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Chapter 12
Muscular System
Points to Ponder
• What are the three types of muscle tissue?
• What are the functions of the muscular system?
• How are muscles named and what are the muscles of the
human body?
• How are skeletal muscles and muscle fibers structured?
• How do skeletal muscles contract?
• How do skeletal muscle cells acquire ATP for contraction?
• What is rigor mortis?
• What are some common muscular disorders?
• What are some serious muscle diseases?
• How do the skeletal and muscular system help maintain
homeostasis?
• How are these 2 systems related to other systems in
maintaining homeostasis?
• Skeletal muscle
Muscle Tissue
– Voluntary striated muscle
• controlled by nerves of the central nervous system
– Multinucleated and tubular
– Attacked to skeleton
• Cardiac muscle
– Involuntary striated muscle
– Uninucleated, tubular, and branched
– Intercalated disks contain gap junctions
• spread contractions quickly throughout the heart wall
– Cardiac fibers relax completely between contraction
• Prevent fatigue
• Smooth muscle
–
–
–
–
–
Involuntary nonstriated muscle
Uninucleated
Cells arranged in parallel lines, forming sheets
Located in the walls of hollow internal organs
Slower to contract, sustain prolonged contractions
• does not fatigue easily
12.1 Overview of the muscular system
Review: 3 types of muscle tissue
Functions of skeletal muscles
1. Support the body by allowing us to stay upright
2. Allow for movement by attaching to the skeleton
3. Help maintain a constant body temperature
-
Contraction causes ATP to break down, releasing heat
4. Assist in movement in the cardiovascular and
lymphatic vessels via muscular contractions
5. Protect internal organs and stabilize joints
-
Muscles pad bones
Tendons hold bones together at joints
12.1 Overview of the muscular system
How are skeletal muscles arranged?
• Attachments:
• Tendon – connective tissue that connects muscle to bone
• Muscles covered with fascis that extends beyond the muscle and becomes
the tendon
• Origin – attachment of a muscle on a stationary bone
• Insertion – attachment of a muscle on a bone that moves
• Muscle contraction pulls on the tendon at its insertion causing the bone to
move
• Action:
• Nervous system does not stimulate a single muscle, it stimulates an
appropriate group of muscles
1. Prime mover – muscle that does most of the work
2. Antagonistic – muscles that work in opposite pairs
- biceps brachii and triceps brachii are antagonist
3. Synergistic – muscles working in groups for a common action
- assist the prime mover, enhances action
An example of muscle arrangement
How to Name Skeletal Muscles
• Size
– Maximus: largerst; Minimus: smallest
– Vastus: huge; Longus: long; Brevis: short
• Shape
– Deltoid: triangular (Greek letter delta is Δ)
– Trapezius: trapezoid; Latissimus: wide; Terres: round
• Location
– Frontalis overlies the frontal bone
– External and internal obliques; pectoralis; gluteus; brachii
How to Name Skeletal Muscles
• Direction of muscle fiber
– Rectus abdominus (rectus means straight)
– Transverse: across; oblique: diagonal
• Attachment
– Brachioradialis: attached to the brachium and radium
– Sternocleidomastoid: attached to sternum, clavicle, and mastoid
• Number of attachments
– the biceps brachii has two attachments
• Action
– extensor digitorum extends the digits
– adductor (to midline), flexor (flexes), levator (lift)
Muscle fibers/cells
• Terminology for cell structure
– The plasma membrane is called the sarcolemma
• Transverse tubules penetrate cell to contact
with sarcoplasmic reticulum
– The cytoplasm is called the sarcoplasm
• Contains glycogen (store energy) and
myoglobin (binds oxygen) needed for muscle
contraction
– The SER of a muscle cell is called the
sarcoplasmic reticulum and stores calcium
Skeletal Muscle Fibers
Sarcoplasm contains networks of SER called
sarcoplasmic reticulum (SR)
• Sarcoplasmic Reticulum:
– Function:
• store calcium and help transmit action potential to
myofibril
– SR forms chambers attached to T-tubules
• Concentrate Ca2+ (via ion pumps)
• Release Ca2+ into sarcomeres to begin muscle
contraction
• All calcium is actively pumped from
sarcoplasm to SR (SR has 1000X more Ca2+
than sarcoplasm)
Skeletal Muscle Fibers
• Terminology for structure within a whole
muscle
– Muscle fibers are arranged in bundles called
fascicles
– Myofibrils are a bundle of myofilaments that
run the length of a fiber
– Myofilaments are proteins (actin and myosin)
that are arranged in repeating units
– Sarcomeres are the repeating units of actin
and myosin found along a myofibril
12.2 Skeletal muscle fiber contraction
12.2 Skeletal muscle fiber contraction
The sarcomere
• Made of two protein myofilaments
– Myosin: are the thick filaments shaped like
a golf club
• Globular head allows for cross-bridges
– Actin: are the thin filaments
• Two other proteins are present
–Tropomyosin and troponin
– These filaments slide over one another during
muscle contraction
Actin
Myosin
Regions of the Sarcomere
1. A-band:
- whole width of thick filaments, looks dark
microscopically
2. M line: at midline of sarcomere
- Center of each thick filament, middle of A-band
- Attaches neighboring thick filaments
3. H-zone:
- Light region on either side of the M line
- Contains thick filaments only
4. Zone of overlap:
- ends of A-bands
- place where thin filaments intercalate between thick
filaments (triads encircle zones of overlap)
Regions of the Sarcomere
3. I-band:
- Contains thin filaments outside zone of overlap
- Not whole width of thin filaments
4. Z lines/disc:
- the centers of the I bands
Sliding Filaments
Figure 10–8
Sliding Filament Theory
Contraction of skeletal muscle is due to thick filaments and thin
filament sliding past each other
– not compression of the filaments
1. H-zones and I-bands decrease width during contraction
2. Zones of overlap increase width
3. Z-lines move closer together
4. A-band remains constant
Sliding causes shortening of every sarcomere in every myofibril in
every fiber
Overall result = shortening of whole skeletal muscle
12.2 Skeletal muscle fiber contraction
The beginning of muscle contraction: The
sliding filament model
1. Nerve impulses travel down motor neurons to a
neuromuscular junction
2. Acetylcholine (ACh) is released from the neurons
and bind to the muscle fibers
3. This binding stimulates fibers causing calcium to
be released from the sarcoplasmic reticulum
Skeletal Muscle:
Neuromuscular Junction
Figure 10–10a, b (Navigator)
12.2 Skeletal muscle fiber contraction
Muscle contraction continued…
4. Released calcium combines with troponin, a
molecule associated with actin
5. This causes the tropomyosin threads around actin
to shift and expose myosin binding sites
6. Myosin heads bind to these sites forming crossbridges
7. ATP bind to the myosin heads and is used as
energy to pull the actin filaments towards the
center of the sarcomere = contraction now occurs
1. ATP is bind to myosin
2. ATP is split to ADP and 2 Phosphates
3. ADP and P remain on the myosin heads
- Heads attach to an actin filament forming cross bridges
4. ADP and P is released, cross-bridges bend sharply
- Power stroke pulls action filament toward the center of the
sarcomere
5. ATP molecule binds again to myosin to break cross bridge
12.2 Skeletal muscle fiber contraction
12.2 Skeletal muscle fiber contraction
What role does ATP play in muscle
contraction and rigor mortis?
•
ATP is needed to attach and detach the myosin heads
from actin
•
After death muscle cells continue to produce ATP
through fermentation and muscle cells can continue to
contract
•
When ATP runs out some myosin heads are still
attached and cannot unattach = rigor mortis
•
Body temperature and rigor mortis helps to estimate
the time of death
Tension Production
• For a single muscle fiber contraction is all–
or–none:
– as a whole, a muscle fiber is either contracted
or relaxed
Resting Length
• Greatest tension produced at optimal resting length
– Optimal resting length = Optimum overlap
– Overlap determines the number of pivoting cross-bridges
Figure 10–14
Frequency of Stimulation
•
Twitch = single contraction due to a single
neural stimulation, 3 phases:
1. Latent period: post stimulation but not tension
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Action potential moves across the sarcolemma
Ca2+ is released
2. Contraction phase: peak tension production
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Ca2+ bind
Active cross bridge formation
3. Relaxation phase: decline in tension
-
Ca2+ is reabsorbed
Cross bridges decline
12.3 Whole muscle contraction
Terms in whole muscle contraction
•
Motor unit – a nerve fiber and all of the muscle fibers it
stimulates
•
Muscle twitch – a single contraction lasting a fraction of
a second
•
Summation – an increase in muscle contraction until
the maximal sustained contraction is reached
•
Tetanus – maximal sustained contraction
•
Tone – a continuous, partial contraction of alternate
muscle fibers causing the muscle to look firm
12.3 Whole muscle contraction
Physiology of skeletal muscle contraction
Total Number of Muscle Fibers
Stimulated
• Each skeletal muscle has thousands of fibers
organized into motor units
• Motor units = all fibers controlled by a single
motor neuron
– Axon branches to contact each fiber
• Number of fibers in a motor unit depends on the
function
– Fine control: 4/unit (e.g. eye muscles)
– Gross control: 2000/unit (e.g. leg muscles)
• Fibers from different units are intermingled in the
muscle so that the activation of one unit will
produce equal tension across the whole muscle
Motor Units in a Skeletal Muscle
Figure 10–17
Recruitment (Multiple
Motor Unit Summation)
• In a whole muscle or group of muscles, smooth
motion and increasing tension is produced by
slowly increasing size or number of motor units
stimulated
• Recruitment = order of activation of a motor unit
– Slower weaker units are activated first
– Strong units are added to produce steady increases
in tension
Contraction Skeletal Muscle
• During sustained contraction of a muscle
– Some units rest while others contract to avoid fatigue
• For maximum tension, all units in complete tetanus
– Leads to rapid fatigue
• Muscle tone = maintaining shape/definition of the
muscle
– Some units are always contracting
– Exercise = Increase # of units contraction 
Increase in metabolic rate 
Increase in speed of recruitment (better tone)
12.3 Whole muscle contraction
Where are the fuel sources for muscle
contraction?
•
Stored in the muscle:
– Glycogen
– Fat
•
In the blood:
– Glucose
– Fatty acids
12.3 Whole muscle contraction
What are the sources of ATP for
muscle contraction?
•
•
Limited amounts of ATP are stored in muscle fibers
Creatine phosphate pathway (CP) –
– fastest way to acquire ATP
– sustains a cell for seconds
– builds up when a muscle is resting
•
Fermentation –
– fast-acting but results in lactate build up
•
Cellular respiration (aerobic) –
– not an immediate source of ATP
– best long term source
12.3 Whole muscle contraction
Acquiring ATP for muscle contraction
Muscle fibers come in two forms
•
Fast-twitch fibers:
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rely on CP and
fermentation (anaerobic)
– Rely on aerobic respiration
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– Designed for endurance
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Lactate buildup is possible
which leads to quick fatigue
• Tire when fuel supply is gone
• Long distance running
• Biking and swimming
Designed for strength
-
-
• Slow-twitch fibers:
Provide explosions of
energy, develop max.
tension more rapidly
(sprinting, weight lifiting)
Light in color
Few mitochondria
Little or no myoglobin
Fewer blood vessels than
slow-twitch
Motor units contain many
fibers
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Dark in color
Many mitochondria
Myoglobin
Many blood vessels
• Provide oxygen
– Low maximum tension but
high resistance to fatigue
12.3 Whole muscle contraction
Types of muscle fibers
12.3 Whole muscle contraction
Health focus: Benefits of exercise
•
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Increases muscle strength, endurance and flexibility
Increases cardiorespiratory endurance
– Heart rate and capacity increase
– Air passages dilate so that the heart and lungs are able to
support prolonged muscular activity
•
HDL increases thus improving cardiovascular health
– Prevents development of plaque
•
•
Proportion of protein to fat increases favorably
May prevent certain cancers :
– colon, breast, cervical, uterine and ovarian
•
•
Improve density of bones thus decreasing the
likelihood of osteoporosis
Enhances mood and may relieve depression
Common muscle disorders
• Spasms
– sudden, involuntary muscle contractions that are
usually painful
• Seizure
– multiple spasms of skeletal muscles
• Cramps
– strong, painful spasms often of the leg and foot
• Strain
– stretching or tearing of a muscle
Common muscle disorders
• Sprain
– twisting of a joint involving muscles,
ligaments, tendons, blood vessels and nerves
• Tendonitis
– inflammation of a tendon usually due to
overuse (i.e. tennis elbow)
• Bursitis
– inflammation of a bursa usually from repetitive
use or frequent pressure
Muscular diseases
•
Fibromyalgia
– chronic achy muscles that is not well understood
•
Muscular dystrophy
– group of genetic disorders in which muscles
progressively degenerate and weaken
•
Myasthenia gravis
– autoimmune disorder that attacks ACh receptor and
weakens muscles of the face, neck and extremities
•
Amyotrophic lateral sclerosis (ALS)
– commonly known as Lou Gehrig’s disease in which
motor neurons degenerate and die leading to loss of
voluntary muscle movement
12.5 Homeostasis
Homeostasis: the skeletal and muscular
systems
• Both systems are involved with movement
that allows us to respond to stimuli, digestion
of food, return of blood to the heart and
moving air in and out of the lungs
• Both systems protect body parts
• Bones store and release calcium need for
muscle contraction and nerve impulse
conduction
• Blood cells are produced in the bone
• Muscles help maintain body temperature
12.5 Homeostasis
Bioethical focus: Anabolic steroids?
• Anabolic steroids are a group of steroids that
usually increase protein production
• Most common side effects are high blood
pressure, jaundice, acne and great increased
risk of cancer
• Abuse of these drugs may also cause
impotence and shrinking of the testicles
• May lead to increased aggressiveness and
violent mood swings
• Are they worth the risk?
• Should they be legal to use in athletics?