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The Muscular
System
Biology 105
Lecture 12
Chapter 6
Outline
I. Characteristics of muscles
II. Three types of muscles
III. Functions of muscles
IV. Structure of skeletal muscles
V. Mechanics of muscle contraction
VI. Energy source for muscle contraction
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Muscular System
 Recall there are different types of muscles:
smooth, cardiac, and skeletal.
 All muscle cells are elongated, and therefore
are called muscle fibers.
 All muscle tissues contract.
 Muscles contain muscle fibers, connective
tissue, blood vessels, and nerves.
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Smooth Muscle
 Smooth muscles are involuntary muscles
found in the walls of many internal organs
(digestive tract, respiratory system, blood
vessels).
 They aid in the function of other organs.
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Cardiac Muscle
 Cardiac muscles are involuntary muscles
found only in the heart wall.
 They function by contracting, which forces
blood from the heart into the arteries.
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Skeletal Muscle
 Skeletal muscles are voluntary muscles
attached to the skeleton.
 They usually work in pairs.
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Skeletal Muscles Work in Pairs
 Most skeletal muscles work in antagonistic
pairs:
 One muscle contracts, while the other relaxes.
 Muscles are attached to the bone by tendons.
 Skeletal muscles are usually attached to two
bones on opposite sides of a joint.
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Skeletal Muscles Work in Pairs
 The origin of the muscle is attached to the
bone that remains stationary during
movement.
 The insertion is attached to the bone that
moves.
 Bones act as levers in working with skeletal
muscles to produce movement.
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Skeletal Muscles Work in Pairs
Origin of muscle:
attachment of muscle
to less moveable bone
The biceps contracts
and pulls the forearm
up, flexing the arm.
The relaxed triceps
is stretched.
(a) Flexion
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Insertion of muscle:
attachment of muscle
to more moveable bone
Figure 6.1a
Functions of Skeletal Muscles
1. Support the body – maintain posture
2. Movement of bones and other tissues
3. Help maintain a constant body temperature –
generate heat
4. Help move blood through the veins and
lymphatic fluid through the lymphatic vessels
5. Help to protect vital organs and stabilize
joints
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Structure of Skeletal Muscles
 Muscles are covered by connective tissue
called fascia.
 A muscle contains bundles of skeletal muscle
fibers (muscle cells):
 The bundles are called fascicles.
 These bundles are covered by connective tissue.
 Blood vessels and nerves are between the
fascicles.
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Structure of Skeletal Muscles
Skeletal muscle
consists of
many bundles
of muscle cells.
A muscle cell
consists of many
myofibrils.
A bundle of
muscle cells is
called a fascicle.
(a) A section of a
skeletal muscle
The striped (striated)
appearance of a skeletal
muscle cell is due to the
regular arrangement of
myofilaments.
(b) A light micrograph of a longitudinal view of skeletal muscle cells
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Figure 6.3a–b
Muscle Cell Components
 Muscle cells (muscle fibers) have many of the
same components as typical cells, but some of
their components have different names…
 Sarcolemma – plasma membrane (cell
membrane).
 Sarcoplasm – similar to cytoplasm, and contains
large amounts of stored glycogen and myoglobin.
 Myoglobin is an oxygen-binding protein similar to
hemoglobin, but found only in muscles.
 Sarcoplasmic reticulum – similar to endoplasmic
reticulum, and functions as a Ca2+ store.
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Muscle Cell Components
 Muscle fibers also have unique features:
 Multiple nuclei
 Transverse tubules (T tubules) – extensions of
the sarcolemma that come into contact with the
sarcoplasmic reticulum.
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c. myofibril
b. Sarcoplasmic reticulum
a. T tubule
d. Z line
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e. sarcomere
f. sarcolemma
Muscle Cells (Fibers)
 The muscle fiber is composed of long, thin
myofibrils.
 Myofibrils are bundles of myofilaments that
contract:
 There are two types of myofilaments: actin and
myosin.
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Muscle Contraction
 A sarcomere is the name for the structural unit
of these myofilaments.
 When you look at the myofibril, the sarcomere
lies between two dark lines called Z lines:
 The Z lines are protein sheets where the actin
filaments attach.
 When muscle fibers are stimulated to contract,
myofilaments slide past one another, causing
sarcomeres to shorten.
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Sarcomeres
The striped (striated)
appearance of a skeletal
muscle cell is due to the
regular arrangement of
myofilaments.
(b) A light micrograph of a longitudinal view of skeletal muscle cells
Z line
One sarcomere
(c) A diagram and electron micrograph of a myofibril
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Figure 6.3b–c
Sarcomeres
Z line
One sarcomere
(c) A diagram and
electron micrograph
of a myofibril
Z line
One sarcomere
Z line
Actin
Myosin
(d) A sarcomere, the contractile unit of a skeletal muscle, contains actin and
myosin myofilaments.
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Figure 6.3c–d
Myofilaments – Actin and Myosin
 The two myofilaments are:
 Actin filaments: thin filaments that are formed by
two intertwining strands of the protein actin.
 Myosin filaments: thick filaments of the protein
myosin that are shaped like a golf club with a
round “head”.
 The myosin heads can bind and detach from
the thin actin filament.
 When bound, they create cross-bridges.
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Muscle Contraction
 A neuron signals the muscle to contract.
 The myosin heads attach to the actin, and
then pull the actin toward the center of the
sarcomere.
 The myosin heads detach.
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Sarcomeres Shorten During Muscle Contraction
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Figure 6.4
Steps of Muscle Contraction
1. Action potentials are transmitted through the
neurons.
2. At the end of the neurons, neurotransmitters
are released into the synaptic cleft.
3. Neurotransmitters bind to receptors on the
sarcolemma.
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Neuromuscular Junction
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Figure 6.7 (1 of 2)
Steps of Muscle Contraction
4. These receptors are ion channels that open.
5. An action potential travels through the T
tubules of the muscle fiber.
6. The action potential goes to the sarcoplasmic
reticulum.
7. The sarcoplasmic reticulum releases Ca2+.
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Steps of Muscle Contraction
8. The calcium binds to the troponin on the actin
filament.
9. This uncovers the binding site for the myosin
to attach.
10. Now the myosin binds to the actin.
11. ATP is needed for the myosin to slide past
the actin.
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Sarcomeres
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Figure 6.6 (1 of 2)
Sarcomeres
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Figure 6.6 (2 of 2)
Tropomyosin-Troponin Complex
 The tropomyosin-troponin complex is
attached to the actin filament.
 Calcium binds to the troponin, causing a shift
in the complex, which opens the sites for
myosin to attach.
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A bundle of muscle cells is called a:
1. Fascicle
2. Fascia
3. Muscle fiber
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33%
33%
33%
What is the oxygen-binding protein found only in
muscles?
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n
M
yo
gl
ob
i
in
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b
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A
yo
si
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25% 25% 25% 25%
Myosin
Actin
Hemoglobin
Myoglobin
n
1.
2.
3.
4.
Which ion is required for the myofilaments to bind
to each other?
di
um
So
hl
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C
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Po
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C
Potassium
Calcium
Chloride
Sodium
si
um
1.
2.
3.
4.
25% 25% 25% 25%
Where is the calcium stored?
1. Nucleus
2. Sarcolemma
3. Sarcoplasmic
reticulum
a
s
Sa
rc
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pl
as
Sa
m
ic
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N
m
33% 33% 33%
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ATP is needed for the myofilaments to slide past each other
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ATP
 ATP is the energy
currency – like money
in the bank!
 The bonds between
the phosphate groups
are high energy
bonds.
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The Energy Sources
 Muscle contractions take a lot of energy in the
form of ATP.
 Muscles get their ATP from three sources:
1. Breakdown of creatine phosphate
2. Cellular respiration
3. Fermentation
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1. Creatine Phosphate
 Creatine phosphate regenerates ADP to make
ATP.
 This gives quick energy for a few seconds (up
to 30 seconds).
 Only 1 ATP is produced per creatine
phosphate.
 Oxygen is not needed.
 When a muscle is resting, the ATP in turn
regenerates creatine phosphate.
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2. Cellular Respiration
 In the mitochondria, glucose is broken down to
produce ATP.
 Remember that oxygen is needed for the
electron transport chain to produce ATP.
 Carbon dioxide is produced as a waste product
during the Krebs Cycle of cellular respiration.
 Can provide energy for hours.
 Produces 36 ATP per glucose molecule.
 Can use glucose, as well as fatty acids and
amino acids, for the energy source.
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3. Fermentation
 This is when the cell only uses glycolysis, and
glucose is broken down to lactic acid.
 Since the Krebs Cycle and the electron
transport chain are skipped, no oxygen is
required.
 No CO2 is produced as a waste product, but
lactic acid is produced.
 Can provide energy for 30 – 60 seconds.
 Only 2 ATP produced per glucose molecule.
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ATP Comes from Many Sources
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Figure 6.10
CP
Breakdown
Cellular
Respiration
Fermentation
Requires O2
No
Yes
No
Produces
CO2
# ATP
produced
No
Yes
No
1
36
2
Duration
30 sec
Hours
30-60 sec
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Which energy source would a long-distance runner
mainly use on a run that lasted for hours?
25% 25% 25% 25%
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1. Fermentation
2. Cellular respiration
3. Creatine phosphate
Which energy source would a sprinter use in the
first 5 seconds of the race?
25% 25% 25% 25%
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Ph
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1. Fermentation
2. Cellular respiration
3. Creatine phosphate
Important Concepts
 Read Chapter 6
 What are the three types of muscles? Where
are they found, and are they under voluntary or
involuntary control?
 What are the functions of skeletal, cardiac, and
smooth muscles?
 How do skeletal muscles work in pairs?
 What is the function of tendons?
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Important Concepts
 What is the overall structure of a muscle? What
are the components of a muscle, and of a
muscle cell (muscle fiber)? What are the
functions of the muscle fiber components?
 You should be able to identify the muscle fiber
components in an illustration, including:
myofibrils, sarcomeres, Z lines, myofilaments
(actin and myosin filaments), cross-bridges,
sarcolemma, sarcoplasm, sarcoplasmic
reticulum, T tubules
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Important Concepts
 What stimulates a muscle to contract?
 You should be able to describe the steps of
how the message is transmitted from the
neuron to the myofilaments…
 What is the role of Ca2+?
 What happens when the message is received
by the myofilaments?
 What are the components and the function of
the tropomyosin-troponin complex?
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Important Concepts
 What are the three energy sources used for
muscle contraction? Which of these require
oxygen and which produce carbon dioxide?
How many ATP are produced, and how long
can each energy source provide energy?
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Definitions
 Muscle fiber, myoglobin, fascia, fascicle,
myofibril, sarcomere, involuntary, voluntary,
origin, insertion
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