actin and myosin filaments• Myosin ATPase activity much slower• Myosin light chain plays regulatory role• Not arranged in sarcomeres• Has less sarcoplasmic reticulum• IP3-receptor channel is the primary calcium
channel• Calcium also enters cell from extracellular fluid
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Cardiac Muscle
• Shares features with both skeletal and smooth muscle• Like skeletal: • Striated; sarcomere structure
• Unlike skeletal: • Muscle fibers shorter; may be branched; have
single nucleus
• Like smooth: • Electrically linked to one another; some exhibit
pacemaker potentials; under sympathetic and parasympathetic control as well as hormone control
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Muscle Summary
Table 12-3
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Muscle Summary
• Skeletal muscles• Origin, insertion, flexors, extensors, and
antagonistic muscles• T-tubules, sarcoplasmic reticulum, myofibrils,
thick filament, thin filament, actin, myosin, and crossbridges
• Sarcomere, Z disks, I bands, A band, H zone, and M line
• Muscle tension, load, sliding filament theory, tropomyosin, troponin, Ca2+-ATPase, myosin ATPase, power stroke, rigor state
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Summary
• Skeletal muscle• Excitation-contraction coupling, DHP receptors,
and Ca2+ release channels• Twitch, latent period, phosphocreatine, and
muscle fatigue• Muscle fiber types, myoglobin, tetanus, and
motor unit
• Mechanics of body movement• Isotonic versus isometric contractions• Series elastic elements, levers, and fulcrums
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Summary
• Smooth muscle• Types of smooth muscle, IP3-receptor channel,
calmodulin, myosin light chain kinase, myosin light protein chains, and myosin phosphatase
• Myogenic contraction, slow wave or pacemaker potentials, and pharmacomechanical coupling
• Cardiac muscle• Comparison to other muscle types