The Muscular System
Chapter 8
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III. Introduction
The 3 types of muscle are:
1. Skeletal (aka: striated, voluntary)
2. Smooth (aka: visceral, involuntary)
3. cardiac
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III. Structure of a Skeletal Muscle
A. Kinds of Tissue
• Connective
• Muscle
• Nervous
• Blood
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III. Structure of a Skeletal Muscle
B. 1 through 3
1. A skeletal muscle is held in position by layers
of fibrous connective tissue called fascia.
2. This tissue extends beyond the end of a
skeletal muscle to form a cordlike tendon.
3. When this tissue extends beyond the muscle
to form a sheet-like structure, it is called an
aponeurosis.
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III. Structure of a Skeletal Muscle
4. Fascicles are bundles
of muscle fibers (cells)
Thick (myosin) & thin (actin)
fibers
Myofibrils
Muscle fiber (cell)
Fascicle
Muscle
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III. Structure of a Skeletal Muscle
C. 1. Describe a single muscle fiber
A single muscle fiber (muscle cell) is a thin,
elongated cylinder
• The cell membrane of a muscle fiber is called the
sarcolemma
• The cytoplasm of a muscle fiber is celled the
sarcoplasm and it contains many nuclei, mitochondria,
7 other cellular structures; also in the sarcoplasm are
many myofibrils that lie parallel to each other.
• Myofibrils are made up of bands of thick myosin fibers
(“A” bands) and thin actin fibers (“I” bands) form
myofibrils.
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III. Structure of a Skeletal Muscle
C. 2. Describe the structure & function of a sarcomere.
A sarcomere is the segment of a myofibril from one “Z”
line to the next “Z” line. The striations of skeletal
muscle are due to the structure of myofibril:
• The light-colored “I” bands are thin actin fibers which are
attached to Z lines
• The dark-colored “A” bands contain the thick myosin
fibers;
• in the part of the “A” band closest to Z lines the myosin
fibers overlap the actin fibers;
• the “H” band portion of the “A” band there are no actin
fibers but there is a central thickening known as the “M”
line
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III. Structure of a Skeletal Muscle
C. 2. Describe the structure & function of a sarcomere.
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III. Structure of a Skeletal Muscle
C. 2. Describe the structure & function of a sarcomere.
• The structure of a
sarcomere allows the actin
& myosin filaments to slide
with respect to each other
when stimulated by the
presence of ATP & Ca++ ions.
• This is called the sliding
filament mechanism
• The contraction of a muscle
results from the
simultaneous contraction of
all of its sarcomeres
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III. Structure of a Skeletal Muscle
C. 3. & C.4.
3. The network of membranous channels in the
cytoplasm/sarcoplasm of muscle fibers is the sarcoplasmic
reticulum. A second set of channels is the transverse tubules
which extend inward from the fiber’s membrane. Each
tubule opens to the outside of the fiber.
4. The sarcoplasmic reticulum and the transverse tubules
activate the muscle contraction mechanism when the fiber is
stimulated.
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III. Structure of a Skeletal Muscle
D. Label see also figure 8.5 on page 172 of text
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IV. Skeletal Muscle Contraction
A. Describe the roles of actin & myosin in muscle contraction.
• Myosin filaments have cross bridges along their
length
• Actin fibers have binding sites for myosin cross
bridges
• The 2 proteins troponin & tropomyosin act together
to expose the actin binding sites in response to an
influx of Ca2+
• The myosin cross bridges bind to actin filaments,
pulling on the actin filaments, causing the sarcomere
to shorten (the Z lines are pulled closer together)
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IV. Skeletal Muscle Contraction
B. Describe the transmission of a nerve impulse across…
• When a nerve impulse
reaches the end of a
motor neuron axon,
vesicles in the neuron’s
cytoplasm release
acetylcholine
(neurotransmitter) into
the synaptic cleft
between the neuron &
the motor end plate of
the muscle
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IV. Skeletal Muscle Contraction
C.1. Describe the interaction between acetylcholine & Ca2+…
• Acetylcholine stimulates a
muscle impulse which passes
in all directions over the
surface of the muscle fiber,
traveling to the sarcoplasmic
reticulum
• The sarcoplasmic reticulum
releases Ca2+ ion allowing
cross bridges of myosin to
bind to actin
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IV. Skeletal Muscle Contraction
C. 2. & C.3.
C.2.
A cross bridge is formed
when the projecting parts of
the myosin filament can
occupy a binding site on the
actin filament and pull itself
along the actin filament.
C. 3.
The action of acetylcholine is
halted by the enzyme
acetylcholinesterase.
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IV. Skeletal Muscle Contraction
D. 1. & D.2.
D.1. ATP (adenosine
triphosphate) stores
energy in a high-energy
chemical bond. When
energy is needed for a
muscle contraction, an
ATP molecule releases
energy and is degraded to
an ADP (adenosine
diphosphate) molecule.
ATP
Energy + ADP + P
D.2. Only small amounts of
ATP are stored in a muscle so
ATP must be regenerated in
order for muscle contraction to
continue. Creatine phosphate
stores energy generated by the
mitochondria then releases
that energy to maintain a
steady supply of ATP
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Creatine phosphate
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IV. Skeletal Muscle Contraction
E. Oxygen supply & cellular respiration
1. Myoglobin is molecule synthesized in muscle cells
that is capable of storing oxygen temporarily.
2. Cellular respiration is an aerobic (requires oxygen)
process that releases energy from glucose & forms
ATP. In other words, oxygen is necessary for
formation of ATP - muscle contraction requires ATP so… oxygen is necessary for muscle contraction.
3. In the absence of oxygen, a form of anaerobic
respiration called lactic acid fermentation supplies
the energy required for muscle contraction.
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IV. Skeletal Muscle Contraction
E. Oxygen supply & cellular respiration
4. Oxygen debt equals the amount of oxygen liver cells
require to convert accumulated lactic acid into
glucose, plus the amount of oxygen muscle cells
need to restore ATP & creatine phosphate to their
original concentrations.
??? How was lactic acid accumulated? During prolonged,
strenuous exercise the muscles so not have sufficient oxygen to
do aerobic respiration & must use anaerobic respiration to
obtain energy. The anaerobic respiration causes an accumulation
of lactic acid in muscles which diffuses into bloodstream &
eventually reaches the liver.
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IV. Skeletal Muscle Contraction
F. Muscle Fatigue
1. When a muscle is exercised strenuously for a
prolonged period it may lose its ability to contract.
Causes?
• Most likely an accumulation of lactic acid as a result of
anaerobic respiration; could also be caused by
• An interruption in the muscle’s blood supply, or
• A lack of acetylcholine in the motor neuron axons
2. Less than half the energy released by cellular
respiration is available for metabolic processes. The
rest is lost as heat.
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V. Muscular Responses
A.
1. Threshold stimulus is the minimum strength
needed for a stimulus to produce a muscle
contraction.
• When a muscle fiber is exposed to a single
stimulus of threshold strength it will contract once,
then relax = “twitch”
2. All-or-nothing response means that when a
muscle contracts, it contracts to its fullest
extent; there is no such thing as a partial
muscle contraction.
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V. Muscular Responses
A.
3. Summation: A muscle fiber receiving a
series of stimuli of increasing frequency
reaches a point when it is unable to relax
completely and the force of individual
twitches combine by the process of
summation.
If the sustained contraction lacks any
relaxation, it is called a tetanic contraction.
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V. Muscular Responses
A.
4. Recruitment : as the stimulus intensity
increases, the number of motor units
responding to the stimulus increases
(remember that a muscle fiber responds
in an all-or-nothing manner).
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V. Muscular Responses
C.
• Fast-twitch fibers
 produce strong
contractions but easily
 are activated during
high-intensity exercise
that requires strength
(ex: weight lifting)
• Slow-twitch fibers
are fatigue resistant
are activated during
low-intensity
exercise when
endurance is more
important than
strength (ex:
swimming, running)
• Typical people are about 50-50;
• Olympic sprinters >80% fast
twitch;
• Olympic marathoners >90%
slow twitch
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V. Muscular Responses
D. Contractions
Sustained Contraction
Muscle Tone
• The result of both recruitment
and summation producing
sustained contractions of
increasing strength. The
smaller motor units contract
first, followed by the larger
motor units (which can
contract more forcefully).
• Sustained contractions of
whole muscles produce the
movement necessary to
perform everyday functions.
• Even when a muscle appears
to be at rest, its fibers
undergo some sustained
contraction as a response to
nerve impulses that originate
repeatedly from the spinal
cord and stimulate a few
muscle fibers.
• Muscle tone is important in
maintaining posture. If muscle
tone is suddenly lost the body
collapses (ex: when a person
loses consciousness)
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VI. Smooth Muscles
A. Compare Smooth & Skeletal Muscle Fibers
The mechanisms of contractions are the same for both
smooth & skeletal muscles.
Both have actin & myosin fibers. But…
• Smooth muscle fibers have a more random arrangement
of the myosin & actin fibers so that smooth muscle lacks
the striated appearance of skeletal muscle.
Also, the sarcoplasmic reticulum of smooth muscle is
less well-develop that that in skeletal muscle
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VI. Smooth Muscles
B. Types of Smooth Muscles
Multi-unit Smooth
Muscle
• muscle fibers are
separate – not
organized in sheets
Visceral Smooth
Muscle
• muscle fibers are
composed of sheets
of spindle-shaped
cells in contact with
each other
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VI. Smooth Muscles
B. Types of Smooth Muscles
Multi-unit Smooth
Muscle
• found in irises of eye
and
• in the walls of blood
vessels
Visceral Smooth
Muscle
• found in the walls of
hollow organs
(stomach, intestines,
bladder, uterus,
intestines)
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VI. Smooth Muscles
B. Types of Smooth Muscles
Multi-unit Smooth
Muscle
• typically contracts
only in response to
stimulation by motor
nerve impulses or
certain hormones
Visceral Smooth
Muscle
• Visceral muscle fibers can
stimulate other visceral
muscle fiber
(self-exciting)
• also display rhythmicity
– a pattern of repeated
contractions
• These 2 properties allow
peristalsis to occur
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Visceral Smooth Muscles & Peristalsis
• Self-excitation and rhythmicity are largely
responsible for a wave-like motion called
peristalsis that occurs in certain tubular
organs like the intestines.
• Peristalsis helps force the contents of
these organs along their lengths.
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VI. Smooth Muscles
C.1 & C.2. Smooth Muscle Contraction
Skeletal muscles
Smooth Muscles
• One neurotransmitter
– acetylcholine
• Two
neurotransmitters –
acetylcholine and
norepinephrine
• Each neurotransmitter
can stimulate
contractions in some
muscles and inhibit
contractions in others
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VI. Smooth Muscles
C.3. Compare Smooth & Skeletal Muscle Contractions
• Smooth muscle is slower to contract & relax
than skeletal muscle
• Smooth muscle can maintain a forceful
contraction longer than skeletal muscle
• Smooth muscle can change length without
changing tautness – this means smooth muscles
in stomach & intestinal walls can stretch as
these organs fill, yet maintain the pressure
inside these organs
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VII. Cardiac Muscle
A. Sarcoplasmic Reticulum
The sarcoplasmic reticulum of cardiac muscle is
distinctive in 2 ways:
• larger transverse tubules and
• less well-developed cisternae that store less
calcium.
• The larger transverse tubules contain calcium
obtained from outside the muscle fibers and
release larger numbers of calcium ions.
This results in a longer cardiac muscle
twitch than found on skeletal muscle.
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VII. Cardiac Muscle
B. & C.
B.
Opposing ends of cardiac muscle fibers are connected by
intercalated discs which allow muscle
impulses to pass freely so that they travel
rapidly from cell-to-cell.
C. When one portion of the cardiac muscle is
stimulated, the impulse spreads to other
fibers of the network and the whole network
contracts as a unit (self-excitation & rhythmicity)
• Cardiac muscle is self-exciting & rhythmic so
that it does not require stimulation outside
the heart to produce regular periods of
contraction & relaxation.
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VIII. Skeletal Muscle Actions
A. Muscle Attachments
• Each muscle has at least two points of
attachment to bone: the origin and the
insertion.
• The origin is the end of a skeletal muscle
that is immovable (or relatively immovable)
• The insertion is the end of a skeletal muscle
that movable end.
• When a muscle contracts, its insertion is
pulled toward its origin.
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VIII. Skeletal Muscle Actions
B. Muscle Movement
• The prime mover (aka agonist) is the
muscle that provides most of the
movement for a particular body movement
• A synergist is a muscle that contracts
and assists the prime mover, making its
action more effective.
• An antagonist is a muscle that can resist
a prime mover’s action and/or cause
movement in the opposite direction.
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VIII. Skeletal Muscle Actions
B. Muscle Movement
• If an agonist and its antagonist contract
simultaneously, the part they act upon
remains rigid.
• Smooth body movements depend on
antagonists relaxing and giving way to
prime movers whenever the prime mover
contracts.
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VIII. Skeletal Muscle Actions
B. Muscle Movement
Flexion & Extension are terms that describe changes in
the angle where two bones meet:
Flexion decreases the
angle where 2 bones meet
Extension increases the
angle where 2 bones meet
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Muscle Functions
1. producing movement- result of contraction
2. maintaining posture- via skeletal muscles
– overcoming gravity effects while sitting or
standing
3. stabilizing joints- pull of skeletal muscles on
bones
4. generating heat- by-product of muscle
activity
– 75% of ATP energy creates heat (only 25% used to
contract muscle)
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Interesting Tidbits
•
•
•
•
•
•
Rigor mortis
Botulinus toxin
Motor unit
Muscle strain
Tendinitis
Muscle pull
• Does it take less
energy (ATP) to smile
or to frown? Explain!
• What is TMJ? What
causes it?
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Do you use more muscles to smile than
to frown?
"It takes one more muscle to smile than to frown, according to
plastic surgeon David H. Song, MD, FACS, assistant professor
at the University of Chicago Hospitals.
Only Cecil's "The Straight Dope" got an expert (Dr. Song) to go through the
motions. A genuine smile takes two muscles to crinkle the eyes, two to
pull up the lip corners and nose, two to elevate the mouth angle, and two
to pull the mouth corners sideways. Total smile: 12.
On the other hand, a frown needs two muscles to pull down the lips and
wrinkles in the lower face, three to furrow the brow, one to purse the lips,
one to depress the lower lip, and two to pull the mouth corners down.
Total frown: 11.
A fake smile, however, only takes two muscles. We detect the fake
because "the eyes aren't smiling."
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Muscle Trivia
• Smallest muscle in the human body (name
& location).
• Largest muscle in the human body (name
& location).
• Longest muscle in the human body (name
& location).
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Muscle Names
• The name of a muscle often describes it:
may indicated size &/or location: pectoralis
major is located in the pectoral (chest)
region and is of large size;
deltiod is shaped like the Greek letter delta
(a triangle);
biceps brachii has two heads (points of
origin) and is located in the arm;
external oblique is located near the outside
with fibers that run obliquely (at an angle)
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