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Chapter 9
Muscular System
Skeletal Muscle
Each skeletal muscle is an organ made up
of skeletal muscle fibers, connective tissue
coverings, blood vessels, and nerve fibers
Structure of a Skeletal Muscle
Outside
In
• Each skeletal muscle is then covered by an
outer, very tough fibrous layer of CT called
deep fascia
– The deep fascia may extend past the length of the
muscle (tendon or aponeurosis), and attach that
muscle to a bone, cartilage or muscle
• Each skeletal muscle is covered by a second
layer of dense, fibrous CT called epimysium
A skeletal muscle is composed of a variety of tissues
Slide number: 2
Muscle
Bone
Tendon
Fascia
(covering muscle)
Epimysium
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Skeletal Muscle cont.
• Skeletal muscles are formed from bundles of
fascicles
– Each fascicle is wrapped in a third layer of CT
made of collagen called perimysium
• Fascicles are formed from bundles of muscle
fibers
– Each muscle fiber (cell) is wrapped in a thin,
delicate (fourth) layer of CT called endomysium
– Cell membrane= Sarcolemma
– Cytoplasm= Sarcoplasm
A skeletal muscle is composed of a variety of tissues
Slide number: 5
Muscle
Bone
Fascicles
Tendon
Muscle fibers (cells)
Fascia
(covering muscle)
Epimysium
Perimysium
Endomysium
Fascicle
Axon of motor neuron
Blood vessel
Nucleus
Sarcoplasmic
reticulum
Muscle fiber
Sarcolemma
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Fascicle
Skeletal Muscle cont.
• Each muscle fiber contains many threadlike
structures called Myofibrils.
– Myofibrils play an important role in muscle
contraction
• They consist of two types of Protein
Filaments
– Thick filament= Myosin
– Thin filament= Actin
A skeletal muscle is composed of a variety of tissues
Slide number: 7
Muscle
Bone
Fascicles
Tendon
Muscle fibers (cells)
Fascia
(covering muscle)
Myofibrils
Epimysium
Perimysium
Thick and thin filaments
Endomysium
Fascicle
Axon of motor neuron
Blood vessel
Myofibril
Nucleus
Sarcoplasmic
reticulum
Muscle fiber
Sarcolemma
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Filaments
Myofibril
Skeletal Muscle Fibers
• Within the sarcoplasm (cytoplasm) of a
muscle fiber, there are two specialized
membranous organelles (“little organs”)
• Sarcoplasmic reticulum (SR)
– Network of membranous channels that surrounds
each myofibril and runs parallel to it
– Similar endoplasmic reticulum in other cells
– SR has high concentrations of calcium ions
compared to the sarcoplasm (maintained by
active transport calcium pump)
– When stimulated by muscle impulse, membranes
become more permeable to calcium ions and
calcium diffuses out of SR and into sarcoplasm
Skeletal Muscle Fibers cont.
• Transverse tubules (TT)
– set of membranous channels that extends into the
sarcoplasm as invaginations continuous with
muscle cell membrane (sarcolemma)
– TTs are filled with extracellular fluid and extend
deep into the cell
– Each TT runs between two enlarged portions of
SR called cisternae
– These structures form a triad near the region
where actin and myosin overlap
Skeletal Muscle Fibers cont.
• SR and TT are involved in activating the
muscle contraction mechanism (discussed in
greater detail later).
• Because one TT is associated with two SR
they are termed the Triad
Skeletal Muscle Fibers cont.
• The organization of think and thick filaments
within muscle fibers produces light and dark
bands (striations) characteristic of skeletal
muscle fibers
• The striations form a repeating pattern of
units called sarcomeres.
– Functional unit of muscle
• Myofibrils are made from sarcomeres in a
row, end-to-end.
Sarcomere characteristics
• I bands=light area =
thin filaments alone
• A bands=dark area =
overlapping of thick and
thin filaments
• Z lines=Sarcomeres
meet one another
Sarcomere characteristics cont.
• H zone=Lighter area in A bands with only
thick filaments
• M line=darker area with proteins to hold
thick filaments in place
Molecules Involved in Contraction
Filaments
• Thick filaments = protein myosin
• Thin filaments = protein actin
Other
• Tropomyosin
• Troponin
Molecules Cont.
• Thick filaments = protein myosin
– rod-like tail (axis) that terminates in two globular
heads or cross bridges
– Cross bridges interact with active sites on thin
filaments
• Thin filaments = Primarily the protein actin
– coiled helical structure (resembles twisted strands
of pearls):
– Tropomyosin = rod-shaped protein spiraling
around actin backbone to stabilize it
Molecules Cont.
– Troponin = complex of polypeptides:
– one binds to actin
– one that binds to tropomyosin
– one that binds to calcium ions
• Both tropomyosin and troponin help control
actin's interaction with myosin during
contraction
Skeletal Muscle Contraction
Neuromuscular Junction
• Neuromuscular Junction (NMJ) = the site
where a motor nerve fiber and a skeletal
muscle fiber meet (also called a synapse or
synaptic cleft)
– In order for a skeletal muscle to contract, its fibers
must first be stimulated by a motor neuron
Neuromuscular Junction cont.
• Motor End Plate = the 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
and muscle
Motor Unit
• Motor Unit =
one motor
neuron and
many skeletal
muscle fibers
Stimulus for Contraction
Introduction
•
•
•
The function of skeletal muscle is to move
bones of the skeleton under voluntary
control.
Contraction of a skeletal muscle fiber is a
complex interaction of several cellular and
chemical constituents.
The final result is a movement whereby actin
and myosin filaments slide past one another.
– The muscle fiber shortens and pulls on its
attachments.
Stimulus for Contraction
• The process begins when a nerve impulse is
initiated by the brain, travels down the spinal
cord, into a motor neuron, which branches
into many motor nerve fibers/endings
• The neuron meets the muscle at the
neuromuscular junction
• Neurotransmitter (Acetylcholine) is released
into the NMJ (via exocytosis)
Stimulus for Contraction cont.
• Acetylcholine diffuses across the NMJ and
creates an electrical signal (similar to a nerve
impulse) at the motor end-plate (sarcolemma)
– The electrical signal is created by the movement
of ions
– It must reach a certain strength for contraction to
be stimulated
• The muscle impulse travels over the surface
of the skeletal muscle fiber and deep into the
muscle fiber by means of the Transverse
Tubules
– This instigates the process of muscle contraction
Excitation Contraction Coupling
Big Picture
• The muscle impulse reaches the sarcoplasmic
reticulum, which releases calcium ions into the
cytosol
• Calcium binds to troponin, moving tropomyosin and
exposing myosin binding sites on actin filament
• Cross-bridges (linkages) form between actin and
myosin
• Actin filaments are pulled inward by myosin crossbridges
• The muscle fiber shortens as contraction occurs
Sliding Filament Theory
• states that muscle contraction involves the
sliding movement of the thin filaments
(actin) past the thick filaments (myosin)
– Resulting in shortening of sarcomeres
• A relaxed muscle cell, overlapping of thick
and thin filaments is only slight
• Changes in muscle during contraction:
– The distance between the Z-lines of the
sarcomeres decreases
– The I-Bands (light bands) shorten
• The A-Bands move closer together, but do not diminish
in length.
When a skeletal muscle contracts
Slide number: 3
Sacromere
A band
Z line
Z line
Actin
filaments
1 Relaxed
Myosin
filaments
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When a skeletal muscle contracts
Slide number: 4
Sacromere
A band
Z line
Z line
Actin
filaments
1 Relaxed
Myosin
filaments
2 Contracting
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When a skeletal muscle contracts
Slide number: 5
Sacromere
A band
Z line
Z line
Actin
filaments
1 Relaxed
Myosin
filaments
2 Contracting
3 Fully contracted
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cross-bridge Cycling
• when calcium ions are present, the myosin
binding sites on actin are exposed
• myosin cross-bridge attaches to actin binding
site
• myosin cross-bridge pulls thin filament
• ADP and phosphate released from myosin
• new ATP binds to myosin
Cross-Bridge Cycling cont.
• linkage between actin and myosin crossbridge break
• ATP splits
• myosin cross-bridge goes back to original
position
* As long as calcium ions and ATP are
present, this walking continues until the
muscle fiber is fully contracted
Slide number: 1
Tropomyosin
Troponin complex
Actin monomers
ADP + P
ADP + P
Myosin filament
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Actin filament
Slide number: 2
Tropomyosin
Troponin complex
Actin monomers
ADP + P
ADP + P
Myosin filament
Ca+2
Muscle contraction
Release of Ca+2 from sarcoplasmic
reticulum exposes binding sites on
thin filament:
Ca+2 binds to troponin complex
Tropomyosin pulled aside
Binding sites on
actin filament
exposed
Ca+2
ADP + P
Ca+2
ADP + P
Ca+2
1 Exposed binding sites on actin allow the muscle
contraction cycle to occur
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Actin filament
Slide number: 3
Contraction cycle
Ca+2
ADP + P
Ca+2
ADP + P
Ca+2
ADP + P
ADP + P
2 Cross-bridge binds actin to myosin
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Slide number: 4
Contraction cycle
ADP + P
ADP
P
ADP
P
ADP + P
3 Cross-bridge pulls actin filament (power stroke),
ADP and P released from myosin
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ADP + P
Slide number: 5
Contraction cycle
ATP
ATP
ATP
4 New ATP binds to myosin, causing linkage to
release
ATP
ADP
P
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ADP
P
Slide number: 6
Contraction cycle
ADP + P
ADP + P
5 ATP splits, which
provides power to
“cock” the myosin
cross-bridge
ATP
ATP
ATP
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Slide number: 7
Contraction cycle
Ca+2
ADP + P
ADP + P
Ca+2
ADP + P
Ca+2
ADP + P
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Slide number: 8
ADP + P
ADP + P
Ca+2
Muscle relaxation
Active transport of Ca+2 into sarcoplasmic
reticulum, which requires ATP, makes
myosin binding sites unavailable.
ATP
Ca+2
ADP + P
Ca+2
ADP + P
Ca+2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Slide number: 9
Ca+2
ADP + P
Ca+2
ADP + P
Ca+2
1 Exposed binding sites on actin allow the muscle
contraction cycle to occur
ADP + P
Contraction
cycle
ADP + P
5 ATP splits, which
provides power to
“cock” the myosin
cross-bridge
ATP
ADP + P
ADP + P
2 Cross-bridge
binds actin to
myosin
ATP
ADP
ATP
P
ATP
4 New ATP binds to myosin, causing linkage to
release
ADP
P
ADP + P
3 Cross-bridge pulls actin filament (power stroke),
ADP and P released from myosin
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Slide number: 10
Tropomyosin
Troponin complex
Actin monomers
ADP + P
ADP + P
Myosin filament
Ca+2
Ca+2
Muscle contraction
Muscle relaxation
Release of Ca+2 from sarcoplasmic
reticulum exposes binding sites on
thin filament:
Ca+2 binds to troponin complex
Active transport of Ca+2 into sarcoplasmic
reticulum, which requires ATP, makes
myosin binding sites unavailable.
ATP
Tropomyosin pulled aside
Binding sites on
actin filament
exposed
Ca+2
ADP + P
Ca+2
ADP + P
Ca+2
1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Actin filament
Relaxation
•
•
Acetylcholinesterase is an enzyme present
in the NMJ
It immediately destroys acetylcholine, so the
motor end-plate is no longer stimulated
– i.e. it cannot cause continuous muscle
contraction
•
•
•
Calcium ions are transported from
sarcoplasm back into sarcoplasmic
reticulum
Linkages between actin and myosin are
broken
The muscle fiber relaxes
Energy Sources for
Contraction
The energy used to power the
interaction between actin and
myosin comes from ATP
Introduction
• ATP stored in skeletal muscle lasts only about
six seconds
• ATP must be regenerated continuously if
contraction is to continue
• There are three pathways in which ATP is
regenerated:
– Coupled Reaction with Creatine Phosphate
– Anaerobic Cellular Respiration (Ch. 4)
– Aerobic Cellular Respiration (Ch. 4)
Coupled Reaction with Creatine
Phosphate (CP)
• CP + ADP <------> creatine + ATP
• Muscle stores a lot of CP
• This coupling reaction allows for about 10
seconds worth of ATP
Oxygen Supply and Cellular
Respiration
• Anaerobic Respiration
– Steps are called glycolysis
– Steps occur in the cytoplasm of the cell
– Results in production of pyruvic acid and 2 ATP
• Aerobic Respiration (bet you thought you were
done with this!)
– Steps are called citric acid cycle and electron
transport chain
– Oxygen is required
– Steps occur in the mitochondrion of the cell
– Results in CO2, water and 36ATP
Muscle Fatigue
•
•
•
Muscle fatigue is a state of physiological
inability to contract
If no oxygen is available in muscle cells to
complete aerobic respiration, pyruvic acid is
converted to lactic acid, which causes
muscle fatigue and soreness
Results from a relative deficit of ATP and/or
accumulation of lactic acid (which decreases
pH)
Oxygen Debt
• The oxygen debt is the amount of oxygen
necessary to support the conversion of lactic
acid to glycogen
• needed to replenish spent glycogen stores
• oxygen not available
– glycolysis continues
– pyruvic acid converted to lactic acid
– liver converts lactic acid to glucose
Heat Production
• Almost half of the energy released during
muscle contraction is lost to heat, which helps
maintain our body temperature at 37o C
• Excessive heat is lost through many negative
feedback mechanisms (discussed in chapter
1) including sweating, dilation of superficial
blood vessels, increased breathing rate, and
increased heart rate
Muscle Responses
Threshold Stimulus
•
•
The minimal strength of stimulation required
to cause contraction
A skeletal muscle fiber’s resting membrane
potential must be depolarized from –100mV
to –70mv before an impulse begins
•
Therefore the threshold stimulus is +30mV
Recording a Muscle Contraction
•
•
•
•
A myogram is a recording of a muscle
contraction
A twitch is a single contraction that lasts a
fraction of a second, followed by relaxation
The delay between stimulation and
contraction is called the latent period
A muscle fiber must return to its resting state
(-100mV) before it can be stimulated again
– This is called the refractory period
All-or-Nothing 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
Summation
•
When several stimuli are delivered in succession to
a muscle fiber, it cannot completely relax between
contractions
–
•
The individual twitches begin to combine and the muscle
contraction becomes sustained
In a sustained contraction, the force of individual
twitches combines in a process called summation
–
can lead to tetanic contractions
Tetanus
• When the resulting sustained contraction lacks even
slight relaxation, it is called titanic contraction
• Incomplete tetanus=the state of a muscle at maximum
tension that is not allowed to relax completely
• Complete tetanus=the muscle is at maximum tension
and there is no relaxation phase at all
Motor Units
•
•
•
Definition: A motor unit is a motor neuron
and the many skeletal muscle fibers it
stimulates
Because the motor neuron branches into
several motor nerve endings, it can
stimulate many skeletal muscles fibers
simultaneously, which then contract
simultaneously
The number of muscle fibers in a motor unit
varies from 10-hundreds
Recruitment
•
•
•
•
A muscle is composed of many motor units,
controlled by many different motor neurons
recruitment - increase in the number of
motor units activated
more precise movements are produced with
fewer muscle fibers within a motor unit
as intensity of stimulation increases,
recruitment of motor units continues until all
motor units are activated
Muscle Tone
• Sustained Contractions
– Even when a muscle is at rest, a certain
amount of sustained contraction is occurring
in its fibers. This is called muscle tone.
• Muscle tone is very important in
maintaining posture
Types of Contractions
• isotonic – muscle contracts and changes
length
• concentric – shortening contraction
• eccentric – lengthening contraction
• isometric – muscle contracts but does not
change length
Fast and Slow Muscle Fibers
• Muscle fibers vary in contraction speed (i.e. slow or
fast twitch)
• Slow-Twitch Fibers are also called red fibers
– contain oxygen carrying pigment, myoglobin, receive a
rich blood supply, and contain many mitochondria
– can generate ATP fast enough to keep up with breakdown
– These fibers contract for long periods without fatiguing
• Fast-twitch fibers are also called white fibers
– contain less myoglobin, blood, and fewer mitochondria.
– contain extensive sarcoplasmic reticulum to store and
reabsorb calcium
– These fibers contract rapidly, but fatigue easily due to
lactic acid accumulation
Smooth Muscle
The contraction mechanism of smooth
muscle is similar to that of skeletal muscle
in that interaction occurs between actin and
myosin, however the transverse tubules
and sarcoplasmic reticula are greatly
reduced, and troponin is absent.
Smooth Muscle Fibers
Multi-unit smooth muscle
• location
– irises of eyes
– walls of blood vessels
• Contraction is rapid and vigorous (similar to
skeletal muscle tissue)
• less organized
• function as separate units
– fibers function separately
Smooth Muscle Fibers cont.
Visceral smooth muscle
• Location = the walls of hollow organs
• Contraction is slow and sustained
– Rhythmicity = pattern of repeated contractions
– Peristalsis = wave-like motion that helps push
substances through passageways
• Structure:
–
–
–
–
–
single-unit smooth muscle
sheets of muscle fibers
fibers held together by gap junctions
random arrangement of actin and myosin filaments
Two layers of muscle surround the passageway
• inner circular layer
• outer longitudinal layer
Smooth 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).
• Norepinephrine and acetylcholine are smooth
muscle neurotransmitters
• Smooth muscle slower to contract and relax
• Smooth muscle more resistant to fatigue
• Stretching can trigger smooth muscle contraction
• Smooth muscle can change length without changing
tautness
CARDIAC MUSCLE
Will be studied in greater detail in
Chapter 15
Cardiac Muscle
• Location=the heart
• Anatomy:
– Striated uninuclear cells joined end-to-end forming a
network
– Cell junctions are called intercalated discs
• Arrangement of actin and myosin not as organized
as skeletal muscle
• Contains sarcoplasmic reticula, transverse tubules,
and numerous mitochondria
• Sarcoplasmic reticulum is less developed than SR in
skeletal muscle and stores much less calcium
Cardiac Muscle cont.
Physiology
• Self-exciting tissue (i.e. “Pacemaker”)
• Rhythmic contractions (60-100 beats/minute)
• Involuntary, all-or-nothing contractions
• Pumps blood to:
– Lungs for oxygenation
– Body for distribution of oxygen and nutrients
SKELETAL MUSCLE
ACTIONS
Skeletal muscles generate a great variety of body
movements. The action of a muscle primarily depends
upon the joint associated with it and the manner in which
the muscle is attached on either side of that joint
Origin and Insertion
• Recall that skeletal muscles are usually
attached to a fixed body part and a movable
body part:
– The origin of a muscle is its immovable
(anchored) end
– The insertion of a muscle is the movable end of
a muscle
• When a muscle contracts and shortens, its
insertion is pulled toward its origin
Skeletal Muscle Actions
• Flexion = decreasing the
angle between 2 bones
– Dorsiflexion = decreasing the
angle between the foot and
shin
– Plantar flexion = pointing toes
• Extension = increasing the
angle between 2 bones
• Abduction = moving a
body part away from the
midline
• Adduction = moving a
body part toward the
midline
• Circumduction =
movement in a circular
(cone-shaped) motion
• Rotation = turning
movement of a bone about
its long axis
– (i.e. atlas/axis)
•
•
•
•
•
Supination = thumbs up
Pronation = thumbs down
Inversion = sole of foot in
Eversion = sole of foot out
Elevation = lifting a body
part
– (i.e. shoulder shrug)
• Depression = returning a
body part to pre-elevated
position
Interactions of Skeletal Muscles
• Prime Mover (agonist) = the primary muscle responsible for
a movement
– The biceps brachii in flexing the arm at the elbow
• Antagonist(s) = the muscle(s) in opposition to the action of
the prime mover. The antagonist relaxes (or stretches)
during the prime movement
– The triceps brachii is the antagonist of the biceps brachii when we flex
the arm at the elbow
• Synergist(s) = muscles that assist the prime mover
– The brachialis helps the biceps brachii during elbow flexion
• Fixators = muscle groups that stabilize the origin of the
prime mover (i.e. hold it in place) so that the prime mover can
act more efficiently
– The scapula is the origin for many arm muscles, but it must be held in
place by fixator muscles in order to function in this way
• serratus anterior
• pectoralis minor
LIFE SPAN CHANGES
• Supplies of ATP, myoglobin, and creatine
phosphate in muscle fibers begin to decline in
one’s forties
• Half of one’s muscle mass has been replaced
by connective and adipose tissue by age 80,
and reflexes are reduced
• Exercise is the best way to maintain muscle
function