Download Human Anatomy & Physiology I

Chapter 8
The Muscular System
Copyright 2010, John Wiley & Sons, Inc.
Chapter 9
Muscular System
Muscle Types
 Skeletal
 Smooth
 Cardiac
Similarities
 All muscle cells are elongated = muscle fibers
 Muscle contraction depends on 2 kinds of
myofilaments (actin & myosin)
 Cell membrane of a muscle cell = sarcolemma –
cytoplasm is called sarcoplasm
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Types of Muscle and Function

Skeletal - 40–50% of total body weight- voluntary
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Cardiac - involuntary
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Mostly movement of bone & body parts
Stabilizing body positions
Heart only
Develops pressure for arterial blood flow
Smooth- grouped in walls of hollow organs
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Sphincters regulate flow in tubes
Maintain diameter of tubes
Move material in GI tract and reproductive organs
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Muscle Functions
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Produce body movements
Stabilize body positions
Store and move substances
Produce heat
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Functional Characteristics of Muscle
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Excitability – to receive & respond to stimuli
Contractility – shorten forcibly when
stimulated
Extensibility – stretched or extended
Elasticity – to bounce back to original length
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Skeletal Muscle Tissue

Muscle includes: muscle fibers, connective
tissue, nerves & blood vessels
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Connective Tissue Coverings
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Endomysium – thin delicate layer of CT that
wraps each muscle fiber
Fascicles – many muscle fibers bundled together
into groups
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Skeletal muscle – many fascicles bundled
together
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Each is wrapped in a 2nd layer of CT made of collagen
– perimysium
Each is covered by a 3rd layer of dense fibrous CT –
epimysium
Deep Fascia – each skeletal muscle is then
covered by a 4th , very tough fibrous layer of CT

May extend past the length of the muscle (tendon or
aponeurosis) and attach that muscle to a bone,
cartilage or muscle
Copyright 2010, John Wiley & Sons, Inc.
Structure of a
Skeletal Muscle
Outside to Inside
• epimysium
• perimysium
• fascicle
• endomysium
Largest to Smallest
• muscle
• fascicles
• muscle fibers
• myofibrils
• thick and thin filaments
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9-4
Skeletal
Muscle
Tissue
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Muscle Histology

Sarcoplasm contains myoglobin
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Red pigmented protein related to Hemoglobin that
carries oxygen
Along entire length are myofibrils
Myofibrils made of protein filaments
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Come in thick and thin filaments
Copyright 2010, John Wiley & Sons, Inc.
Skeletal Muscle Fibers

2 types of protein filaments (cytoskeletal)
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Striations are caused by the arrangement of
thick & thin filaments
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Thick – protein myosin
Thin – protein actin
A-Band – dark area – overlapping of thick & thin
I-Band – light area – thin filaments only
Length of each myofibril is divided into
sarcomeres

Sarcomeres meet one another at an area called –
Z-line
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Sarcomere
• I band
• A band
• H zone
• Z line
• M line
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9-6
Muscle Histology
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Sarcomere
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Thick filaments – myosin
 Rod-like tail (axis) terminates in 2 globular heads or
cross bridges
 Cross bridges interact with active sites on thin
filaments
Thin filaments – actin
 Coiled helical structure (resembles twisted strands of
pearls)
 Tropomyosin – rod-shaped protein spiraling around
actin backbone to stabilize it
 Troponin – complex polypeptides
 One binds to actin
 One that binds to tropomyosin
 One that binds to calcium ions
 Both help control actin’s interaction with myosin
during contraction
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Myofilaments
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9-7
Sarcomere
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Sarcomere
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Sarcomere
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Functional Structure

Tropomyosin blocks myosin binding site
when muscle is at rest
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Within sarcoplasm – 2 specialized membranous
organelles
Sarcoplasmic reticulum (SR)
 Network of membranous channels that surrounds
each myofibril & runs parallel to it
 Same as ER in other cells
 SR has high concentrations of Ca ions compared to
the sarcoplasm (maintained by active transport –
calcium pump)
 When stimulated by muscle impulse, membranes
become more permeable to Ca ions and Ca
diffuses out of SR & into sarcoplasm
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Within sarcoplasm – 2 specialized membranous
organelles
Transverse tubules (TT)
 Set of membranous channels that extends into
the sarcoplasm as invaginations continuous with
muscle cell membrane (sarcolemma)
 TT’s are filled with extracellular fluid & extend
deep into the cell
 Each TT runs between 2 enlarged portions of
SR – cisternae
 Form a triad near the region where actin &
myosin overlap
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Skeletal Muscle Contraction
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Neuromuscular Junction (NMJ) – site where a motor
nerve fiber & a skeletal muscle fiber meet (synapse
or synaptic cleft)
Fibers must first be stimulated by a motor neuron for
a skeletal muscle to contract
Motor Unit – 1 motor neuron & many skeletal muscle
fibers
Motor End Plate – specific part of a skeletal muscle
fiber’s sarcolemma directly beneath the NMJ
Neurotransmitter – chemical substance released from
a motor end fiber, causing stimulation of the
sarcolemma of muscle fiber – acetylcholine (ACh)
Synaptic Cleft - small space between neuron &
muscle
Copyright 2010, John Wiley & Sons, Inc.
Axon collateral of
somatic motor neuron
Axon terminal
Nerve impulse
Synaptic vesicle
containing
acetylcholine
(ACh)
Synaptic
end bulb
Sarcolemma
Axon terminal
Synaptic
end bulb
Motor
end
plate
Neuromuscular
junction (NMJ)
Synaptic cleft
(space)
Sarcolemma
Myofibril
(b) Enlarged view of the
neuromuscular junction
(a) Neuromuscular junction
1 1ACh is released
from synaptic vesicle
Synaptic end bulb
Synaptic cleft
(space)
2
2 ACh binds to Ach
receptor
Motor end plate
Na+
Junctional fold
3 Muscle action
potential is produced
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(c) Binding of acetylcholine to ACh receptors in the motor end plate
Stimulus for contraction
Begins – motor impulse reaches the end of the motor
nerve fiber/ending – membrane depolarized (-70mV
to -55mV)
 Calcium ions rush into motor nerve fiber
 Neurotransmitter (acetylcholine) released in to NMJ
(exocytosis)
Acetylcholine diffuses across the NMJ &
stimulates/depolarizes the motor end-plate
(sarcolemma) 100mV to -70mV
The muscle impulse travels over the surface of the
skeletal muscle fiber & deep into the muscle fiber by
means of the TT
Copyright 2010, John Wiley & Sons, Inc.
Excitation Contraction Coupling
• muscle impulses cause sarcoplasmic reticulum to
release calcium ions into sarcoplasm of the muscle
fiber
• calcium binds to troponin to change its shape,
moving tropomyosin and exposing myosin binding
sites on actin filament
• cross bridges (linkages) form between actin &
myosin
•Actin filaments are pulled inward by myosin crossbridges
•Muscle fiber shortens as contraction occurs
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9-11
Neuromuscular Junction
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Synaptic end bulbs (at neuron terminal)
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Release neurotransmitter
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Neuromuscular
Junction
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Neuromuscular Junctions
Interactions Animations

Neuromuscular Junctions
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Contraction Cycle
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Myosin binds to actin & releases phosphate
group (forming crossbridges)
Crossbridge swivels releasing ADP and
shortening sarcomere (power stroke)
ATP binds to Myosin → release of myosin
from actin
ATP broken down to ADP & Pi → activates
myosin head to bind and start again
Repeats as long as Ca2+ concentration is
high
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Contraction Cycle
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Sliding Filament Theory
Changes in muscle during contraction
 H zones and the I bands narrow
 Regions of overlap widen
 Z lines move closer together
 Shortening the sarcomere
Copyright 2010, John Wiley & Sons, Inc.
Cross-bridge Cycling
When calcium ions are present, myosin binding sites on
actin are exposed
 Cross-bridge attaches
 ATP breakdown provides E to “cock” myosin head
 “Cocked” myosin attaches to exposed actin binding
site
 Cross-bridge (myosin head) springs from cocked
position and pulls on actin filament
 Cross-bridges break
 ATP binds to cross-bridge (but is not yet broken
down)
 Myosin heads are released from actin
*As long as Ca ions and ATP are present, this walking
continues until muscle fiber is fully contracted
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9-13
Relaxation
• acetylcholinesterase – enzyme present in NMJ
• breaks down acetylcholine, so the motor end-plate is no
longer stimulated (cannot cause continuous muscle
contraction)
• muscle impulse stops
• calcium moves back from sarcoplasm into sarcoplasmic
reticulum
• myosin and actin binding sites are broken
•Muscle fiber relaxes
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9-14
Energy Sources for Contraction
The E used to power the interaction between actin &
myosin comes from ATP
ATP stored in skeletal muscle lasts only about six
seconds
 ATP must be regenerated continuously if contraction
is to continue
 3 pathways in which ATP is regenerated
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Coupled reaction with Creatine Phosphate (CP)
Anaerobic Cellular Respiration
Aerobic Cellular Respiration
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Energy Sources for Contraction
CP + ADP  creatine + ATP
• creatine phosphate – stores energy that quickly converts
ADP to ATP
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9-15
Oxygen Supply and
Cellular Respiration
• Anaerobic Phase
• glycolysis
• occur in cytoplasm of cell
• produces pyruvic acid & little ATP
• Aerobic Phase
• citric acid cycle and electron
transport chain
• occurs in mitochondria
• produces CO2 & most ATP
• myoglobin stores extra oxygen
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9-16
Muscle Fatigue
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Physiological inability to contract
No O2 is available in muscle cells to complete
aerobic respiration
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Pyruvic acid is converted to lactic acid
Muscle fatigue & soreness
Results form a relative deficit of ATP and/or
accumulation of lactic acid (decreases pH)
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Muscle Fatigue
•cramp – sustained, involuntary contraction
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9-18
Oxygen Debt
Oxygen debt – amount of oxygen needed by liver to convert
lactic acid to glucose
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9-17
Heat Production
• Almost half of E released during muscle contraction is
lost to heat, which helps maintain our body temperature
at 37 degrees celsius
• by-product of cellular respiration
• muscle cells are major source of body heat
• blood transports heat throughout body
• Heat lost through negative feedback mechanisms –
sweating, dilation of superficial blood vessels, increased
breathing rate & increased heart rate
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9-19
Muscle Tone
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Even at rest some motor neuron activity
occurs = Muscle Tone
If nerves are cut fiber becomes flaccid (very
limp)
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Metabolism
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Rapid changes from very low ATP
consumption to high levels of consumption
Creatine phosphate (high energy store)
Fast and good for ~ 15 sec
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Nerve
impulse
1 Nerve impulse arrives at
axon terminal of motor
neuron and triggers release
of acetylcholine (ACh).
2 ACh diffuses across
synaptic cleft, binds
to its receptors in the
motor end plate, and
triggers a muscle
action potential (AP).
ACh receptor
3 Acetylcholinesterase in
Synaptic vesicle
synaptic cleft destroys
filled with ACh
ACh so another muscle
action potential does not
arise unless more ACh is
released from motor neuron.
Muscle action
potential
Transverse tubule
4 Muscle AP travelling along
transverse tubule opens Ca2+
release channels in the
sarcoplasmic reticulum (SR)
membrane, which allows
calcium ions to flood into the
sarcoplasm.
SR
Ca2+
9 Muscle relaxes.
8 Troponin–tropomyosin
complex slides back
into position where it
blocks the myosin
binding sites on actin.
5 Ca2+ binds to troponin on
the thin filament, exposing
the binding sites for myosin.
Elevated Ca2+
Ca2+ active
transport pumps
7 Ca2+ release channels in
SR close and Ca2+ active
transport pumps use ATP
to restore low level of
Ca2+ in sarcoplasm.
6 Contraction: power strokes
use ATP; myosin heads bind
to actin, swivel, and release;
thin filaments are pulled toward
center of sarcomere.
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Production of ATP for Muscle
Contraction
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Glycolysis
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Break down glucose to 2 pyruvates getting 2
ATPs
If insufficient mitochondria or oxygen,
pyruvate → lactic acid
Get about 30–40 seconds more activity
maximally
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Production of ATP for Muscle
Contraction
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Aerobic Cellular Respiration
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Production of ATP in mitochondria
Requires oxygen and carbon substrate
Produces CO2 and H2O and heat.
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Fatigue
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Inability to contract forcefully after prolonged
activity
Limiting factors can include:
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Ca2+
Creatine Phosphate
Oxygen
Build up of acid
Neuronal failure
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Control of Muscle Contraction
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Single action potential(AP) → twitch
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Total tension of fiber depends on
frequency of APs (number/second)
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Smaller than maximum muscle force
Require wave summation
Maximum = tetanus
Total tension of muscle depends on
number of fibers contracting in unison
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Increasing numbers = Motor unit recruitment
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Muscular Responses
Threshold Stimulus
• minimal strength required to cause contraction
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9-20
Recording a Muscle Contraction
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Myogram – recording of a muscle
contraction
Twitch- single contraction that lasts a
fraction of a second, followed by
relaxation
latent period – delay between stimulation
& contraction
refractory period – muscle fiber must
return to its resting state (-100mV)
before it can be stimulated again
all-or-none response
 If a muscle fiber is brought to threshold
or above, it responds with a complete
twitch
 If the stimulus is sub-threshold, the
muscle fiber will not respond
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Myogram
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Myogram
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Staircase Effect (treppe)
Most muscle fiber contraction is “all or nothing”
A muscle fiber that has been inactive can be subjected
to a series of stimuli &
 The fiber undergoes a series of twitches with
relaxation between &
 The strength of each successive contraction
increases slightly
Phenomenon is small & brief & involves excess
calcium in sarcoplasm
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Summation
•When several stimuli are delivered
in succession to a muscle fiber, it
cannot completely relax between
contractions
•individual twitches combine and
muscle contraction becomes
sustained
•When resulting sustained
contraction lacks even slight
relaxation, it is called tetanic
contractions
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9-21
Motor Units
Motor neuron & the many skeletal muscle fibers
it stimulates
 Motor neuron branches into several motor
nerve endings, it can stimulate many skeletal
muscles fibers simultaneously, which then
contract simultaneously
 The # of muscle fibers in a motor unit varies
from 10 - hundreds
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Recruitment of Motor Units
• recruitment - increase in the number of motor units
activated
• whole muscle composed of many motor units, controlled
by many different motor neurons, simultaneous
contraction of all units does not necessarily occur
• as intensity of stimulation increases, recruitment of
motor units continues until all motor units are
activated
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9-22
Sustained Contractions
•Even when a muscle is at rest, a certain
amount of sustained contraction is occurring
in its fibers - muscle tone
• very important in maintaining posture
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9-23
Types of Contractions
• concentric – shortening contraction
• eccentric – lengthening contraction
• isometric – muscle contracts but does not change
length – attachments do not move, tensing a muscle
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9-24
Fiber Types
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Slow oxidative (SO)- small diameter and red
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Fast oxidative- glycolytic (FOG)
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Large amounts of myoglobin and mitochondria
ATP production primarily oxidative
Fatigue resistant
Large diameter = many myofibrils
Many mitochondria and high glycolytic capacity
Fast glycolytic fibers (FG)
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White, fast & powerful and fast fatiguing
For strong, short term use
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Recruitment

Recruited in order: SO → FOG → FG
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Fast and Slow Twitch Muscle Fibers
Fast-twitch fatigueSlow-twitch fibers (type I)
resistant fibers (type IIb)
• always oxidative
• intermediate fibers
• resistant to fatigue
• oxidative
• red fibers
• oxygen containing pigment - myoglobin • intermediate amount of
myoglobin
• good blood supply
• pink to red in color
• contain many mitochondria
Fast-twitch glycolytic fibers (type II)
• white fibers (less myoglobin)
• fewer mitochondria
• contain extensive sarcoplasmic reticulum to store and reabsorb
calcium
• poorer blood supply
• contract rapidly, but fatigue easily due to lactic acid
accumulation
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9-25
Effects of Exercise
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SO/FG fiber ratio genetically determined
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Endurance exercise gives FG → FOG
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High FG → sprinters
High SO → marathoners
Increased diameter and numbers of
mitochondria
Strength exercise increases size and
strength of FG fibers
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Cardiac Muscle
Location
• only in the heart
Anatomy
•Arrangement of actin & myosin not as organized as skeletal
Physiology
• Self-exciting tissue (pacemaker)
• Involuntary control – all or nothing contractions
•Function as a “syncytium” (all or nothing)
• rhythmic contractions (60-100 beats/minute)
• longer refractory period than skeletal muscle
•Pumps blood to
•Lungs for oxygenation
•Body for distribution of oxygen & nutrients
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9-29
Cardiac Muscle
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Involuntary muscle found only in heart wall
Striated, branched short fibers with single,
central nucleus in each fiber
Fibers connected by:
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Intercalated discs (thickened cell membranes)
Gap junctions that allow spread of action
potentials
ATP generated by abundant mitochondria
and by lactic acid when cells lack oxygen
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Cardiac Muscle
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Does not require nerve stimulation nerve
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Has its own intrinsic pacemaker (and conduction
system within cardiac muscle) that initiates cardiac
contraction
Known as auto-rhythmicity
Ca2+released from S.R. and extracellular
spaces
Intercalated discs with gap junctions transmit
action potentials from ne muscle cell to the
next
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Cardiac Muscle
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Smooth Muscle Fibers
Compared to skeletal muscle fibers
• shorter
• single nucleus
• elongated with tapering ends
• myofilaments randomly organized
• no striations
• lack transverse tubules
• sarcoplasmic reticula are reduced
• walls of hollow visceral organs & blood vessels
• Involuntary control
• Contractions are slow & sustained
• troponin is absent
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9-26
Types of Smooth Muscle
Visceral Smooth Muscle
• single-unit smooth muscle
• sheets of muscle fibers
• fibers held together by gap junctions
• exhibit rhythmicity – pattern of repeated
contractions
• exhibit peristalsis – wave-like motion that
helps push substances through passageways
• walls of most hollow organs
• contraction is slow & sustained
•Structure
•Random arrangement of actin &
myosin filaments
•2 layers of muscle surround
passageway
•Inner circular layer
•Outer longitudinal layer
Multiunit Smooth Muscle
• fibers function separately
• irises of eye
• walls of blood vessels
• contraction is rapid & vigorous
(similar to skeletal muscle tissue
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9-27
Smooth Muscle Contraction
Resembles skeletal muscle contraction
• interaction between actin and myosin
• both use calcium and ATP
• both depend on impulses
Different from skeletal muscle contraction
• A protein,
calmodulin binds to calcium ions (no troponin) and
activates the contraction mechanism
• most calcium diffuses in to smooth muscle cells from the
extracellular fluid (reduced SR)
• two neurotransmitters affect smooth muscle
• acetlycholine and norepinephrine
• hormones affect smooth muscle
• stretching can trigger smooth muscle contraction
• contraction is slow & sustained
• smooth muscle more resistant to fatigue
9-28
Copyright 2010, John Wiley & Sons, Inc.
Smooth Muscle
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Involuntary
Found in internal organs such as stomach,
bladder, walls of arteries
Structure
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Tapered cells each with single nucleus
Filaments not regular so tissue does not appear
striated
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Smooth Muscle
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Types
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Visceral (single unit) type or
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Multi-unit type
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Form sheets and are auto-rhythmic
Contract as a unit
Each has own nerve and can contract independently
Graded contractions and slow responses
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Often sustain long term tone
Often triggered by autonomic nerves
Modulated chemically, by nerves, by mechanical
events (stretching)
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Smooth Muscle
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Life-Span Changes
• myoglobin, ATP, and creatine phosphate
decline in ones forties
• by age 80, half of muscle mass has
atrophied – replaced by connective &
adipose tissue
•Reflexes are reduced
• exercise helps to maintain muscle mass and
function
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9-65
Aging
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As with bone there is a slow progressive loss
of skeletal muscle mass
Relative number of SO fibers tends to
increase
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Homeostatic Imbalances/Disorders
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Tendinitis
Compartment Syndrome
Poliomyelitis
Myasthenia Gravis
Duchenne Muscular Dystrophy
Rigor Mortis
Botulism
TMJ
Parkinson’s Disease
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Clinical Application
Myasthenia Gravis
• autoimmune disorder
• receptors for acetylcholine on muscle cells are attacked
• weak and easily fatigued muscles result
• difficulty swallowing and chewing
• ventilator needed if respiratory muscles are affected
• treatments include
• drugs that boost acetylcholine
• removing thymus gland
• immunosuppressant drugs
• antibodies
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9-66
Movement
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Muscles move one bone relative to another
around one or more joint(s)
Origin → most stationary end
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Insertion → most mobile end
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Location where the tendon attaches
Location where tendon inserts
Action → the motion or function of the
muscle
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Movement

Generally arranged in opposing pairs
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Flexors - extensors; abductors - adductors
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The major actor: prime mover or agonist
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Muscle with opposite effect: antagonist
Synergists - help prime mover
Fixators - stabilize origin of prime mover
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Role of muscle varies with motion
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Basis of Muscle Names: Table 8.2
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Direction of fibers relative to body axes
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Size of muscle
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Examples: lateralis, medialis (medius),
intermedius, rectus
Examples: alba, brevis, longus, magnus, vastus
Shape of muscle
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Examples: deltoid, orbicularis, serratus,
trapezius
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Basis of Muscle Names: Table 8.2
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Action of muscle
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Number of tendons (heads) of origin
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Examples: abductor, adductor, flexor, extensor
Examples: biceps, triceps, quadriceps
Location of muscle

Examples: abdominus, brachialis, cleido,
oculo-, uro-,
Copyright 2010, John Wiley & Sons, Inc.