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© 2014 Pearson Education, Inc.
Lecture Presentation
Anne Gasc
Hawaii Pacific University and
University of Hawaii–Honolulu Community College
BIOLOGY OF HUMANSConcepts, Applications, and Issues
Fifth Edition
Judith Goodenough Betty McGuire
6The Muscular System
© 2014 Pearson Education, Inc.
The Muscular System
OUTLINE:
Function and Characteristics of Muscles
Skeletal Muscles Working in Pairs
Contraction of Muscles
Voluntary Movement
Energy for Muscle Contraction
Slow-Twitch and Fast-Twitch Muscle Cells
Building Muscle
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Function and Characteristics of Muscles
All muscles are
Excitable (they respond to stimuli)
Contractile (they can shorten)
Extensible (they can stretch)
Elastic (they can return to their original length after
being shortened or stretched)
Three types of muscle
Skeletal
Cardiac
Smooth
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Function and Characteristics of Muscles
Skeletal muscles are voluntary muscles responsible
for
Moving our body
Maintaining posture
Supporting internal organs
Pushing against veins and lymphatic vessels to move
blood and lymph along
Generating heat
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Skeletal Muscles Working in Pairs
The body has more than 600 skeletal muscles
Synergistic muscles
Muscles that must contract at the same time to cause
movement
Antagonistic muscles
Movement is produced when one muscle of the pair
contracts and the other relaxes
Example: the biceps muscle and triceps muscle of the
upper arm
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Figure 6.1 Skeletal muscle.
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Figure 6.2 Some major muscles of the body.
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Damage to Skeletal Muscles or Tendons
Most of the major muscles we use for locomotion, manipulation, and other voluntary movements are attached to bones
Tendon
Band of connective tissue that attaches a muscle to a bone
Origin of a muscle
The end attached to the bone that remains relatively stationary during movement
Insertion of a muscle
The end attached to the bone that moves
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Damage to Skeletal Muscles or Tendons
Tendinitis
Condition of having an inflamed tendon
Caused by overuse, misuse, or age
Healing is slow because tendons have a poor blood
supply
Most effective treatment is rest
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Damage to Skeletal Muscles or Tendons
Muscle pull
Also called a muscle strain or tear
Caused by overstretching that damages the muscle or
tendon
Treatment includes ice to reduce swelling and
keeping the muscle stretched
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Contraction of Muscles
An entire, intact muscle is formed from individual
muscle fibers grouped in increasingly larger bundles,
each wrapped in a connective tissue sheath
Fascicle: a bundle of muscle cells
A skeletal muscle has many fascicles
Each fascicle is surrounded by its own connective
tissue sheath
The connective tissue sheaths of fascicles merge at
the ends of muscles to form tendons that attach the
muscle to bone
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Contraction of Muscles
A muscle cell a muscle fiber
When skeletal muscle cells are viewed under a
microscope, they have distinct bands called
striations formed by the arrangement of myofibrils
within the cell
Myofibrils are specialized bundles of proteins
Each myofibril contains two types of myofilaments
Myosin (thick) filaments
Actin (thin) filaments are more numerous
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Contraction of Muscles
Each myofibril has tens of thousands of contractile
units, called sarcomeres. The ends of each
sarcomere are marked by dark protein bands called
Z lines. Within each sarcomere the actin and myosin
filaments are specifically arranged
One end of each actin filament is attached to a Z line
Myosin filaments lie in the middle of the sarcomere,
and their ends partially overlap with surrounding actin
filaments. The degree of overlap increases when the
muscle contracts
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Figure 6.3 The structure of a skeletal muscle.
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Contraction of Muscles
Muscle contraction occurs at the molecular level
According to the sliding filament model, a muscle
contracts when actin filaments slide past myosin
filaments, shortening the sarcomere
Myosin molecules are shaped like two-headed golf
clubs. The club-shaped myosin heads are key to
moving actin filaments
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Contraction of Muscles
The myosin head, also known as a cross-bridge,
attaches to a nearby actin filament
Then the head bends and swivels, pulling the actin
filament toward the midline of the sarcomere
The myosin head disengages from the actin filament
The movements of myosin require ATP
The cycle begins again
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Figure 6.4 When a muscle contracts, actin filaments slide past
myosin filaments.
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Contraction of Muscles
Muscle contraction is controlled by the availability of
calcium ions
Muscle cells contain the proteins troponin and
tropomyosin
The troponin-tropomyosin complex and calcium ions
regulate muscle contraction at the actin-myosin
binding sites
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Contraction of Muscles
When a muscle is relaxed, the troponin-tropomyosin
complex covers the actin-myosin binding sites
Muscle contraction occurs when calcium ions bind to
troponin, causing it to change shape
This change in shape moves tropomyosin, exposing
the actin-myosin binding sites
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Figure 6.5 Calcium ions initiate muscle contraction.
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Calcium Ions and Regulatory Proteins
Calcium ions are stored in the sarcoplasmic
reticulum, a form of smooth endoplasmic reticulum
found in muscle cells
Transverse tubules (T tubules)
Pockets in the plasma membrane of a muscle cell
Carry signals from motor neurons deep into the
muscle cell to every sarcomere
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Calcium Ions and Regulatory Proteins
Rigor mortis
Muscle contraction will occur as long as ATP is
present
Without ATP, cross-bridges cannot be broken
Within 3 to 4 hours after death, the muscles become
stiff rigor mortis
Actin and myosin gradually break down and muscles
relax again after 2 to 3 days
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Role of Nerves
Neuromuscular junction
Junction between the tip of a motor neuron and a
skeletal muscle cell
A nerve impulse travels down a motor neuron to the
neuromuscular junction, where it causes the release
of acetylcholine (a neurotransmitter) from the motor
neuron
Acetylcholine diffuses across a small gap and binds
to receptors on the plasma membrane of the muscle
cell
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Role of Nerves
The acetylcholine causes changes in the
permeability of the muscle cell, resulting in an
electrochemical message similar to a nerve impulse
The message travels along the plasma membrane
into the T tubules and then to the sarcoplasmic
reticulum, releasing calcium ions for muscle
contraction
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Role of Nerves
Web Activity: Muscle Structure and Function
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Figure 6.6 Neuromuscular junction.
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Muscular Dystrophy (MD)
A group of inherited conditions in which muscles
weaken: if too many calcium ions enter a muscle
cell, proteins may be destroyed, eventually causing
the cell to die; on a large scale, muscles weaken
Duchenne muscular dystrophy
One of the most common forms
The gene for production of the protein dystrophin is
defective
Lack of dystrophin allows excess calcium ions to enter
muscle cells, eventually killing the cells
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Voluntary Movement
Motor unit: a motor neuron and all the muscle cells it
stimulates
All the muscle cells in a given motor unit contract
together
The number of muscle cells in a motor unit is highly
variable
Muscles responsible for precise movements have
fewer muscle cells in each motor unit than do muscles
responsible for less precise movements
On average, there are 150 muscle cells in a motor unit
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Figure 6.7 A motor unit.
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Motor Units and Recruitment
Increasing the number of motor units that are
stimulated increases the strength of muscle
contraction. This process, performed by the nervous
system, is called recruitment.
Muscle twitch
Contraction of a muscle in response to a single
stimulus
Twitches are very brief and typically not part of normal
movements
If a second stimulus is received before the muscle is
fully relaxed, the second twitch will be stronger than the
first, due to summation
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Muscle Twitches, Summation, and Tetanus
A sustained, powerful contraction caused by very
frequent stimuli
Fatigue sets in when a muscle is unable to contract
even when stimulated
Changing the frequency of stimulation is another
way to vary the contraction of muscles
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Figure 6.8 Muscle contraction shown graphically.
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Energy for Muscle Contraction
Muscle contraction requires an enormous amount of
energy
ATP for muscle contraction comes from many
sources, typically used in sequence
ATP stored in muscle cells
Creatine phosphate stored in muscle cells
Anaerobic metabolic pathways
Aerobic respiration
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Figure 6.9 Energy sources for muscle contraction.
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Slow-Twitch and Fast-Twitch Muscle Cells
Slow-twitch muscle cells
Contract slowly, with great endurance
Abundant mitochondria
Packed with myoglobin (oxygen-binding pigment)
Dark, reddish appearance
Myoglobin
Rich blood supply
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Slow-Twitch and Fast-Twitch Muscle Cells
Fast-twitch muscle cells
Contract rapidly and powerfully but with much less
endurance
Can make and break cross-bridge attachments more
rapidly
Have more actin and myosin
Rely on anaerobic metabolic pathways to generate
ATP and therefore tire quickly
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Figure 6.10 Slow- and fast-twitch muscle cells.
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Building Muscle
Aerobic exercise
Enough oxygen is delivered to the muscles to keep
them going for long periods
Increases endurance and coordination
Promotes development of new blood vessels
Increases the number of mitochondria
Typically does not increase size of muscles
Examples: walking, jogging, swimming
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Building Muscle
Resistance exercise
Builds strength
Muscles increase in size when they are repeatedly
made to exert more than 75% of their maximum force
Increases in muscle size reflect increases in the
diameter of existing muscle cells
Example: weight lifting
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Building Muscle
| From Star to Suspicion:
Did Cycling Champ Use Steroids?
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You Should Now Be Able To:
Describe the function and characteristics of muscles
Know how muscles contract and explain voluntary
movement, muscle twitch, tetanus and rigor mortis
Describe the possible damage to skeletal muscles or
tendons
Describe where the energy comes from for muscle
contraction
Understand the difference between slow-twitch and
fast-twitch muscle cells, how to build muscle, and
the risks associated with the use of steroids