Muscular System
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Muscle Tissue
Chapter # 4
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Muscular System
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Muscles
“muscle” = myo- or myssarco- = “flesh” - also refers to muscles
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Muscular System
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Main Functions of Muscles
1. Produce movement
2. Maintain posture & body position
3. Stabilize joints
4. Generate heat
Additional: protect organs, valves, dilate
pupils, raise hairs
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Muscular System
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Types of Muscle Tissue
Skeletal: voluntary, striated, multinucleated
Cardiac: (heart) striated, involuntary
Smooth: visceral (lines hollow organs),
nonstriated, involuntary
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Muscular System
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Special Characteristics
 Excitability – can receive and respond to
stimuli
 Contractility – can shorten forcibly
 Extensibility – can be stretched or extended
 Elasticity – can recoil and resume resting
length after being stretched
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Muscular System
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Gross Anatomy of Skeletal Muscle
 1 muscle = 1 organ
 Each muscle served by a nerve, artery, & vein (1+)
 Rich blood supply – need energy & O2
 Connective tissue sheaths: wraps each cell and
reinforce whole muscle
 Attachment: (1) directly to bone, (2) by tendons or
aponeuroses to bone, cartilage, or other muscles
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Organization of Skeletal Muscle
Muscular System
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Muscle
• Muscle cells + blood vessels + nerve fibers
• Covered by epimysium (connective tissue)
Fascicle
• Bundle of muscle cells
• Surrounded by perimysium
Muscle fiber (cell)
• Surrounded by endomysium
Myofibril
• Complex organelle
Sarcomere
• Contractile unit
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Muscular System
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Muscular System
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Anatomy of Muscle Fiber
 Multinucleate cell
 Up to 30 cm long
 Sarcolemma (plasma
membrane)
 Sarcoplasm (cytoplasm)
 Myofibril = rodlike organelle
 Contains contractile element
(sarcomeres)
 Alternating light (I) and dark (A)
bands
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Muscular System
Sarcomere
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 Smallest contractile unit of muscle fiber
 Region between 2 successive Z discs
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Muscular System
Sarcomere
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 Protein myofilaments:
 Thick filaments = myosin protein
 Thin filaments = actin protein
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Myofilaments
Muscular System
Thick Filaments
 Myosin head: forms
cross bridges with thin
filaments to contract
muscle cell
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Thin Filaments
 Tropomyosin: protein
strand stabilizes actin
 Troponin: bound to actin,
affected by Ca2+
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Muscular System
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 Sarcoplasmic Reticulum (SR): specialized smooth
ER, surrounds each myofibril
 Stores and releases calcium
 T Tubule: part of sarcolemma, conduct nerve
impulses to every sarcomere
 Triggers release of calcium from SR
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Muscular System
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Sliding Filament Model
 During contractions: thin
filaments slide past thick
ones so they overlap more
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Muscular System
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Sliding Filament Model
 Myosin heads latch onto active sites on actin to form a
cross-bridge
 Attachments made to propel thin filaments to center of
sarcomere
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Muscular System
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Basic Muscle Contraction
1. Stimulation by nerve impulse
2. Generate and send electrical current (action
potential) along sarcolemma
3. Rise in calcium ion levels to trigger contraction
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Muscular System
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Nerve Impulse
 1 nerve cell (motor neuron) stimulates a few or
hundreds of muscle cells
 Motor unit = 1 neuron + muscle cells stimulated
 Axon: extension of neuron
 Axon terminal: end of axon
 Neuromuscular junction (NMJ): where axon terminal
meets muscle fiber
 Synpatic cleft: space between neuron & muscle fiber
 Acetylcholine (ACh): neurotransmitter
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Muscular System
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Excitation of Muscle Cell
1. Action potential travels down axon and arrives at
neuromuscular junction
2. Release of acetylcholine (ACh) into synaptic cleft
3. ACh diffuses across cleft & attaches to ACh receptors
on sarcolemma of muscle fiber
4. Rush of sodium (Na+) into sarcoplasm produces action
potential in sarcolemma
5. ACh broken down
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Muscular System
Contraction of Muscle Cell
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1. Action potential travels down sarcolemma along TTubules
2. Calcium is released from SR
3. Calcium binds to troponin  changes shape  myosin
binding sites exposed on actin
4. Myosin cross-bridge forms with actin
5. Myosin head pivots and pulls actin filament toward M
line
6. ATP attaches to myosin and cross-bridge detaches
7. Myosin can be reactivated
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Muscular System
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Muscular System
Homeostatic Imbalances
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 Myasthenia gravis: loss of ACh receptors
in sarcolemma by immune system attack
 progressive muscular paralysis
 Botulism: from bacterial toxin; prevents
release of ACh at synaptic terminals 
muscular paralysis
 Rigor mortis: “death stiffness” = no ATP
production, myosin cross-bridges “stuck”
until proteins break down (peak: 12 hrs,
fades: 48-60 hrs later)
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Muscular System
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Muscle Responses
Twitches (single, brief, jerky contractions) =
problem, Healthy muscle shows smooth
contraction
Graded muscle responses: different degrees
of muscle shortening
Greater force by:
1. Increase frequency of muscle stimulation
 Contractions are summed (max tension =
complete tetanus)
2. Increase number of muscle cells being
stimulated
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Muscle response to changes in stimulation frequency
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Increasing stimulus  Increasing muscle tension
Muscular System
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Muscular System
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Energy
 ATP = only energy source for muscles
 Regenerated by:
1. Creatine phosphate (CP): transfers energy to
ATP
2. Aerobic respiration: complete glucose
breakdown with O2 present
3. Lactic acid fermentation: glucose breakdown
without O2
 Muscle fatigue: lack of O2, ATP supply low, lactic
acid accumulates, soreness  muscle contracts
more weakly until it stops
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Muscular System
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Types of Contractions
Isotonic
Isometric
 “same tension”
 “same length”
 Muscle length changes
 Muscle length stays same
 Concentric: shortens
 Tension increases
 Eccentric: lengthens
 Moving against heavy
load or immovable
object
 Eg. bicep curl, bend
knee, smiling
 Eg. lifting heavy weights
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Muscular System
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Isotonic Contractions
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Sequence the
Contraction:
Muscular System
events
for
Muscle
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Put the following events in order:
A. Calcium binds to troponin  changes shape  myosin
binding sites exposed on actin
B. Myosin head pivots and pulls actin filament toward M
line
C. ATP attaches to myosin and cross-bridge detaches
D. Action potential travels down sarcolemma along TTubules
E. Myosin cross-bridge forms with actin
F. Calcium is released from sarcoplasmic reticulum (SR)
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