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The Muscular System
Biol 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
 Remember there were different types of
muscles: cardiac, smooth and skeletal.
 All muscle cells are elongated and therefore
are called muscle fibers.
 All muscle tissues contract.
 Muscles contain muscle cells (called muscle
fibers), connective tissue, blood vessels, and
nerves
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Types of Muscles
1. Smooth muscle
2. Cardiac muscle
3. Skeletal muscle
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11-2
Smooth muscle
 Smooth muscles are involuntary muscles found
in the walls of many internal organs (digestive
tract, respiratory system, blood vessels).
 Function to aid in the function of other organs
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11-2
Cardiac muscle
 Cardiac muscles are involuntary muscles found
only in the heart wall.
 Functions by contracting to force blood from the
heart into the arteries
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11-2
Skeletal muscle
 Skeletal muscle are voluntary muscles attached
to the skeleton.
 Usually work in pairs
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Skeletal Muscles Work in Pairs
 Most skeletal muscles are antangonistic pairs.
 One muscle contracts, 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 our posture
2. Movement of bones, and other tissues
3. Help maintain a constant body temperature
– generates heat
4. Helps 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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Smooth muscles are under this kind of control
1. Voluntary
2. Involuntary
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Smooth muscles are found in
1. The heart
2. Digestive tract
3. Attached to bones
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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
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
A bundle of muscle cells is called:
1. Fascicles
2. Fascia
3. Muscle Fibers
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Muscle Cells
 Muscle cells are long cells called muscle
fibers.
 The muscle fiber is composed of long thin
myofibrils
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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 cont
 Myofibrils are bundles of myofilaments that
contracts.
 Myofilaments are made of actin and myosin
filaments.
 When muscle fibers are stimulated to
contract, myofilaments slide past one
another, causing sarcomeres to shorten.
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Muscle Cell Components
 Muscle cells (muscle fibers) have many of the
same components as typical cells have but
some of their components have different names
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Muscle Cell Components
 Sarcolemma – plasma membrane (cell
membrane)
 Sarcoplasm – similar to cytoplasm, contains
large amount 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, one of its functions is to store Ca2+
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Muscle Cell Components
 Muscle cells (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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Muscle Contraction
 The small myofibrils that make up the muscle
fiber (muscle cell) contain two types of
myofilaments: actin and myosin filaments
 Sarcomere is the name for the structural unit
of these myofilaments
 The sarcomere goes between two dark lines
= Z lines. The Z lines are protein sheets
where the actin filaments attach
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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 formed by two
intertwining strands of the protein actin.
 Myosin filaments: Thick filaments of the protein
myosin shaped like a golf club, with a round
“head”.
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Myofilaments – actin and myosin
 The myosin heads can bind and detach from
the thin actin filament. When bound it creates
cross-bridges.
 When the muscle is stimulated, these filaments
slide past each other, making the sarcomere to
shorten
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Muscle Contraction cont
 A neuron signals the muscle to contract
 The myosin heads attach to the actin then
pull the actin toward the center of the
sarcomere
 Then the myosin heads detach
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Sarcomeres
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Figure 6.4
Neuromuscular Junction
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Figure 6.7 (1 of 2)
Steps of Muscle Contraction
1. Action potentials are transmitted through the
neurons.
2. At the end of the neurons neurotransmitters are
released
3. Neurotransmitters bind to receptor on the
sarcolemma
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Steps of Muscle Contraction
4. The receptors are ion channels that open
5. An action potential travels through the T
tubules
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 opens up 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, opening the sites for
myosin to attach.
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ATP is needed for the myofilaments to slide past each other
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What is the an oxygen binding protein found only
in muscles?
1.
2.
3.
4.
Myosin
Actin
Hemoglobin
Myoglobin
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What ion is required for the myofilaments to bind
to each other?
1.
2.
3.
4.
Potassium
Calcium
Chloride
Sodium
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Where is the calcium stored?
1. Nucleus
2. Sarcolemma
3. 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
ATP
 ATP is the currency. Like money in the bank.
 The bonds between the phosphate groups
are high energy bonds
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The Energy Source
 Muscle contractions take a lot of energy in
the form of ATP.
 Muscles get their ATP from three sources:
 1. The 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 sec)
 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 on the electron
transport chain to produce the ATP.
 Carbon dioxide is produced as a waste product
during the Krebs cycle step in 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 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 is skipped, no oxygen is
required.
 No CO2 is produced as a waste produce but
lactic acid is produced
 Can provide energy for 30 – 60 sec
 2 ATP produced per glucose molecule
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ATP Comes from Many Sources
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Figure 6.10
ATP Comes from Many Sources
6 seconds
ATP stored
in muscles
10 seconds
30–40 seconds
ATP formed from
creatine phosphate
and ADP
ATP generated from
glycogen stored in muscles
and broken down to form
glucose
Oxygen limited
• Glucose oxidized
to lactic acid
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Figure 6.10 (1 of 2)
ATP Comes from Many Sources
End of exercise
ATP generated from glycogen
stored in muscles and broken
down to form glucose
Oxygen present
• Heart beats faster to deliver
oxygen more quickly
• Myoglobin releases oxygen
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After prolonged exercise
Oxygen debt paid back
Breathe heavily to deliver oxygen
• Lactic acid used to produce ATP
• Creatine phosphate restored
• Oxygen restored to myoglobin
• Glycogen reserves restored
Figure 6.10 (2 of 2)
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?
1. Fermentation
2. Cellular respiration
3. Creatine
Phosphate
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Which energy source would a sprinter use in the
first 5 seconds of the race?
1. Fermentation
2. Cellular respiration
3. Creatine
Phosphate
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Important Concepts
 Read Chapter 10 for next lecture
 What are the three types of muscles, where are
they found, are they under vol. or invol control
 What are the functions of skeletal, cardiac and
smooth muscles
 How do skeletal muscles work in pairs?
 What is the structure and the components of a
muscle, and of a muscle cell (muscle fiber) and the
functions of the muscle cell components.
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Important Concepts
 What is the function of tendons?
 What stimulates a muscle to contract
 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?
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Important Concepts
 What are the components of the muscle fibers,
their functions, be able to identify them in an
illustration, including: myofibrils, sarcomeres, Z
lines, the myofilaments - actin and myosin
filaments, cross-bridges, sarcolemma,
sarcoplasm, sarcoplasmic reticulum, T-tubules
 What are the components and the function of the
tropomyosin-troponin complex
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Important Concepts
 What are the three energy sources for muscle
contraction, which require oxygen, which produce
carbon dioxide, how many ATP are produced,
how long can it provide energy
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Definitions
 muscle fibers, Myoglobin, fascia, fascicles,
myofibrils, sarcomere, involuntary, voluntary,
origin, insertion
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